Chapter 4

**Defining and Using
Simple
Data Types**

This chapter covers the concepts essential for working with simple data types in assembly-language programs. The first section shows how to declare integer variables. The second section describes basic operations including moving, loading, and sign-extending numbers, as well as calculating. The last section describes how to do various operations with numbers at the bit level, such as using bitwise logical instructions and shifting and rotating bits.

The complex data types introduced in the next
chapter - arrays, strings, structures, unions, and records -
use many of the operations illustrated in this chapter.
Floating-point operations require a different set of
instructions and techniques. These are covered in Chapter 6,
"Using Floating-Point and Binary Coded Decimal

Numbers."

**Declaring Integer
Variables**

An integer is a whole number, such as 4 or
4,444. Integers have no fractional part, as do the real numbers
discussed in Chapter 6. You can initialize integer variables in
several ways with the data allocation directives. This section
explains how to use the **SIZEOF** and **TYPE** operators
to provide information to the assembler about the types in your
program. For information on symbolic integer constants, see
"Integer Constants and Constant Expressions" in Chapter
1.

**Allocating Memory for
Integer Variables**

When you declare an integer variable by assigning a label to a data allocation directive, the assembler allocates memory space for the integer. The variable’s name becomes a label for the memory space. The syntax is:

[[*name*]] *directive* *
initializer*

The following directives indicate the integer’s size and value range:

**Directive Description of
Initializers **

**BYTE**, **
DB** (byte) Allocates
unsigned numbers from 0 to 255.

**SBYTE** (signed
byte) Allocates signed numbers from -128 to +127.

**WORD, DW** (word =
2 bytes) Allocates unsigned numbers from 0 to 65,535
(64K).

**SWORD** (signed
word) Allocates signed numbers from -32,768 to +32,767.

**DWORD, DD**
(doubleword = 4 bytes), Allocates unsigned numbers from 0 to
4,294,967,295 (4 megabytes).

**SDWORD** (signed
doubleword) Allocates signed numbers from -2,147,483,648 to
+2,147,483,647.

**FWORD, DF**
(farword = 6 bytes) Allocates 6-byte (48-bit) integers. These
values are normally used only as pointer variables on the
80386/486 processors.

**QWORD, DQ**
(quadword = 8 bytes) Allocates 8-byte integers used
with 8087-family coprocessor instructions.

**TBYTE, DT** (10
bytes), Allocates 10-byte (80-bit) integers if the initializer
has a radix specifying the base of the number.

See Chapter 6 for information on the **
REAL4**, **REAL8**, and **REAL10** directives that
allocate real numbers.

The **SIZEOF** and **TYPE** operators,
when applied to a type, return the size of an integer of that
type. The size attribute associated with each data type
is:

**Data Type Bytes **

**BYTE, SBYTE** 1

**WORD, SWORD** 2

**DWORD, SDWORD** 4

**FWORD** 6

**QWORD** 8

**TBYTE** 10

The data types **SBYTE**, **SWORD**,
and **SDWORD** tell the assembler to treat the initializers
as signed data. It is important to use these signed types with
high-level constructs such as **.IF**, **.WHILE**, and
**.REPEAT**, and with **PROTO** and **INVOKE**
directives. For descriptions of these directives, see the
sections "Loop-Generating Directives," "Declaring Procedure
Prototypes," and "Calling Procedures with INVOKE" in Chapter
7.

The assembler stores integers with the least significant bytes lowest in memory. Note that assembler listings and most debuggers show the bytes of a word in the opposite order - high byte first.

Figure 4.1 illustrates the integer formats.

**Figure 4.7 Integer Formats**

Although the **TYPEDEF** directive’s
primary purpose is to define pointer variables (see "Defining
Pointer Types with TYPEDEF" in Chapter 3), you can also use **
TYPEDEF** to create an alias for any integer type. For
example, these declarations

char TYPEDEF
SBYTE

long TYPEDEF DWORD

float TYPEDEF REAL4

double TYPEDEF REAL8

allow you to use char, long, float, or double in your programs if you prefer the C data labels.

**Data
Initialization**

You can initialize variables when you declare them with constants or expressions that evaluate to constants. The assembler generates an error if you specify an initial value too large for the variable type.

