CHAPTER 7

Very few programs execute all lines sequentially from .STARTUP to .EXIT. Rather, complex program logic and efficiency dictate that you control the flow of your program - jumping from one point to another, repeating an action until a condition is reached, and passing control to and from procedures. This chapter describes various ways for controlling program flow and several features that simplify coding program-control constructs.

The first section covers jumps from one point in the program to another. It explains how MASM 6.1 optimizes both unconditional and conditional jumps under certain circumstances, so that you do not have to specify every attribute. The section also describes instructions you can use to test conditional jumps.

The next section describes loop structures that repeat actions or evaluate conditions. It discusses MASM directives, such as .WHILE and .REPEAT, that generate appropriate compare, loop, and jump instructions for you, and the .IF, .ELSE, and .ELSEIF directives that generate jump instructions.

The "Procedures" section in this chapter explains how to write an assembly-language procedure. It covers the extended functionality for PROC, a PROTO directive that lets you write procedure prototypes similar to those used in C, an INVOKE directive that automates parameter passing, and options for the stack-frame setup inside procedures.

Jumps are the most direct way to change program control from one location to another. At the processor level, jumps work by changing the value of the IP (Instruction Pointer) register to a target offset and, for far jumps, by changing the CS register to a new segment address. Jump instructions fall into only two categories: conditional and unconditional.

The JMP instruction transfers control unconditionally to another instruction. JMP’s single operand contains the address of the target instruction.

; Handle one case
label1: .
.
.
jmp continue

; Handle second case
label2: .
.
.
jmp continue
.
.
.
continue:

The distance of the target from the jump instruction and the size of the operand determine the assembler’s encoding of the instruction. The longer the distance, the more bytes the assembler uses to code the instruction. In versions of MASM prior to 6.0, unconditional NEAR jumps sometimes generated inefficient code, but MASM can now optimize unconditional jumps.

The assembler determines the smallest encoding possible for the direct unconditional jump. MASM does not require a distance operator, so you do not have to determine the correct distance of the jump. If you specify a distance, it overrides any assembler optimization. If the specified distance falls short of the target address, the assembler generates an error. If the specified distance is longer than the jump requires, the assembler encodes the given distance and does not optimize it.

You do not specify SHORT, NEAR, FAR, NEAR16, NEAR32, FAR16, FAR32, or PROC as the distance of the target.

The target of the jump is not external and is in the same segment as the jump instruction. If the target is in a different segment (but in the same group), it is treated as though it were external.

If these two conditions are met, MASM uses the instruction, distance, and size of the operand to determine how to optimize the encoding for the jump. No syntax changes are necessary.

This information about jump optimizing also applies to conditional jumps on the 80386/486.

An indirect operand provides a pointer to the target address, rather than the address itself. A pointer is a variable that contains an address. The processor distinguishes indirect (pointer) operands from direct (address) operands by the instruction’s context.

You can specify the pointer’s size with the WORD, DWORD, or FWORD attributes. Default sizes are based on .MODEL and the default segment size.

jmp [bx] ; Uses .MODEL and segment size defaults
jmp WORD PTR [bx] ; A NEAR16 indirect call

If the indirect operand is a register, the jump is always a NEAR16 jump for a 16-bit register, and NEAR32 for a 32-bit register:

jmp DWORD PTR [var] ; A NEAR32 jump in a 32-bit segment;
; a FAR16 jump in a 16-bit segment

In this case, your code must clear the ambiguity with the NEAR32 or FAR16 keywords. The following example shows how to use TYPEDEF to define NEAR32 and FAR16 pointer types.

NFP TYPEDEF PTR NEAR32
FFP TYPEDEF PTR FAR16
jmp NFP PTR [var] ; NEAR32 indirect jump
jmp FFP PTR [var] ; FAR16 indirect jump

You can use an unconditional jump as a form of conditional jump by specifying the address in a register or indirect memory operand. Also, you can use indirect memory operands to construct jump tables that work like C switch statements, Pascal CASE statements, or Basic ON GOTO, ON GOSUB, or SELECT CASE statements, as shown in the following example.

NPVOID TYPEDEF NEAR PTR
.DATA
ctl_tbl NPVOID extended, ; Null key (extended code)
ctrla, ; Address of CONTROL-A key routine
ctrlb ; Address of CONTROL-B key routine
.CODE
.
.
.
mov ah, 8h ; Get a key
int 21h
cbw ; Stretch AL into AX
mov bx, ax ; Copy
shl bx, 1 ; Convert to address
jmp ctl_tbl[bx] ; Jump to key routine

extended:
mov ah, 8h ; Get second key of extended key
int 21h
. ; Use another jump table
. ; for extended keys
.
jmp next
ctrla: . ; CONTROL-A code here
.
.
jmp next
ctrlb: . ; CONTROL-B code here
.
.
jmp next
.
.
next: . ; Continue

In this instance, the indirect memory operands point to addresses of routines for handling different keystrokes.

The most common way to transfer control in assembly language is to use a conditional jump. This is a two-step process:

All conditional jumps except two (JCXZ and JECXZ) use the processor flags for their criteria. Thus, any statement that sets or clears a flag can serve as a test basis for a conditional jump. The jump statement can be any one of 30 conditional-jump instructions. A conditional-jump instruction takes a single operand containing the target address. You cannot use a pointer value as a target as you can with unconditional jumps.

JCXZ and JECXZ are special conditional jumps that do not consult the processor flags. Instead, as their names imply, these instructions cause a jump only if the CX or ECX register is zero. The use of JCXZ and JECXZ with program loops is covered in the next section, "Loops."

The remaining conditional jumps in the processor’s repertoire all depend on the status of the flags register. As the following list shows, several conditional jumps have two or three names - JE (Jump if Equal) and JZ (Jump if Zero), for example. Shared names assemble to exactly the same machine instruction, so you may choose whichever mnemonic seems more appropriate. Jumps that depend on the status of the flags register include:

The last two jumps in the list, JPE (Jump if Parity Even) and JPO (Jump if Parity Odd), are useful only for communications programs. The processor sets the parity flag if an operation produces a result with an even number of set bits. A communications program can compare the flag against the parity bit received through the serial port to test for transmission errors.

The conditional jumps in the preceding list can follow any instruction that changes the processor flags, as these examples show:

; Uses JO to handle overflow condition
add ax, bx ; Add two values
jo overflow ; If value too large, adjust

; Uses JNZ to check for zero as the result of subtraction
sub ax, bx ; Subtract
mov cx, Count ; First, initialize CX
jnz skip ; If the result is not zero, continue
call zhandler ; Else do special case

As the second example shows, the jump does not have to immediately follow the instruction that alters the flags. Since MOV does not change the flags, it can appear between the SUB instruction and the dependent jump.

Comparison of two values

Individual bit settings in a value

Whether a value is zero or nonzero

The CMP instruction is the most common way to test for conditional jumps. It compares two values without changing either, then sets or clears the processor flags according to the results of the comparison.

Internally, the CMP instruction is the same as the SUB instruction, except that CMP does not change the destination operand. Both set flags according to the result of the subtraction.

You can compare signed or unsigned values, but you must choose the subsequent conditional jump to reflect the correct value type. For example, JL (Jump if Less Than) and JB (Jump if Below) may seem conceptually similar, but a failure to understand the difference between them can result in program bugs. Table 7.1 shows the correct conditional jumps for comparisons of signed and unsigned values. The table shows the zero, carry, sign, and overflow flags as ZF, CF, SF, and OF, respectively.

