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
|file: /Techref/microchip/math/neg/8bit.htm, 2KB, , updated: 2004/12/6 11:48, local time: 2018/4/20 07:15,
|©2018 These pages are served without commercial sponsorship. (No popup ads, etc...).Bandwidth abuse increases hosting cost forcing sponsorship or shutdown. This server aggressively defends against automated copying for any reason including offline viewing, duplication, etc... Please respect this requirement and DO NOT RIP THIS SITE. Questions?|
<A HREF="http://www.piclist.com/techref/microchip/math/neg/8bit.htm"> PIC Microcontoller Math Method Two's complement 8 bit</A>
|Did you find what you needed?|
PICList 2018 contributors:
o List host: MIT, Site host massmind.org, Top posters @20180420 RussellMc, Van Horn, David, Sean Breheny, Isaac M. Bavaresco, Bob Blick, Neil, David C Brown, alan.b.pearce, Brent Brown, John Gardner,
* Page Editors: James Newton, David Cary, and YOU!
* Roman Black of Black Robotics donates from sales of Linistep stepper controller kits.
* Ashley Roll of Digital Nemesis donates from sales of RCL-1 RS232 to TTL converters.
* Monthly Subscribers: Gregg Rew. on-going support is MOST appreciated!
* Contributors: Richard Seriani, Sr.
Welcome to www.piclist.com!