A **?** in place of an initializer
indicates you do not require the assembler to initialize the
variable. The assembler allocates the space but does not write
in it. Use **?** for buffer areas or variables your program
will initialize at run time.

You can declare and initialize variables in one step with the data directives, as these examples show.

integer BYTE 16
; Initialize byte to 16

negint SBYTE -16 ; Initialize signed byte to -16

expression WORD 4*3 ; Initialize word to 12

signedexp SWORD 4*3 ; Initialize signed word to 12

empty QWORD ? ; Allocate uninitialized long int

BYTE 1,2,3,4,5,6 ; Initialize six unnamed bytes

long DWORD 4294967295 ; Initialize doubleword to

; 4,294,967,295

longnum SDWORD -2147433648 ; Initialize signed doubleword

; to -2,147,433,648

tb TBYTE 2345t ; Initialize 10-byte binary number

For information on arrays and on using the
**DUP** operator to allocate initializer lists, see "Arrays
and Strings" in Chapter 5.

**Working with Simple
Variables**

Once you have declared integer variables in your program, you can use them to copy, move, and sign-extend integer variables in your MASM code. This section shows how to do these operations as well as how to add, subtract, multiply, and divide numbers and do bit-level manipulations with logical, shift, and rotate instructions.

Since MASM instructions require operands to
be the same size, you may need to operate on data in a size
other than that originally declared. You can do this with the
**PTR** operator. For example, you can use the **PTR**
operator to access the high-order word of a **DWORD**-size
variable. The syntax for the **PTR** operator is

*type* **PTR** *
expression*

where the **PTR** operator forces *
expression* to be treated as having the type specified. An
example of this use is

.DATA

num DWORD 0

.CODE

mov ax, WORD
PTR num[0] ; Loads a word-size value from

mov dx, WORD PTR num[2] ; a doubleword variable

**Copying
Data**

The primary instructions for moving data from
operand to operand and loading them into registers are **
MOV** (Move), **XCHG** (Exchange), **CWD** (Convert
Word to Double), and **CBW** (Convert Byte to
Word).

**Moving
Data**

The most common method of moving data, the
**MOV** instruction, is essentially a copy instruction,
since it always copies the source operand to the destination
operand without affecting the source. After a **MOV**
instruction, the source and destination operands contain the
same value.

The following example illustrates the **
MOV** instruction. As explained in "General-Purpose
Registers," Chapter 1, you cannot move a value from one
location in memory to another in a single operation.

; Immediate
value moves

mov ax, 7 ; Immediate to register

mov mem, 7 ; Immediate to memory direct

mov mem[bx], 7 ; Immediate to memory indirect

; Register moves

mov mem, ax ; Register to memory direct

mov mem[bx], ax ; Register to memory indirect

mov ax, bx ; Register to register

mov ds, ax ; General register to segment register

; Direct memory moves

mov ax, mem ; Memory direct to register

mov ds, mem ; Memory to segment register

; Indirect memory moves

mov ax, mem[bx] ; Memory indirect to register

mov ds, mem[bx] ; Memory indirect to segment register

; Segment register moves

mov mem, ds ; Segment register to memory

mov mem[bx], ds ; Segment register to memory indirect

mov ax, ds ; Segment register to general register

The following example shows several common types of moves that require two instructions.

; Move
immediate to segment register

mov ax, DGROUP ; Load AX with immediate value

mov ds, ax ; Copy AX to segment register

; Move memory to memory

mov ax, mem1 ; Load AX with memory value

mov mem2, ax ; Copy AX to other memory

; Move segment register to segment register

mov ax, ds ; Load AX with segment register

mov es, ax ; Copy AX to segment register

The **MOVSX** and **MOVZX**
instructions for the 80386/486 processors extend and copy
values in one step. See "Extending Signed and Unsigned
Integers," following.

**Exchanging
Integers**

The **XCHG** (Exchange) instruction
exchanges the data in the source and destination operands. You
can exchange data between registers or between registers and
memory, but not from memory to memory:

xchg ax, bx ;
Put AX in BX and BX in AX

xchg memory, ax ; Put "memory" in AX and AX in "memory"

; xchg mem1, mem2 ; Illegal- can't exchange memory
locations

**Extending Signed and
Unsigned Integers**

Since moving data between registers of different sizes is illegal, you must "sign-extend" integers to convert signed data to a larger size. Sign-extending means copying the sign bit of the unextended operand to all bits of the operand’s next larger size. This widens the operand while maintaining its sign and value.