Signed Comparisons Instruction Jump if True Unsigned Comparisons Instruction Jump if True

The mnemonic names of jumps always refer to the comparison of CMP’s first operand (destination) with the second operand (source). For instance, in this example, JG tests whether the first operand is greater than the second.

cmp ax, bx ; Compare AX and BX
jg next1 ; Equivalent to: If ( AX > BX ) goto next1

The individual bit settings in a single value can also serve as the criteria for a conditional jump. The TEST instruction tests whether specific bits in an operand are on or off (set or clear), and sets the zero flag accordingly.

The TEST instruction is the same as the AND instruction, except that TEST changes neither operand. The following example shows an application of TEST.

.DATA
bits BYTE ?
.CODE
.
.
.
; If bit 2 or bit 4 is set, then call task_a
; Assume "bits" is 0D3h 11010011
test bits, 10100y ; If 2 or 4 is set AND 00010100
jz skip1 ; --------
call task_a ; Then call task_a 00010000
skip1: ; Jump taken
.
.
.
; If bits 2 and 4 are clear, then call task_b
; Assume "bits" is 0E9h 11101001
test bits, 10100y ; If 2 and 4 are clear AND 00010100
jnz skip2 ; --------
call task_b ; Then call task_b 00000000
skip2: ; Jump taken

The source operand for TEST is often a mask in which the test bits are the only bits set. The destination operand contains the value to be tested. If all the bits set in the mask are clear in the destination operand, TEST sets the zero flag. If any of the flags set in the mask are also set in the destination operand, TEST clears the zero flag.

The 80386/486 processors provide additional bit-testing instructions. The BT (Bit Test) series of instructions copy a specified bit from the destination operand to the carry flag. A JC or JNC can then route program flow depending on the result. For variations on the BT instruction, see the Reference.

A program often needs to jump based on whether a particular register contains a value of zero. We’ve seen how the JCXZ instruction jumps depending on the value in the CX register. You can test for zero in other data registers nearly as efficiently with the OR instruction. A program can OR a register with itself without changing the register’s contents, then act on the resulting flags status. For example, the following example tests whether BX is zero:

but produces smaller and faster code, since it does not use an immediate number as an operand. The same technique also lets you test a register’s sign bit:

Unlike an unconditional jump, a conditional jump cannot reference a label more than 128 bytes away. For example, the following statement is valid as long as target is within a distance of 128 bytes:

; Jump to target less than 128 bytes away
jz target ; If previous operation resulted
; in zero, jump to target

However, if target is too distant, the following sequence is necessary to enable a longer jump. Note this sequence is logically equivalent to the preceding example:

; Jumps to distant targets previously required two steps
jnz skip ; If previous operation result is
; NOT zero, jump to "skip"
jmp target ; Otherwise, jump to target
skip:

MASM can automate jump-extending for you. If you target a conditional jump to a label farther than 128 bytes away, MASM rewrites the instruction with an unconditional jump, which ensures that the jump can reach its target. If target lies within a 128-byte range, the assembler encodes the instruction jz target as is. Otherwise, MASM generates two substitute instructions:

The assembler generates this same code sequence if you specify the distance with NEAR PTR, FAR PTR, or SHORT. Therefore,

MASM enables automatic jump expansion by default, but you can turn it off with the NOLJMP form of the OPTION directive. For information about the OPTION directive, see page 24.

If the assembler generates code to extend a conditional jump, it issues a level 3 warning saying that the conditional jump has been lengthened. You can set the warning level to 1 for development and to level 3 for a final optimizing pass to see if you can shorten jumps by reorganizing.

If you specify the distance for the jump and the target is out of range for that distance, a "Jump out of Range" error results.

Since the JCXZ and JECXZ instructions do not have logical negations, expansion of the jump instruction to handle targets with unspecified distances cannot be performed for those instructions. Therefore, the distance must always be short.

The size and distance of the target operand determines the encoding for conditional or unconditional jumps to externals or targets in different segments. The jump-extending and optimization features do not apply in this case.

Conditional jumps on the 80386 and 80486 processors can be to targets up to 32K away, so jump extension occurs only for targets greater than that distance.

When you code jumps in assembly language, you must invent many label names. One alternative to continually thinking up new label names is to use anonymous labels, which you can use anywhere in your program. But because anonymous labels do not provide meaningful names, they are best used for jumping over only a few lines of code. You should mark major divisions of a program with actual named labels.

Use two at signs (@@) followed by a colon (:) as an anonymous label. To jump to the nearest preceding anonymous label, use @B (back) in the jump instruction’s operand field; to jump to the nearest following anonymous label, use @F (forward) in the operand field.

The items @B and @F always refer to the nearest occurrences of @@:, so there is never any conflict between different anonymous labels.

The high-level structures you can use for decision-making are the .IF, .ELSEIF, and .ELSE statements. These directives generate conditional jumps. The expression following the .IF directive is evaluated, and if true, the following instructions are executed until the next .ENDIF, .ELSE, or .ELSEIF directive is reached. The .ELSE statements execute if the expression is false. Using the .ELSEIF directive puts a new expression inside the alternative part of the original .IF statement to be evaluated. The syntax is:

.IF condition1
statements
[[.ELSEIF condition2
statements]]
[[.ELSE
statements]]
.ENDIF

.IF cx == 20
0017 83 F9 14 * cmp cx, 014h
001A 75 05 * jne @C0001
001C BA 0014 mov dx, 20
.ELSE
001F EB 03 * jmp @C0003
0021 *@C0001:
0021 BA 001E mov dx, 30
.ENDIF
0024 *@C0003:

Loops repeat an action until a termination condition is reached. This condition can be a counter or the result of an expression’s evaluation. MASM 6.1 offers many ways to set up loops in your programs. The following list compares MASM loop structures:

LOOP Automatically decrements CX. When CX = 0, the loop ends. The top of the loop cannot be greater than 128 bytes from the LOOP instruction. (This is true for all LOOP instructions.)

LOOPE/LOOPZ, LOOPNE/LOOPNZ Loops while equal or not equal. Checks both CX and the state of the zero flag. LOOPZ ends when either CX=0 or the zero flag is clear, whichever occurs first. LOOPNZ ends when either CX=0 or the zero flag is set, whichever occurs first. LOOPE and LOOPZ assemble to the same machine instruction, as do LOOPNE and LOOPNZ. Use whichever mnemonic best fits the context of your loop. Set CX to a number out of range if you don’t want a count to control the loop.

JCXZ, JECXZ Branches to a label only if CX = 0 or ECX = 0. Unlike other conditional-jump instructions, which can jump to either a near or a short label under the 80386 or 80486, JCXZ and JECXZ always jump to a short label.

Conditional jumps Acts only if certain conditions met. Necessary if several conditions must be tested. See "Conditional Jumps," page 164.

; The LOOP instruction: For 200 to 0 do task
mov cx, 200 ; Set counter
next: . ; Do the task here
.
.
loop next ; Do again
; Continue after loop

; The LOOPNE instruction: While AX is not 'Y', do task
mov cx, 256 ; Set count too high to interfere
wend: . ; But don't do more than 256 times
. ; Some statements that change AX
.
cmp al, 'Y' ; Is it Y or too many times?
loopne wend ; No? Repeat
; Yes? Continue

The JCXZ and JECXZ instructions provide an efficient way to avoid executing loops when the loop counter CX is empty. For example, consider the following loops:

mov cx, LoopCount ; Load loop counter
next: . ; Iterate loop CX times
.
.
loop next ; Do again

If LoopCount is zero, CX decrements to -1 on the first pass. It then must decrement 65,535 more times before reaching 0. Use a JCXZ to avoid this problem:

mov cx, LoopCount ; Load loop counter
jcxz done ; Skip loop if count is 0
next: . ; Else iterate loop CX times
.
.
loop next ; Do again
done: ; Continue after loop

The high-level control structures generate loop structures for you. These directives are similar to the while and repeat loops of C or Pascal, and can make your assembly programs easier to code and to read. The assembler generates the appropriate assembly code. These directives are summarized as follows:

.WHILE ... .ENDW The statements between .WHILE condition and .ENDW execute while the condition is true.

