from Andy Warren
If [...] your original number is in a register, you can do it with this bit of code:
not REG ;REG = two's-complement of REG. inc REG ;
or this one:
mov W, /REG ;W = two's-complement of REG ;*** WARNING: ADDLW was expanded in three instructions! Check if previous instruction is a skip instruction. ; ADDLW 1 ;(REG is unchanged). mov Hack, W mov W, #1 ;(REG is unchanged). add W, Hack
If the original number is in W, the easiest way is:
;*** WARNING: Manual replacement required for "SUBLW k" instruction (w = k - w). Check if previous instruction is a skip instruction. 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.
|file: /Techref/scenix/lib/math/neg/8bit_sx.htm, 1KB, , updated: 2004/6/10 14:40, local time: 2021/10/26 00:44,
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<A HREF="http://www.sxlist.com/techref/scenix/lib/math/neg/8bit_sx.htm"> PIC Microcontroller Math Method Two's complement 8 bit</A>
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