8086-based processors provide four instructions specifically for sign-extending. The four instructions act only on the accumulator register (AL, AX, or EAX), as shown in the following list.

**Instruction Sign-extend **

**CBW** (convert
byte to word) AL to AX

**CWD** (convert
word to doubleword) AX to DX:AX

**CWDE** (convert
word to doubleword extended)* AX to EAX

**CDQ** (convert
doubleword to quadword)* EAX to EDX:EAX

*Requires an extended register and applies only to 80386/486 processors.

On the 80386/486 processors, the **CWDE**
instruction converts a signed 16-bit value in AX to a signed
32-bit value in EAX. The **CDQ** instruction converts a
signed 32-bit value in EAX to a signed 64-bit value in the
EDX:EAX register pair.

This example converts signed integers using
**CBW**, **CWD**, **CWDE**, and **CDQ**.

.DATA

mem8 SBYTE -5

mem16 SWORD +5

mem32 SDWORD -5

.CODE

.

.

.

mov al, mem8 ; Load 8-bit -5 (FBh)

cbw ; Convert to 16-bit -5 (FFFBh) in AX

mov ax, mem16 ; Load 16-bit +5

cwd ; Convert to 32-bit +5 (0000:0005h) in DX:AX

mov ax, mem16 ; Load 16-bit +5

cwde ; Convert to 32-bit +5 (00000005h) in EAX

mov eax, mem32 ; Load 32-bit -5 (FFFFFFFBh)

cdq ; Convert to 64-bit -5

; (FFFFFFFF:FFFFFFFBh) in EDX:EAX

These four instructions efficiently convert unsigned values as well, provided the sign bit is zero. This example, for instance, correctly widens mem16 whether you treat the variable as signed or unsigned.

The processor does not differentiate between signed and unsigned values. For instance, the value of mem8 in the previous example is literally 251 (0FBh) to the processor. It ignores the human convention of treating the highest bit as an indicator of sign. The processor can ignore the distinction between signed and unsigned numbers because binary arithmetic works the same in either case.

If you add 7 to mem8, for example, the result is 258 (102h), a value too large to fit into a single byte. The byte-sized mem8 can accommodate only the least-significant digits of the result (02h), and so receives the value of 2. The result is the same whether we treat mem8 as a signed value (-5) or unsigned value (251).

This overview illustrates how the programmer,
not the processor, must keep track of which values are signed
or unsigned, and treat them accordingly. If AL=127 (01111111y),
the instruction **CBW** sets AX=127 because the sign bit is
zero. If AL=128 (10000000y), however, the sign bit is 1. **
CBW** thus sets AX=65,280

(FF00h), which may not be what you had in mind if you assumed AL originally held an unsigned value.To widen unsigned values, explicitly set the higher register to zero, as shown in the following example:

.DATA

mem8 BYTE 251

mem16 WORD 251

.CODE

.

.

.

mov al, mem8 ; Load 251 (FBh) from 8-bit memory

sub ah, ah ; Zero upper half (AH)

mov ax, mem16 ; Load 251 (FBh) from 16-bit memory

sub dx, dx ; Zero upper half (DX)

sub eax, eax ; Zero entire extended register (EAX)

mov ax, mem16 ; Load 251 (FBh) from 16-bit memory

The 80386/486 processors provide instructions
that move and extend a value to a larger data size in a single
step. **MOVSX** moves a signed value into a register and
sign-extends it. **MOVZX** moves an unsigned value into a
register and zero-

extends it.

; 80386/486
instructions

movzx dx, bl ; Load unsigned 8-bit value into

; 16-bit register and zero-extend

These special 80386/486 instructions usually execute much faster than the equivalent 8086/286 instructions.

**Adding and Subtracting
Integers**

You can use the **ADD**, **ADC**, **
INC**, **SUB**, **SBB**, and **DEC** instructions
for adding, incrementing, subtracting, and decrementing values
in single registers. You can also combine them to handle larger
values that require two registers for storage.