.REPEAT ... .UNTIL The loop executes at least once and continues until the condition given after .UNTIL is true. Generates conditional jumps.

.REPEAT ... .UNTILCXZ Compares label to an expression and generates appropriate loop instructions.

These constructs work much as they do in a high-level language such as C or Pascal. Keep in mind the following points:

These directives generate appropriate processor instructions. They are not new instructions.

They require proper use of signed and unsigned data declarations.

These directives cause a set of instructions to execute based on the evaluation of some condition. This condition can be an expression that evaluates to a signed or unsigned value, an expression using the binary operators in C (&&, ||, or !), or the state of a flag. For more information about expression operators, see page 178.

The evaluation of the condition requires the assembler to know if the operands in the condition are signed or unsigned. To state explicitly that a named memory
location contains a signed integer, use the signed data allocation directives SBYTE, SWORD, and SDWORD.

As with while loops in C or Pascal, the test condition for .WHILE is checked before the statements inside the loop execute. If the test condition is false, the loop does not execute. While the condition is true, the statements inside the loop repeat.

Use the .ENDW directive to mark the end of the .WHILE loop. When the condition becomes false, program execution begins at the first statement following the .ENDW directive. The .WHILE directive generates appropriate compare and jump statements. The syntax is:

For example, this loop copies the contents of one buffer to another until a ‘$’ character (marking the end of the string) is found:

.DATA
buf1 BYTE "This is a string",'$'
buf2 BYTE 100 DUP (?)
.CODE
sub bx, bx ; Zero out bx
.WHILE (buf1[bx] != '$')
mov al, buf1[bx] ; Get a character
mov buf2[bx], al ; Move it to buffer 2
inc bx ; Count forward
.ENDW

MASM’s .REPEAT directive allows for loop constructions like the do loop of C and the REPEAT loop of Pascal. The loop executes until the condition following the .UNTIL (or .UNTILCXZ) directive becomes true. Since the condition is checked at the end of the loop, the loop always executes at least once. The .REPEAT directive generates conditional jumps. The syntax is:

where condition can also be expr1 == expr2 or expr1 != expr2. When two conditions are used, expr2 can be an immediate expression, a register, or (if expr1 is a register) a memory location.

For example, the following code fills a buffer with characters typed at the keyboard. The loop ends when the ENTER key (character 13) is pressed:

.DATA
buffer BYTE 100 DUP (0)
.CODE
sub bx, bx ; Zero out bx
.REPEAT
mov ah, 01h
int 21h ; Get a key
mov buffer[bx], al ; Put it in the buffer
inc bx ; Increment the count
.UNTIL (al == 13) ; Continue until al is 13

The .UNTIL directive generates conditional jumps, but the .UNTILCXZ directive generates a LOOP instruction, as shown by the listing file code for these examples. In a listing file, assembler-generated code is preceded by an asterisk.

ASSUME bx:PTR SomeStruct

.REPEAT
*@C0001:
inc ax
.UNTIL ax==6
* cmp ax, 006h
* jne @C0001

.REPEAT
*@C0003:
mov ax, 1
.UNTILCXZ
* loop @C0003

.REPEAT
*@C0004:
.UNTILCXZ [bx].field != 6
* cmp [bx].field, 006h
* loope @C0004

The .BREAK and .CONTINUE directives terminate a .REPEAT or .WHILE loop prematurely. These directives allow an optional .IF clause for conditional breaks. The syntax is:

Note that .ENDIF is not used with the .IF forms of .BREAK and .CONTINUE in this context. The .BREAK and .CONTINUE directives work the same way as the break and continue instructions in C. Execution continues at the instruction following the .UNTIL, .UNTILCXZ, or .ENDW of the nearest enclosing loop.

Instead of ending the loop execution as .BREAK does, .CONTINUE causes loop execution to jump directly to the code that evaluates the loop condition of the nearest enclosing loop.

The following loop accepts only the keys in the range ‘0’ to ‘9’ and terminates when you press ENTER.

.WHILE 1 ; Loop forever
mov ah, 08h ; Get key without echo
int 21h
.BREAK .IF al == 13 ; If ENTER, break out of the loop
.CONTINUE .IF (al < '0') || (al > '9')
; If not a digit, continue looping
mov dl, al ; Save the character for processing
mov ah, 02h ; Output the character
int 21h
.ENDW

If you assemble the preceding source code with the /Fl and /Sg command-line options and then view the results in the listing file, you will see this code:

.WHILE 1
0017 *@C0001:
0017 B4 08 mov ah, 08h
0019 CD 21 int 21h
.BREAK .IF al == 13
001B 3C 0D * cmp al, 00Dh
001D 74 10 * je @C0002
.CONTINUE .IF (al '0') || (al '9')
001F 3C 30 * cmp al, '0'
0021 72 F4 * jb @C0001
0023 3C 39 * cmp al, '9'
0025 77 F0 * ja @C0001
0027 8A D0 mov dl, al
0029 B4 02 mov ah, 02h
002B CD 21 int 21h
.ENDW
002D EB E8 * jmp @C0001
002F *@C0002:

The high-level control structures can be nested. That is, .REPEAT or .WHILE loops can contain .REPEAT or .WHILE loops as well as .IF statements.

If the code generated by a .WHILE loop, .REPEAT loop, or .IF statement generates a conditional or unconditional jump, MASM encodes the jump using the jump extension and jump optimization techniques described in "Unconditional Jumps," page 162, and "Conditional Jumps," page 164.

You can express the conditions of the .IF, .REPEAT, and .WHILE directives using relational operators, and you can express the attributes of the operand with the PTR operator. To write loop conditions, you also need to know how the assembler evaluates the operators and operands in the condition. This section explains the operators, attributes, precedence level, and expression evaluation order for the conditions used with loop-generating directives.

The binary relational operators in MASM 6.1 are the same binary operators used in C. These operators generate MASM compare, test, and conditional jump instructions. High-level control instructions include:

A condition without operators (other than !) tests for nonzero as it does in C. For example, .WHILE (x) is the same as .WHILE (x != 0), and .WHILE (!x) is the same as .WHILE (x == 0).

You can also use the flag names (ZERO?, CARRY?, OVERFLOW?, SIGN?, and PARITY?) as operands in conditions with the high-level control structures. For example, in .WHILE (CARRY?), the value of the carry flag determines the outcome of the condition.

Expression operators generate unsigned jumps by default. However, if either side of the operation is signed, the assembler considers the entire operation signed.

You can use the PTR operator to tell the assembler that a particular operand in a register or constant is a signed number, as in these examples:

Without the PTR operator, the assembler would treat the contents of BX as an unsigned value.

You can also specify the size attributes of operands in memory locations with SBYTE, SWORD, and SDWORD, for use with .IF, .WHILE, and .REPEAT.

.DATA
mem1 SBYTE ?
mem2 WORD ?
.IF mem1 > 0
.WHILE mem2 < bx
.WHILE SWORD PTR ax < count

As with C, you can concatenate conditions with the && operator for AND, the || operator for OR, and the ! operator for negate. The precedence level is !, &&, and ||, with ! having the highest priority. Like expressions in high-level languages, precedence is evaluated left to right.

