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 htsoft.com] 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 http://www.myke.com/basic.htm: 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
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