**Adding and Subtracting
Integers Directly**

The **ADD**, **INC** (Increment), **
SUB**, and **DEC** (Decrement) instructions operate on 8-
and 16-bit values on the 8086-80286 processors, and on 8-, 16-,
and 32-bit values on the 80386/486 processors. They can be
combined with the **ADC** and **SBB** instructions to
work on 32-bit values on the 8086 and 64-bit values on the
80386/486 processors. (See "Adding and Subtracting in Multiple
Registers," following.)

These instructions have two requirements:

1. If there are two operands, only one operand can be a memory operand.

2. If there are two operands, both must be the same size.

To meet the second requirement, you can use
the **PTR** operator to force an operand to the size
required. (See "Working with Simple Variables," previous.) For
example, if Buffer is an array of bytes and BX
points to an element of the array, you can add a word
from
Buffer with

add ax, WORD
PTR Buffer[bx] ; Add word from byte array

The next example shows 8-bit signed and unsigned addition and subtraction.

.DATA

mem8 BYTE 39

.CODE

; Addition

; signed unsigned

mov al, 26 ; Start with register 26 26

inc al ; Increment 1 1

add al, 76 ; Add immediate 76 + 76

; ---- ----

; 103 103

add al, mem8 ; Add memory 39 + 39

; ---- ----

mov ah, al ; Copy to AH -114 142

+overflow

add al, ah ; Add register 142

; ----

; 28+carry

; Subtraction

; signed unsigned

mov al, 95 ; Load register 95 95

dec al ; Decrement -1 -1

sub al, 23 ; Subtract immediate -23 -23

; ---- ----

; 71 71

sub al, mem8 ; Subtract memory -122 -122

; ---- ----

; -51 205+sign

mov ah, 119 ; Load register 119

sub al, ah ; and subtract -51

; ----

; 86+overflow

The **INC** and **DEC** instructions
treat integers as unsigned values and do not update the carry
flag for signed carries and borrows.

When the sum of 8-bit signed operands exceeds
127, the processor sets the overflow flag. (The overflow flag
is also set if both operands are negative and the sum is less
than or equal to -128.) Placing a **JO** (Jump on Overflow)
or **INTO** (Interrupt on Overflow) instruction in your
program at this point can transfer control to error-recovery
statements. When the sum exceeds 255, the processor sets the
carry flag. A **JC** (Jump on Carry) instruction at this
point can transfer control to error-recovery
statements.

In the previous subtraction example, the
processor sets the sign flag if the result goes below 0. At
this point, you can use a **JS** (Jump on Sign) instruction
to transfer control to error-recovery statements. Jump
instructions are described in the "Jumps" section in Chapter
7.

**Adding and Subtracting
in Multiple Registers**

You can add and subtract numbers larger than
the register size on your processor with the **ADC** (Add
with Carry) and **SBB** (Subtract with Borrow) instructions.
If the operations prior to an **ADC** or **SBB**
instruction do not set the carry flag, these instructions are
identical to **ADD** and **SUB**. When you operate on
large values in more than one register, use **ADD** and **
SUB** for the least significant part of the number and **
ADC** or **SBB** for the most significant part.

The following example illustrates multiple-register addition and subtraction. You can also use this technique with 64-bit operands on the 80386/486 processors.

.DATA

mem32 DWORD 316423

mem32a DWORD 316423

mem32b DWORD 156739

.CODE

.

.

.

; Addition

mov ax, 43981 ; Load immediate 43981

sub dx, dx ; into DX:AX

add ax, WORD PTR mem32[0] ; Add to both + 316423

adc dx, WORD PTR mem32[2] ; memory words ------

; Result in DX:AX 360404

; Subtraction

mov ax, WORD PTR mem32a[0] ; Load mem32 316423

mov dx, WORD PTR mem32a[2] ; into DX:AX

sub ax, WORD PTR mem32b[0] ; Subtract low - 156739

sbb dx, WORD PTR mem32b[2] ; then high ------

; Result in DX:AX 159684

For 32-bit registers on the 80386/486 processors, only two steps are necessary. If your program needs to be assembled for more than one processor, you can assemble the statements conditionally, as shown in this example:

.DATA

mem32 DWORD 316423

mem32a DWORD 316423

mem32b DWORD 156739

p386 TEXTEQU (@Cpu AND 08h)

.CODE

.