The assembler evaluates conditions created with high-level control structures according to short-circuit evaluation. If the evaluation of a particular condition automatically determines the final result (such as a condition that evaluates to false in a compound statement concatenated with AND), the evaluation does not continue.

the assembler evaluates the first condition. If this condition is false (that is, if AX is less than or equal to 0), the evaluation is finished. The second condition is not checked and the loop does not execute, because a compound condition containing && requires both expressions to be true for the entire condition to be true.

Organizing your code into procedures that execute specific tasks divides large programs into manageable units, allows for separate testing, and makes code more efficient for repetitive tasks.

Assembly-language procedures are similar to functions, subroutines, and procedures in high-level languages such as C, FORTRAN, and Pascal. Two instructions control the use of assembly-language procedures. CALL pushes the return address onto the stack and transfers control to a procedure, and RET pops the return address off the stack and returns control to that location.

The PROC and ENDP directives mark the beginning and end of a procedure. Additionally, PROC can automatically:

Preserve register values that should not change but that the procedure might otherwise alter.

Set up a local stack pointer, so that you can access parameters and local variables placed on the stack.

Adjust the stack when the procedure ends.

Procedures require a label at the start of the procedure and a RET instruction at the end. Procedures are normally defined by using the PROC directive at the start of the procedure and the ENDP directive at the end. The RET instruction normally is placed immediately before the ENDP directive. The assembler makes sure the distance of the RET instruction matches the distance defined by the PROC directive. The basic syntax for PROC is:

label PROC [[NEAR | FAR]]
.
.
.
RET [[constant]]
label ENDP

The CALL instruction pushes the address of the next instruction in your code onto the stack and passes control to a specified address. The syntax is:

The operand contains a value calculated at run time. Since that operand can be a register, direct memory operand, or indirect memory operand, you can write call tables similar to the example code on page 164.

Calls can be near or far. Near calls push only the offset portion of the calling address and therefore must target a procedure within the same segment or group. You can specify the type for the target operand. If you do not, MASM uses the declared distance (NEAR or FAR) for operands that are labels and for the size of register or memory operands. The assembler then encodes the call appropriately, as it does with unconditional jumps. (See previous "Unconditional Jumps" and "Conditional Jumps.")

MASM optimizes a call to a far non-external label when the label is in the current segment by generating the code for a near call, saving one byte.

You can define procedures without PROC and ENDP, but if you do, you must make sure that the size of the CALL matches the size of the RET. You can specify the RET instruction as RETN (Return Near) or RETF (Return Far) to override the default size:

call NEAR PTR task ; Call is declared near
. ; Return comes to here
.
.
task: ; Procedure begins with near label
.
. ; Instructions go here
.
retn ; Return declared near

The RET instruction (and its RETF and RETN variations) allows an optional constant operand that specifies a number of bytes to be added to the value of the SP register after the return. This operand adjusts for arguments passed to the procedure before the call, as shown in the example in "Using Local Variables," following.

When you define procedures without PROC and ENDP, you must make sure that calls have the same size as corresponding returns. For example, RETF pops two words off the stack. If a NEAR call is made to a procedure with a far return, the popped value is meaningless, and the stack status may cause the execution to return to a random memory location, resulting in program failure.

An extended PROC syntax automates many of the details of accessing arguments and saving registers. See "Declaring Parameters with the PROC Directive," later in this chapter.

Each time you call a procedure, you may want it to operate on different data. This data, called "arguments," can be passed to the procedure in various ways. Although you can pass arguments to a procedure in registers or in variables, the most common method is the stack. Microsoft languages have specific conventions for passing arguments. These conventions for assembly-language modules shared with modules from high-level languages are explained in Chapter 12, "Mixed-Language Programming."

This section describes how a procedure accesses the arguments passed to it on the stack. Each argument is accessed as an offset from BP. However, if you use the PROC directive to declare parameters, the assembler calculates these offsets for you and lets you refer to parameters by name. The next section, "Declaring Parameters with the PROC Directive," explains how to use PROC this way. This example shows how to pass arguments to a procedure. The procedure expects to find those arguments on the stack. As this example shows, arguments must be accessed as offsets of BP.

; C-style procedure call and definition

mov ax, 10 ; Load and
push ax ; push constant as third argument
push arg2 ; Push memory as second argument
push cx ; Push register as first argument
call addup ; Call the procedure
add sp, 6 ; Destroy the pushed arguments
. ; (equivalent to three pops)
.
.
addup PROC NEAR ; Return address for near call
; takes two bytes
push bp ; Save base pointer - takes two bytes
; so arguments start at fourth byte
mov bp, sp ; Load stack into base pointer
mov ax, [bp+4] ; Get first argument from
; fourth byte above pointer
add ax, [bp+6] ; Add second argument from
; sixth byte above pointer
add ax, [bp+8] ; Add third argument from
; eighth byte above pointer
pop bp ; Restore BP
ret ; Return result in AX
addup ENDP

Starting with the 80186 processor, the ENTER and LEAVE instructions simplify the stack setup and restore instructions at the beginning and end of procedures. However, ENTER uses a lot of time. It is necessary only with nested, statically-scoped procedures. Thus, a Pascal compiler may sometimes generate ENTER. The LEAVE instruction, on the other hand, is an efficient way to do the stack cleanup. LEAVE reverses the effect of the last ENTER instruction by restoring BP and SP to their values before the procedure call.

With the PROC directive, you can specify registers to be saved, define param-
eters to the procedure, and assign symbol names to parameters (rather than as offsets from BP). This section describes how to use the PROC directive to automate the parameter-accessing techniques described in the last section.

For example, the following diagram shows a valid PROC statement for a procedure called from C. It takes two parameters, var1 and arg1, and uses (and must save) the DI and SI registers:

label PROC [[attributes]] [[USES reglist]] [[, ]] [[parameter[[:tag]]... ]]

attributes Any of several attributes of the procedure, including the distance, langtype, and visibility of the procedure. The syntax for attributes is given on the following page.

reglist A list of registers following the USES keyword that the procedure uses, and that should be saved on entry. Registers in the list must be separated by blanks or tabs, not by commas. The assembler generates prologue code to push these registers onto the stack. When you exit, the assembler generates epilogue code to pop the saved register values off the stack.

parameter The list of parameters passed to the procedure on the stack. The list can have a variable number of parameters. See the discussion following for the syntax of parameter. This list can be longer than one line if the continued line ends with a comma.

distance Controls the form of the RET instruction generated. Can be NEAR or FAR. If distance is not specified, it is determined from the model declared with the .MODEL directive. NEAR distance is assumed for TINY, SMALL, COMPACT, and FLAT. The assembler assumes FAR distance for MEDIUM, LARGE, and HUGE. For 80386/486 programming with 16- and 32-bit segments, you can specify NEAR16,NEAR32,FAR16, or FAR32.

langtype Determines the calling convention used to access parameters and restore the stack. The BASIC, FORTRAN, and PASCAL langtypes convert procedure names to uppercase, place the last parameter in the parameter list lowest on the stack, and generate a RET num instruction to end the procedure. The RET adjusts the stack upward by num, which represents the number of bytes in the argument list. This step, called "cleaning the stack," returns the stack pointer SP to the value it had before the caller pushed any arguments.

The C and STDCALL langtype prefixes an underscore to the procedure name when the procedure’s scope is PUBLIC or EXPORT and places the first parameter lowest on the stack. SYSCALL is equivalent to the C calling convention with no underscore prefixed to the procedure’s name. STDCALL uses caller stack cleanup when :VARARG is specified; otherwise the called routine must clean up the stack (see Chapter 12).

visibility Indicates whether the procedure is available to other modules. The visibility can be PRIVATE, PUBLIC, or EXPORT. A procedure name is PUBLIC unless it is explicitly declared as PRIVATE. If the visibility is EXPORT, the linker places the procedure’s name in the export table for segmented executables. EXPORT also enables PUBLIC visibility.

