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PIC Microcontoller Math Method

Two's complement 8 bit

from Andy Warren

If [...] your original number is in a register, you can do it with this bit of code:

        COMF    REG     ;REG = two's-complement of REG.
        INCF    REG     ;

or this one:

        COMF    REG,W   ;W = two's-complement of REG
        ADDLW   1       ;(REG is unchanged).

If the original number is in W, the easiest way is:

        SUBLW   0       ;W = two's-complement of W.

Clyde Smith-Stubbs [clyde at] of HI-TECH Software says

[These snippits do not handle the problem: REG = 0x80] because there is no correct result in this case. The value 0x80 interpreted as 2's complement is legal, and equal to -128. The negative of this, 128, is NOT representable as an 8 bit 2's complement number.

From Negating the Contents of "w"

If you have to Negate the contents of the "w" register, you could use the code above (after saving the value in "w" into a register) or you could use the code below for low-end devices (PIC16C5x, PIC12C5xx, PIC16C505). Any file register can be used for this code because its contents are never changed.
  addwf  Reg, w           ;  w = w + Reg
  subwf  Reg, w           ;  w = Reg - w
                          ;  w = Reg - ( w + Reg )
                          ;  w = -w

file: /Techref/microchip/math/neg/8bit.htm, 2KB, , updated: 2004/12/6 11:48, local time: 2024/5/18 20:02,

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