.

.

; Addition

IF p386

mov eax, 43981 ; Load immediate

add eax, mem32 ; Result in EAX

ELSE

.

. ; do steps in previous example

.

ENDIF

; Subtraction

IF p386

mov eax, mem32a ; Load memory

sub eax, mem32b ; Result in EAX

ELSE

.

. ; do steps in previous example

.

ENDIF

Since the status of the carry flag affects
the results of calculations with **ADC** and **SBB**, be
sure to turn off the carry flag with the **CLC** (Clear
Carry Flag) instruction or use **ADD** or **SUB** for the
first calculation, when appropriate.

**Multiplying and
Dividing Integers**

The 8086 family of processors uses different multiplication and division instructions for signed and unsigned integers. Multiplication and division instructions also have special requirements depending on the size of the operands and the processor the code runs on.

**Using Multiplication
Instructions**

The **MUL** instruction multiplies
unsigned numbers. **IMUL** multiplies signed numbers. For
both instructions, one factor must be in the accumulator
register (AL for 8-bit numbers, AX for 16-bit numbers, EAX for
32-bit numbers). The other factor can be in any single register
or memory operand. The result overwrites the contents of the
accumulator register.

Multiplying two 8-bit numbers produces a 16-bit result returned in AX. Multiplying two 16-bit operands yields a 32-bit result in DX:AX. The 80386/486 processor handles 64-bit products in the same way in the EDX:EAX pair.

This example illustrates multiplication of signed 16- and 32-bit integers.

.DATA

mem16 SWORD -30000

.CODE

.

.

.

; 8-bit unsigned multiply

mov al, 23 ; Load AL 23

mov bl, 24 ; Load BL * 24

mul bl ; Multiply BL -----

; Product in AX 552

; overflow and carry set

; 16-bit signed multiply

mov ax, 50 ; Load AX 50

; -30000

imul mem16 ; Multiply memory -----

; Product in DX:AX -1500000

; overflow and carry set

A nonzero number in the upper half of the result (AH for byte, DX or EDX for word) sets the overflow and carry flags.

On the 80186-80486 processors, the **
IMUL** instruction supports three additional operand
combinations. The first syntax option allows for 16-bit
multipliers producing a 16-bit product or 32-bit multipliers
for 32-bit products on the 80386/486. The result overwrites the
destination. The syntax for this operation is:

**IMUL** *register16*, *
immediate*

The second syntax option specifies three
operands for **IMUL**. The first operand must be a 16-bit
*register* operand, the second a 16-bit *memory* (or
*register*) operand, and the third a 16-bit *
immediate* operand. **IMUL** multiplies the memory (or
register) and immediate operands and stores the product in the
register operand with this syntax:

**IMUL register16,{
memory16 | register16}, immediate**

For the 80386/486 only, a third option for
**IMUL** allows an additional operand for multiplication of
a register value by a register or memory value. The syntax
is:

**IMUL** *register*,{*register*
**|** *memory*}

The destination can be any 16-bit or 32-bit register. The source must be the same size as the destination.

In all of these options, products too large
to fit in 16 or 32 bits set the overflow and carry flags. The
following examples show these three options for **
IMUL**.

imul dx, 456 ;
Multiply DX times 456 on 80186-80486

imul ax, [bx],6 ; Multiply the value pointed to by BX

; by 6 and put the result in AX

imul dx, ax ; Multiply DX times AX on 80386

imul ax, [bx] ; Multiply AX by the value pointed to

; by BX on 80386

The **IMUL** instruction with multiple
operands can be used for either signed or unsigned
multiplication, since the 16-bit product is the same in either
case. To get a 32-bit result, you must use the single-operand
version of **MUL** or **IMUL**.

**Using Division
Instructions**

The **DIV** instruction divides unsigned
numbers, and **IDIV** divides signed numbers. Both return a
quotient and a remainder.

Table 4.1 summarizes the division operations. The dividend is the number to be divided, and the divisor is the number to divide by. The quotient is the result. The divisor can be in any register or memory location except the registers where the quotient and remainder are returned.