You can explicitly set the default visibility with the OPTION directive. OPTION PROC:PUBLIC sets the default to public. For more information, see Chapter 1, "Using the Option Directive."

prologuearg Specifies the arguments that affect the generation of prologue and epilogue code (the code MASM generates when it encounters a PROC directive or the end of a procedure). For an explanation of prologue and epilogue code, see "Generating Prologue and Epilogue Code," later in this chapter.

The comma that separates parameters from reglist is optional, if both fields appear on the same line. If parameters appears on a separate line, you must end the reglist field with a comma. In the syntax:

parmname is the name of the parameter. The tag can be the qualifiedtype or the keyword VARARG. However, only the last parameter in a list of param-
eters can use the VARARG keyword. The qualifiedtype is discussed in "Data Types," Chapter 1. An example showing how to reference VARARG param-
eters appears later in this section. You can nest procedures if they do not have parameters or USES register lists. This diagram shows a procedure definition with one parameter definition.

The procedure presented in "Passing Arguments on the Stack," page 182, is here rewritten using the extended PROC functionality. Prior to the procedure call, you must push the arguments onto the stack unless you use INVOKE. (See "Calling Procedures with INVOKE," later in this chapter.)

addup PROC NEAR C,
arg1:WORD, arg2:WORD, count:WORD
mov ax, arg1
add ax, count
add ax, arg2
ret
addup ENDP

If the arguments for a procedure are pointers, the assembler does not generate any code to get the value or values that the pointers reference; your program must still explicitly treat the argument as a pointer. (For more information about using pointers, see Chapter 3, "Using Addresses and Pointers.")

In the following example, even though the procedure declares the parameters as near pointers, you must code two MOV instructions to get the values of the param-
eters. The first MOV gets the address of the parameters, and the second MOV gets the parameter.

; Call from C as a FUNCTION returning an integer

.MODEL medium, c
.CODE
myadd PROC arg1:NEAR PTR WORD, arg2:NEAR PTR WORD

mov bx, arg1 ; Load first argument
mov ax, [bx]
mov bx, arg2 ; Add second argument
add ax, [bx]

ret

myadd ENDP

You can use conditional-assembly directives to make sure your pointer parameters are loaded correctly for the memory model. For example, the following version of myadd treats the parameters as FAR parameters, if necessary.

.MODEL medium, c ; Could be any model
.CODE
myadd PROC arg1:PTR WORD, arg2:PTR WORD

IF @DataSize
les bx, arg1 ; Far parameters
mov ax, es:[bx]
les bx, arg2
add ax, es:[bx]
ELSE
mov bx, arg1 ; Near parameters
mov ax, [bx]
mov bx, arg2
add ax, [bx]
ENDIF

ret
myadd ENDP

In the PROC statement, you can append the :VARARG keyword to the last parameter to indicate that the procedure accepts a variable number of arguments. However, :VARARG applies only to the C, SYSCALL, or STDCALL calling conventions (see Chapter 12). A symbol must precede :VARARG so the procedure can access arguments as offsets from the given variable name, as this example illustrates:

addup3 PROTO NEAR C, argcount:WORD, arg1:VARARG

invoke addup3, 3, 5, 2, 4

addup3 PROC NEAR C, argcount:WORD, arg1:VARARG
sub ax, ax ; Clear work register
sub si, si

.WHILE argcount > 0 ; Argcount has number of arguments
add ax, arg1[si] ; Arg1 has the first argument
dec argcount ; Point to next argument
inc si
inc si
.ENDW

ret ; Total is in AX
addup3 ENDP

You can pass non-default-sized pointers in the VARARG portion of the parameter list by separately passing the segment portion and the offset portion of the address.

When you use the extended PROC features and the assembler encounters a RET instruction, it automatically generates instructions to pop saved registers, remove local variables from the stack, and, if necessary, remove parameters. It generates this code for each RET instruction it encounters. You can reduce code size by having only one return and jumping to it from various locations.

In high-level languages, local variables are visible only within a procedure. In Microsoft languages, these variables are usually stored on the stack. In assembly-language programs, you can also have local variables. These variables should not be confused with labels or variable names that are local to a module, as described in Chapter 8, "Sharing Data and Procedures Among Modules and Libraries."

This section outlines the standard methods for creating local variables. The next section shows how to use the LOCAL directive to make the assembler

automatically generate local variables. When you use this directive, the assembler generates the same instructions as those demonstrated in this section but handles some of the details for you.

If your procedure has relatively few variables, you can usually write the most efficient code by placing these values in registers. Use local (stack) data when you have a large amount of temporary data for the procedure.

To use a local variable, you must save stack space for it at the start of the procedure. A procedure can then reference the variable by its position in the stack. At the end of the procedure, you must clean the stack by restoring the stack pointer. This effectively throws away all local variables and regains the stack space they occupied.

This example subtracts 2 bytes from the SP register to make room for a local word variable, then accesses the variable as [bp-2].

push ax ; Push one argument
call task ; Call
.
.
.

task PROC NEAR
push bp ; Save base pointer
mov bp, sp ; Load stack into base pointer
sub sp, 2 ; Save two bytes for local variable
.
.
.
mov WORD PTR [bp-2], 3 ; Initialize local variable
add ax, [bp-2] ; Add local variable to AX
sub [bp+4], ax ; Subtract local from argument
. ; Use [bp-2] and [bp+4] in
. ; other operations
.
mov sp, bp ; Clear local variables
pop bp ; Restore base
ret 2 ; Return result in AX and pop
task ENDP ; two bytes to clear parameter

Notice the instruction mov sp,bp at the end of the procedure restores the original value of SP. The statement is required only if the value of SP changes inside the procedure (usually by allocating local variables). The argument passed to the procedure is removed with the RET instruction. Contrast this to the example in "Passing Arguments on the Stack," page 182, in which the calling code adjusts the stack for the argument.

MASM’s LOCAL directive automates the process for creating local variables on the stack. LOCAL frees you from having to count stack words, and it makes your code easier to write and maintain. This section illustrates the advantages of creating temporary data with the LOCAL directive.

To use the LOCAL directive, list the variables you want to create, giving a type for each one. The assembler calculates how much space is required on the stack. It also generates instructions to properly decrement SP (as described in the previous section) and to reset SP when you return from the procedure.

When you create local variables this way, your source code can refer to each local variable by name rather than as an offset of the stack pointer. Moreover, the assembler generates debugging information for each local variable. If you have programmed before in a high-level language that allows scoping, local variables will seem familiar. For example, a C compiler sets up variables with automatic storage class in the same way as the LOCAL directive.

task PROC NEAR arg:WORD
LOCAL loc:WORD
.
.
.
mov loc, 3 ; Initialize local variable
add ax, loc ; Add local variable to AX
sub arg, ax ; Subtract local from argument
. ; Use "loc" and "arg" in other operations
.
.
ret
task ENDP

The LOCAL directive must be on the line immediately following the PROC statement with the following syntax:

Each vardef defines a local variable. A local variable definition has this form:

label The name given to the local variable. You can use this name to access the variable.

count The number of elements of this name and type to allocate on the stack. You can allocate a simple array on the stack with count. The brackets around count are required. If this field is omitted, one data object is assumed.

qualifiedtype A simple MASM type or a type defined with other types and attributes. For more information, see "Data Types" in Chapter 1.