**Table 4.1 Division
Operations**

**Size of Operand Dividend Register Size of
Divisor Quotient Remainder **

16 bits AX 8 bits AL AH

32 bits DX:AX 16 bits AX DX

64 bits (80386 and 80486) EDX:EAX 32 bits EAX EDX

Unsigned division does not require careful attention to flags. The following examples illustrate signed division, which can be more complex.

.DATA

mem16 SWORD -2000

mem32 SDWORD 500000

.CODE

.

.

.

; Divide 16-bit unsigned by 8-bit

mov ax, 700 ; Load dividend 700

mov bl, 36 ; Load divisor DIV 36

div bl ; Divide BL ------

; Quotient in AL 19

; Remainder in AH 16

; Divide 32-bit signed by 16-bit

mov ax, WORD PTR mem32[0] ; Load into DX:AX

mov dx, WORD PTR mem32[2] ; 500000

idiv mem16 ; DIV -2000

; Divide memory ------

; Quotient in AX -250

; Remainder in DX 0

; Divide 16-bit signed by 16-bit

mov ax, WORD PTR mem16 ; Load into AX -2000

cwd ; Extend to DX:AX

mov bx,-421 ; DIV -421

idiv bx ; Divide by BX -----

; Quotient in AX 4

; Remainder in DX -316

If the dividend and divisor are the same size, sign-extend or zero-extend the dividend so that it is the length expected by the division instruction. See "Extending Signed and Unsigned Integers," earlier in this chapter.

**Manipulating Numbers
at the Bit Level**

The instructions introduced so far in this chapter access numbers at the byte or word level. The logical, shift, and rotate instructions described in this section access individual bits in a number. You can use logical instructions to evaluate characters and do other text and screen operations. The shift and rotate instructions do similar tasks by shifting and rotating bits through registers. This section reviews some applications of these bit-level operations.

**Logical
Instructions**

The logical instructions **AND**, **
OR**, and **XOR** compare bits in two operands. Based on
the results of the comparisons, the instructions alter bits in
the first (destination) operand. The logical instruction **
NOT** also changes bits, but operates on a single
operand.

The following list summarizes these four logical instructions. The list makes reference to the "destination bit," meaning the bit in the destination operand. The terms "both bits" and "either bit" refer to the corresponding bits in the source and destination operands. These instructions include:

**Instruction Sets Destination Bit If Clears
Destination Bit If **

**AND** Both bits
set Either or both bits clear

**OR** Either or
both bits set Both bits clear

**XOR** Either bit
(but not both) set Both bits set or both clear

**NOT** Destination
bit clear Destination bit set

**Note**

Do not confuse logical instructions with the logical operators, which perform these operations at assembly time, not run time. Although the names are the same, the assembler recognizes the difference.

The following example shows the result of the
**AND**, **OR**, **XOR**, and **NOT** instructions
operating on a value in the AX register and in a mask. A mask
is any number with a pattern of bits set for an intended
operation.

mov ax, 035h ;
Load value 00110101

and ax, 0FBh ; Clear bit 2 AND 11111011

; --------

; Value is now 31h 00110001

or ax, 016h ; Set bits 4,2,1 OR 00010110

; --------

; Value is now 37h 00110111

xor ax, 0ADh ; Toggle bits 7,5,3,2,0 XOR 10101101

; --------

; Value is now 9Ah 10011010

not ax ; Value is now 65h 01100101

The **AND** instruction clears unmasked
bits - that is, bits not protected by 1 in the mask. To mask
off certain bits in an operand and clear the others, use an
appropriate masking value in the source operand. The bits of
the mask should be 0 for any bit positions you want to clear
and 1 for any bit positions you want to remain
unchanged.

The **OR** instruction forces specific
bits to 1 regardless of their current settings. The bits of the
mask should be 1 for any bit positions you want to set and 0
for any bit positions you want to remain unchanged.

The **XOR** instruction toggles the value
of specific bits on and off - that is, reverses them from their
current settings. This instruction sets a bit to 1 if the
corresponding bits are different or to 0 if they are the same.
The bits of the mask should be 1 for any bit positions you want
to toggle and 0 for any bit positions you want to remain
unchanged.

The following examples show an application
for each of these instructions. The code illustrating the **
AND** instruction converts a "y" or "n" read from the
keyboard to uppercase, since bit 5 is always clear in uppercase
letters. In the example for **OR**, the first statement is
faster and uses fewer bytes than cmp bx, 0. When the operands for **XOR** are identical, each bit
cancels itself, producing 0.

;AND example -
converts characters to uppercase

mov ah, 7 ; Get character without echo

int 21h

and al, 11011111y ; Convert to uppercase by clearing bit 5

cmp al, 'Y' ; Is it Y?

je yes ; If so, do Yes actions

. ; Else do No actions

.

yes: .

;OR example - compares operand to 0

or bx, bx ; Compare to 0

jg positive ; BX is positive

jl negative ; BX is negative

; else BX is zero

;XOR example - sets a register to 0

xor cx, cx ; 2 bytes, 3 clocks on 8088

sub cx, cx ; 2 bytes, 3 clocks on 8088

mov cx, 0 ; 3 bytes, 4 clocks on 8088

On the 80386/486 processors, the **BSF**
(Bit Scan Forward) and the **BSR** (Bit Scan Reverse)
instructions perform operations like those of the logical
instructions. They scan the contents of a register to find the
first-set or last-set bit. You can use **BSF** or **BSR**
to find the position of a set bit in a mask or to check if a
register value is 0.

**Shifting and Rotating
Bits**

The 8086-based processors provide a complete set of instructions for shifting and rotating bits. Shift instructions move bits a specified number of places to the right or left. The last bit in the direction of the shift goes into the carry flag, and the first bit is filled with 0 or with the previous value of the first bit.

Rotate instructions also move bits a
specified number of places to the right or left. For each bit
rotated, the last bit in the direction of the rotate operation
moves into the first bit position at the other end of the
operand. With some variations, the carry bit is used as an
additional bit of the operand. Figure 4.2 illustrates the eight
variations of shift and rotate instructions for 8-bit operands.
Notice that **SHL** and **SAL** are identical.

**Figure 4.8 Shifts
and Rotates**

All shift instructions use the same format. Before the instruction executes, the destination operand contains the value to be shifted; after the instruction executes, it contains the shifted operand. The source operand contains the number of bits to shift or rotate. It can be the immediate value 1 or the CL register. The 8088 and 8086 processors do not accept any other values or registers with these instructions.

Starting with the 80186 processor, you can use 8-bit immediate values larger than 1 as the source operand for shift or rotate instructions, as shown here:

shr bx, 4 ; 9
clocks, 3 bytes on 80286

The following statements are equivalent if the program must run on the 8088 or 8086 processor:

mov cl, 4 ; 2
clocks, 3 bytes on 80286

shr bx, cl ; 9 clocks, 2 bytes on 80286

; 11 clocks, 5 bytes total

Masks for logical instructions can be shifted to new bit positions. For example, an operand that masks off a bit or group of bits can be shifted to move the mask to a different position, allowing you to mask off a different bit each time the mask is used. This technique, illustrated in the following example, is useful only if the mask value is unknown until run time.

.DATA

masker BYTE 00000010y ; Mask that may change at run time

.CODE

.

.

.

mov cl, 2 ; Rotate two at a time

mov bl, 57h ; Load value to be changed 01010111y

rol masker, cl ; Rotate two to left 00001000y

or bl, masker ; Turn on masked values ---------

; New value is 05Fh 01011111y

rol masker, cl ; Rotate two more 00100000y

or bl, masker ; Turn on masked values ---------

; New value is 07Fh 01111111y

**Multiplying and
Dividing with Shift Instructions**

You can use the shift and rotate instructions
(**SHR**, **SHL**, **SAR**, and **SAL**) for
multiplication and division. Shifting a value right by one bit
has the effect of dividing by two; shifting left by 1 bit has
the effect of multiplying by two. You can take advantage of
shifts to do fast multiplication and division by powers of two.
For example, shifting left twice multiplies by four, shifting
left three times multiplies by eight, and so on.

Use **SHR** (Shift Right) to divide
unsigned numbers. You can use **SAR** (Shift Arithmetic
Right) to divide signed numbers, but **SAR** rounds negative
numbers down - **IDIV** always rounds negative numbers up
(toward 0). Division using **SAR** must adjust for this
difference. Multiplication by shifting is the same for signed
and unsigned numbers, so you can use either **SAL** or **
SHL**.