If the number of local variables exceeds one line, you can place a comma at the end of the first line and continue the list on the next line. Alternatively, you can use several consecutive LOCAL directives.

The assembler does not initialize local variables. Your program must include code to perform any necessary initializations. For example, the following code fragment sets up a local array and initializes it to zero:

arraysz EQU 20

aproc PROC USES di
LOCAL var1[arraysz]:WORD, var2:WORD
.
.
.
; Initialize local array to zero
push ss
pop es ; Set ES=SS
lea di, var1 ; ES:DI now points to array
mov cx, arraysz ; Load count
sub ax, ax
rep stosw ; Store zeros
; Use the array...
.
.
.
ret
aproc ENDP

Even though you can reference stack variables by name, the assembler treats them as offsets of BP, and they are not visible outside the procedure. In the following procedure, array is a local variable.

index EQU 10
test PROC NEAR
LOCAL array[index]:WORD
.
.
.
mov bx, index
; mov array[bx], 5 ; Not legal!

The second MOV statement may appear to be legal, but since array is an
offset of BP, this statement is the same as

BP and BX can be added only to SI and DI. This example would be legal, however, if the index value were moved to SI or DI. This type of error in your program can be difficult to find unless you keep in mind that local variables in procedures are offsets of BP.

MASM provides the INVOKE directive to handle many of the details important to procedure calls, such as pushing parameters according to the correct calling conventions. To use INVOKE, the procedure called must have been declared previously with a PROC statement, an EXTERNDEF (or EXTERN) statement, or a TYPEDEF. You can also place a prototype defined with PROTO before the INVOKE if the procedure type does not appear before the INVOKE. Procedure prototypes defined with PROTO inform the assembler of types and numbers of arguments so the assembler can check for errors and provide automatic conversions when INVOKE calls the procedure.

Declaring procedure prototypes is good programming practice, but is optional. Prototypes in MASM perform the same function as prototypes in C and other high-level languages. A procedure prototype includes the procedure name, the types, and (optionally) the names of all parameters the procedure expects. Prototypes usually are placed at the beginning of an assembly program or in a separate include file so the assembler encounters the prototype before the actual procedure.

Prototypes enable the assembler to check for unmatched parameters and are especially useful for procedures called from other modules and other languages. If you write routines for a library, you may want to put prototypes into an include file for all the procedures used in that library. For more information about using include files, see Chapter 8, "Sharing Data and Procedures among Modules and Libraries."

The PROTO directive provides one way to define a procedure prototype. The syntax for a prototype definition is the same as for a procedure declaration (see "Declaring Parameters with the PROC Directive," earlier in this chapter), except that you do not include the list of registers, prologuearg list, or the scope of the procedure.

Also, the PROTO keyword precedes the langtype and distance attributes. The attributes (like C and FAR) are optional. However, if they are not specified, the defaults are based on any .MODEL or OPTION LANGUAGE statement. The names of the parameters are also optional, but you must list parameter types. A label preceding :VARARG is also optional in the prototype but not in the PROC statement.

If a PROTO and a PROC for the same function appear in the same module, they must match in attribute, number of parameters, and parameter types. The easiest way to create prototypes with PROTO is to write your procedure and then copy the first line (the line that contains the PROC keyword) to a location in your program that follows the data declarations. Change PROC to PROTO and remove the USES reglist, the prologuearg field, and the visibility field. It is important that the prototype follow the declarations for any types used in it to avoid any forward references used by the parameters in the prototype.

The following example illustrates how to define and then declare two typical procedures. In both prototype and declaration, the comma before the argument list is optional only when the list does not appear on a separate line:

; Procedure prototypes.

addup PROTO NEAR C argcount:WORD, arg2:WORD, arg3:WORD
myproc PROTO FAR C, argcount:WORD, arg2:VARARG

; Procedure declarations

addup PROC NEAR C, argcount:WORD, arg2:WORD, arg3:WORD
.
.
.
myproc PROC FAR C PUBLIC <callcount> USES di si,
argcount:WORD,
arg2:VARARG

When you call a procedure with INVOKE, the assembler checks the arguments given by INVOKE against the parameters expected by the procedure. If the data types of the arguments do not match, MASM reports an error or converts the type to the expected type. These conversions are explained in the next section.

INVOKE generates a sequence of instructions that push arguments and call a procedure. This helps maintain code if arguments or langtype for a procedure are changed. INVOKE generates procedure calls and automatically:

Converts arguments to the expected types.

Pushes arguments on the stack in the correct order.

Cleans the stack when the procedure returns.

If arguments do not match in number or if the type is not one the assembler can convert, an error results.

If the procedure uses VARARG, INVOKE can pass a number of arguments different from the number in the parameter list without generating an error or warning. Any additional arguments must be at the end of the INVOKE argument list. All other arguments must match those in the prototype parameter list.

where expression can be the procedure’s label or an indirect reference to a procedure, and arguments can be an expression, a register pair, or an expression preceded with ADDR. (The ADDR operator is discussed later in this chapter.)

addup PROTO NEAR C argcount:WORD, arg2:WORD, arg3:WORD
myproc PROTO FAR C, argcount:WORD, arg2:VARARG

addup PROC NEAR C, argcount:WORD, arg2:WORD, arg3:WORD
.
.
.
myproc PROC FAR C PUBLIC <callcount> USES di si,
argcount:WORD,
arg2:VARARG

The assembler can convert some arguments and parameter type combinations so that the correct type can be passed. The signed or unsigned qualities of the arguments in the INVOKE statements determine how the assembler converts them to the types expected by the procedure.

The addup procedure, for example, expects parameters of type WORD, but the arguments passed by INVOKE to the addup procedure can be any of these types:

BYTE, SBYTE, WORD, or SWORD

An expression whose type is specified with the PTR operator to be one of those types

An 8-bit or 16-bit register

An immediate expression in the range -32K to +64K

A NEAR PTR

If the type is smaller than that expected by the procedure, MASM widens the argument to match.

For INVOKE to correctly handle type conversions, you must use the signed data types for any signed assignments. MASM widens an argument to match the type expected by a procedure’s parameters in these cases:

The assembler can extend a segment if far data is expected, and it can convert the type given in the list to the types expected. If the assembler cannot convert the type, however, it generates an error.

If the assembler needs to widen an argument, it first copies the value to AL or AX. It widens an unsigned value by placing a zero in the higher register area, and widens a signed value with a CBW, CWD, or CWDE instruction as required. Similarly, the assembler copies a constant argument value into AL or AX when the .8086 directive is in effect. You can see these generated instructions in the listing file when you include the /Sg command-line option.

Using the accumulator register to widen or copy an argument may lead to an error if you attempt to pass AX as another argument. For example, consider the following INVOKE statement for a procedure with the C calling convention

where arg is a BYTE variable and myproc expects four arguments of type WORD. The assembler widens and then pushes arg like this:

The generated code thus overwrites the last argument (AX) passed to the procedure. The assembler generates an error in this case, requiring you to rewrite the INVOKE statement.

To summarize, the INVOKE directive overwrites AX and perhaps DX when widening arguments. It also uses AX to push constants on the 8088 and 8086. If you use these registers (or EAX and EDX on an 80386/486) to pass arguments, they may be overwritten. The assembler’s error detection prevents this from ever becoming a run-time bug, but AX and DX should remain your last choice for holding arguments.

You can pass a FAR pointer in a segment::offset pair, as shown in the following. Note the use of double colons to separate the register pair. The registers could be any other register pair, including a pair that an MS-DOS call uses to return values.

FPWORD TYPEDEF FAR PTR WORD
SomeProc PROTO var1:DWORD, var2:WORD, var3:WORD

pfaritem FPWORD faritem
.
.
.
les bx, pfaritem
INVOKE SomeProc, ES::BX, arg1, arg2

However, INVOKE cannot combine into a single address one argument for the segment and one for the offset.

You can use the ADDR operator to pass the address of an expression to a procedure that expects a NEAR or FAR pointer. This example generates code to pass a far pointer (to arg1) to the procedure proc1.

PBYTE TYPEDEF FAR PTR BYTE
arg1 BYTE "This is a string"
proc1 PROTO NEAR C fparg:PBYTE
.
.
.
INVOKE proc1, ADDR arg1

For information on defining pointers with TYPEDEF, see "Defining Pointer Types with TYPEDEF" in Chapter 3.

You can make an indirect procedure call such as call [bx + si] by using a pointer to a function prototype with TYPEDEF, as shown in this example:

FUNCPROTO TYPEDEF PROTO NEAR ARG1:WORD
FUNCPTR TYPEDEF PTR FUNCPROTO

.DATA
pfunc FUNCPTR OFFSET proc1, OFFSET proc2

.CODE
.
.
.
mov bx, OFFSET pfunc ; BX points to table
mov si, Num ; Num contains 0 or 2
INVOKE FUNCPTR PTR [bx+si], arg1 ; Call proc1 if Num=0
; or proc2 if Num=2

You can also use ASSUME to accomplish the same task. The following ASSUME statement associates the type FUNCPTR with the BX register.

Code generated by the INVOKE directive may vary depending on the processor mode and calling conventions in effect. You can check your listing files to see the code generated by the INVOKE directive if you use the /Sg command-line
option.

When you use the PROC directive with its extended syntax and argument list, the assembler automatically generates the prologue and epilogue code in your procedure. "Prologue code" is generated at the start of the procedure. It sets up a stack pointer so you can access parameters from within the procedure. It also saves space on the stack for local variables, initializes registers such as DS, and pushes registers that the procedure uses. Similarly, "epilogue code" is the code at the end of the procedure that pops registers and returns from the procedure.

The assembler automatically generates the prologue code when it encounters the first instruction or label after the PROC directive. This means you cannot label the prologue for the purpose of jumping to it. The assembler generates the epilogue code when it encounters a RET or IRET instruction. Using the assembler-generated prologue and epilogue code saves time and decreases the number of repetitive lines of code in your procedures.

Local variables defined.

Arguments passed to the procedure.

Current processor selected (affects epilogue code only).

Current calling convention.

Options passed in the prologuearg of the PROC directive.

Registers being saved.

The prologuearg list contains options specifying how to generate the prologue or epilogue code. The next section explains how to use these options, gives the standard prologue and epilogue code, and explains the techniques for defining your own prologue and epilogue code.

The standard prologue and epilogue code handles parameters and local variables. If a procedure does not have any parameters or local variables, the prologue and epilogue code that sets up and restores a stack pointer is omitted, unless
FORCEFRAME is included in the prologuearg list. (FORCEFRAME is discussed later in this section.) Prologue and epilogue code also generates a push and pop for each register in the register list.

1. Point BP to top of stack.

2. Make space on stack for local variables.

3. Save registers the procedure must preserve.

The epilogue cancels these three steps in reverse order, then cleans the stack, if necessary, with a RET num instruction. For example, the procedure declaration

myproc PROC NEAR PASCAL USES di si,
arg1:WORD, arg2:WORD, arg3:WORD
LOCAL local1:WORD, local2:WORD

push bp ; Step 1:
mov bp, sp ; point BP to stack top
sub sp, 4 ; Step 2: space for 2 local words
push di ; Step 3:
push si ; save registers listed in USES

pop si ; Undo Step 3
pop di
mov sp, bp ; Undo Step 2
pop bp ; Undo Step 1
ret 6 ; Clean stack of pushed arguments

Notice the RET 6 instruction cleans the stack of the three word-sized arguments. The instruction appears in the epilogue because the procedure does not use the C calling convention. If myproc used C conventions, the epilogue would end with a RET instruction without an operand.

The assembler generates standard epilogue code when it encounters a RET instruction without an operand. It does not generate an epilogue if RET has a nonzero operand. To suppress generation of a standard epilogue, use RETN or RETF with or without an operand, or use RET 0.

The standard prologue and epilogue code recognizes two operands passed in the prologuearg list, LOADDS and FORCEFRAME. These operands modify the prologue code. Specifying LOADDS saves and initializes DS. Specifying
FORCEFRAME as an argument generates a stack frame even if no arguments are sent to the procedure and no local variables are declared. If your procedure has any parameters or locals, you do not need to specify FORCEFRAME.

For example, adding LOADDS to the argument list for myproc creates this prologue:

push bp ; Step 1:
mov bp, sp ; point BP to stack top
sub sp, 4 ; Step 2: space for 2 locals
push ds ; Save DS and point it
mov ax, DGROUP ; to DGROUP, as
mov ds, ax ; instructed by LOADDS
push di ; Step 3:
push si ; save registers listed in USES

pop si ; Undo Step 3
pop di
pop ds ; Restore DS
mov sp, bp ; Undo Step 2
pop bp ; Undo Step 1
ret 6 ; Clean stack of pushed arguments

If you want a different set of instructions for prologue and epilogue code in your procedures, you can write macros that run in place of the standard prologue and epilogue code. For example, while you are debugging your procedures, you may want to include a stack check or track the number of times a procedure is called. You can write your own prologue code to do these things whenever a procedure executes. Different prologue code may also be necessary if you are writing applications for Windows. User-defined prologue macros will respond correctly if you specify FORCEFRAME in the prologuearg of a procedure.

To write your own prologue or epilogue code, the OPTION directive must appear in your program. It disables automatic prologue and epilogue code generation. When you specify

the assembler calls the macro specified in the OPTION directive instead of generating the standard prologue and epilogue code. The prologue macro must be a macro function, and the epilogue macro must be a macro procedure.

macroname MACRO procname, \
flag, \
parmbytes, \
localbytes, \
<reglist>, \
userparms

Your macro must have formal parameters to match all the actual arguments passed. The arguments passed to your macro include:

Bit 0, 1, 2 For calling conventions (000=unspecified language type, 001=C, 010=SYSCALL, 011=STDCALL, 100=PASCAL, 101=FORTRAN, 110=BASIC).

Bit 8 Set if the epilogue is generated as a result of an IRET instruction and cleared if the epilogue is generated as a result of a RET instruction.

parmbytes The accumulated count in bytes of all parameters given in the PROC statement.

localbytes The count in bytes of all locals defined with the LOCAL directive.

reglist A list of the registers following the USES operator in the procedure declaration. Enclose this list with angle brackets (< >) and separate each item with commas. Reverse the list for epilogues.

userparms Any argument you want to pass to the macro. The prologuearg (if there is one) specified in the PROC directive is passed to this argument.

Your macro function must return the parmbytes parameter. However, if the prologue places other values on the stack after pushing BP and these values are not referenced by any of the local variables, the exit value must be the number of bytes for procedure locals plus any space between BP and the locals. Therefore, parmbytes is not always equal to the bytes occupied by the locals.

The following macro is an example of a user-defined prologue that counts the number of times a procedure is called.

ProfilePro MACRO procname, \
flag, \
bytecount, \
numlocals, \
regs, \
macroargs

.DATA
procname&count WORD 0
.CODE
inc procname&count ; Accumulates count of times the
; procedure is called
push bp
mov bp, sp
; Other BP operations
IFNB <regs>
FOR r, regs
push r
ENDM
ENDIF
EXITM %bytecount
ENDM

Your program must also include this statement before calling any procedures that use the prologue:

If you define either a prologue or an epilogue macro, the assembler uses the standard prologue or epilogue code for the one you do not define. The form of the code generated depends on the .MODEL and PROC options used.