Multiply and divide instructions are relatively slow, particularly on the 8088 and 8086 processors. When multiplying or dividing by a power of two, use shifts to speed operations by a factor of 10 or more. For example, these statements take only four clocks on an 8088 or 8086 processor:

sub ah, ah ;
Clear AH

shl ax, 1 ; Multiply byte in AL by 2

The following statements produce the same results, but take between 74 and 81 clocks on the 8088 or 8086 processors. The same statements take 15 clocks on the 80286 and between 11 and 16 clocks on the 80386. (For a discussion about instruction timings, see "A Word on Instruction Timings" in the Introduction.)

mov bl, 2 ;
Multiply byte in AL by 2

mul bl

As the following macro shows, it’s
possible to multiply by any number - in this case, 10 - without
resorting to the **MUL** instruction. However, such a
procedure is no more than an interesting arithmetic exercise,
since the additional code almost certainly takes more time to
execute than a single **MUL**. You should consider using
shifts in your program only when multiplying or dividing by a
power of two.

mul_10 MACRO
factor ; Factor must be unsigned

mov ax, factor ; Load into AX

shl ax, 1 ; AX = factor * 2

mov bx, ax ; Save copy in BX

shl ax, 1 ; AX = factor * 4

shl ax, 1 ; AX = factor * 8

add ax, bx ; AX = (factor * 8) + (factor * 2)

ENDM ; AX = factor * 10

Here’s another macro that divides by
512. In contrast to the previous example, this macro uses
little code and operates faster than an equivalent **DIV**
instruction.

div_512 MACRO
dividend ; Dividend must be unsigned

mov ax, dividend ; Load into AX

shr ax, 1 ; AX = dividend / 2 (unsigned)

xchg al, ah ; XCHG is like rotate right 8

; AL = (dividend / 2) / 256

cbw ; Clear upper byte

ENDM ; AX = (dividend / 512)

If you need to shift a value that is too
large to fit in one register, you can shift each part
separately. The **RCR** (Register Carry Right) and **
RCL** (Register Carry Left) instructions carry values from
the first register to the second by passing the leftmost or
rightmost bit through the carry flag.

This example shifts a multiword value.

.DATA

mem32 DWORD 500000

.CODE

; Divide 32-bit unsigned by 16

mov cx, 4 ; Shift right 4 500000

again: shr WORD PTR mem32[2], 1 ; Shift into carry DIV 16

rcr WORD PTR mem32[0], 1 ; Rotate carry in ------

loop again ; 31250

Since the carry flag is treated as part of
the operand (it’s like using a 9-bit or 17-bit operand),
the flag value before the operation is crucial. The carry flag
can be adjusted by a previous instruction, but you can also set
or clear the flag directly with the **CLC** (Clear Carry
Flag), **CMC** (Complement Carry Flag), and **STC** (Set
Carry Flag) instructions.

On the 80386 and 80486 processors, an
alternate method for multiplying quickly by constants takes
advantage of the **LEA** (Load Effective Address)
instruction and the scaling of indirect memory operands. By
using a 32-bit value as both the index and the base register in
an indirect memory operand, you can multiply by the constants
2, 3, 4, 5, 8, and 9 more quickly than you can by using the **
MUL** instruction. **LEA** calculates the offset of the
source operand and stores it into the destination register,
EBX, as this example shows:

lea ebx,
[eax*2] ; EBX = 2 * EAX

lea ebx, [eax*2+eax] ; EBX = 3 * EAX

lea ebx, [eax*4] ; EBX = 4 * EAX

lea ebx, [eax*4+eax] ; EBX = 5 * EAX

lea ebx, [eax*8] ; EBX = 8 * EAX

lea ebx, [eax*8+eax] ; EBX = 9 * EAX

Scaling of 80386 indirect memory operands is
reviewed in "Indirect Memory Operands with 32-Bit Registers" in
Chapter 3. **LEA** is introduced in "Loading Addresses into
Registers" in Chapter 3.

The next chapter deals with more complex data types - arrays, strings, structures, unions, and records. Many of the operations presented in this chapter can also be applied to the data structures covered in Chapter 5, "Defining and Using Complex Data Types."

Questions:

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