If you want to revert to the standard prologue or epilogue code, use
PROLOGUEDEF or EPILOGUEDEF as the macroname in the OPTION statement.

In this case, no user-defined macro is called, and the assembler does not generate a default code sequence. This state remains in effect until the next OPTION
PROLOGUE or OPTION EPILOGUE is encountered.

For additional information about writing macros, see Chapter 9, "Using Macros." The PROLOGUE.INC file provided in the MASM 6.1 distribution disks can create the prologue and epilogue sequences for the Microsoft C professional development system.

In addition to jumps, loops, and procedures that alter program execution, interrupt routines transfer execution to a different location. In this case, control goes to an interrupt routine.

You can write your own interrupt routines, either to replace an existing routine or to use an undefined interrupt number. For example, you may want to replace an MS-DOS interrupt handler, such as the Critical Error (Interrup 24h) and CONTROL+C (Interrupt 23h) handlers. The BOUND instruction checks array bounds and calls Interrupt 5 when an error occurs. If you use this instruction, you need to write an interrupt handler for it.

How to call interrupts

How the processor handles interrupts

How to redefine an existing interrupt routine

The example routine in this section handles addition or multiplication overflow and illustrates the steps necessary for writing an interrupt routine. For additional information about MS-DOS and BIOS interrupts, see Chapter 11, "Writing Memory-Resident Software."

Interrupts provide a way to access MS-DOS and ROM-BIOS from assembly language. They are called with the INT instruction, which takes an immediate value between 0 and 255 as its only operand.

MS-DOS and ROM-BIOS interrupt routines accept data through registers. For instance, most MS-DOS routines (and many BIOS routines) require a function number in the AH register. Many handler routines also return values in registers. To use an interrupt, you must know what data the handler routine expects and what data, if any, it returns. For information, consult Help or one of the other references mentioned in the Introduction.

The following fragment illustrates a simple call to MS-DOS Function 9, which displays the string msg on the screen:

.DATA
msg BYTE "This writes to the screen$"
.CODE
mov ax, SEG msg ; Necessary only if DS does not
mov ds, ax ; already point to data segment
mov dx, offset msg ; DS:DX points to msg
mov ah, 09h ; Request Function 9
int 21h

1. Looks up the address of the interrupt routine in the Interrupt Vector Table. This table starts at the lowest point in memory (segment 0, offset 0) and consists of a series of far pointers called vectors. Each vector comprises a 4-byte address (segment:offset) pointing to an interrupt handler routine. The table sequence implies the number of the interrupt the vector references: the first vector points to the Interrupt 0 handler, the second vector to the Interrupt 1 handler, and so forth. Thus, the vector at 0000:i*4 holds the address of the handler routine for Interrupt i.

2. Clears the trap flag (TF) and interrupt enable flag (IF).

3. Pushes the flags register, the current code segment (CS), and the current instruction pointer (IP), in that order. (The current instruction is the one following the INT statement.) As with a CALL, this ensures control returns to the next logical position in the program.

4. Jumps to the address of the interrupt routine, as specified in the Interrupt Vector Table.

5. Executes the code of the interrupt routine until it encounters an IRET
instruction.

6. Pops the instruction pointer, code segment, and flags.

Provide a new routine to handle the interrupt.

Replace the old routine’s address in the Interrupt Vector Table with the address of your new routine.

Replace the old address back into the vector table before your program ends.

You can write an interrupt routine as a procedure by using the PROC and ENDP directives. The routine should always be defined as FAR and should end with an IRET instruction instead of a RET instruction.

You can use the full extended PROC syntax (described in "Declaring Parameters with the PROC Directive," earlier in this chapter) to write interrupt procedures. However, you should not make interrupt procedures NEAR or specify arguments for them. You can use the USES keyword, however, to correctly generate code to save and restore a register list in interrupt procedures.

The IRET instruction in MASM 6.1 has two forms that suppress epilogue code. This allows an interrupt to have local variables or use a user-defined prologue. IRETF pops a FAR16 return address, and IRETFD pops a FAR32 return address.

The following example shows how to replace the handler for Interrupt 4. Once registered in the Interrupt Vector Table, the new routine takes control when the processor encounters either an INT 4 instruction or its special variation INTO (Interrupt on Overflow). INTO is a conditional instruction that acts only when the overflow flag is set. With INTO after a numerical calculation, your code can automatically route control to a handler routine if the calculation results in a numerical overflow. By default, the routine for Interrupt 4 simply consists of an IRET, so it returns without doing anything. Using INTO is an alternative to using JO (Jump on Overflow) to jump to another set of instructions.

The following example program first executes INT 21h to invoke MS-DOS Function 35h (Get Interrupt Vector). This function returns the existing vector for Interrupt 4. The program stores the vector, then invokes MS-DOS Function 25h (Set Interrupt Vector) to place the address of the ovrflow procedure in the Interrupt Vector Table. From this point on, ovrflow gains control whenever the processor executes INTO while the overflow flag is set. The new routine displays a message and returns with AX and DX set to 0.

.MODEL LARGE, C
FPFUNC TYPEDEF FAR PTR
.DATA
msg BYTE "Overflow - result set to 0",13,10,'$'
vector FPFUNC ?
.CODE
.STARTUP

mov ax, 3504h ; Load Interrupt 4 and call DOS
int 21h ; Get Interrupt Vector
mov WORD PTR vector[2],es ; Save segment
mov WORD PTR vector[0],bx ; and offset

push ds ; Save DS
mov ax, cs ; Load segment of new routine
mov ds, ax
mov dx, OFFSET ovrflow ; Load offset of new routine
mov ax, 2504h ; Load Interrupt 4 and call DOS
int 21h ; Set Interrupt Vector
pop ds ; Restore
.
.
.
add ax, bx ; Do arithmetic
into ; Call Interrupt 4 if overflow
.
.
.
lds dx, vector ; Load original address
mov ax, 2504h ; Restore it to vector table
int 21h ; with DOS set vector function
mov ax, 4C00h ; Terminate function
int 21h

ovrflow PROC FAR
sti ; Enable interrupts
; (turned off by INT)
mov ah, 09h ; Display string function
mov dx, OFFSET msg ; Load address
int 21h ; Call DOS
sub ax, ax ; Set AX to 0
cwd ; Set DX to 0
iret ; Return
ovrflow ENDP
END

Before the program ends, it again uses MS-DOS Function 25h to reset the original Interrupt 4 vector back into the Interrupt Vector Table. This reestablishes the original routine as the handler for Interrupt 4.

The first instruction of the ovrflow routine warrants further discussion. When the processor encounters an INT instruction, it clears the interrupt flag before branching to the specified interrupt handler routine. The interrupt flag serves a crucial role in smoothing the processor’s tasks, but must not be abused. When clear, the flag inhibits hardware interrupts such as the keyboard or system timer. It should be left clear only briefly and only when absolutely necessary. Unless you have a

compelling reason to leave the flag clear, always include an STI (Set Interrupt Flag) instruction at the beginning of your interrupt handler routine to reenable hardware interrupts.

CLI (Clear Interrupt Flag) and its corollary STI are designed to protect small sections of time-dependent code from interruptions by the hardware. If you use CLI in your program, be sure to include a matching STI instruction as well. The sample interrupt handlers in Chapter 11, "Writing Memory-Resident Software," illustrate how to use these important instructions.

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