Two's complement
You are encouraged to solve this task according to the task description, using any language you may know.
Two's complement is an important concept in representing negative numbers. To turn a positive integer negative, flip the bits and add one.
- Task
Show how to calculate the two's complement of an integer. If the technique is different depending on the size of the integer, give an example of each.
6502 Assembly
8-Bit
<lang 6502asm>LDA #%01010101 EOR #255 CLC ADC #1 ;result: #%10101011</lang>
Z80 Assembly
8-Bit
Zilog Z80
<lang z80>ld a,%00001111 neg ;returns %11110001 in a</lang>
Game Boy <lang z80>ld a,%00001111 cpl ;game boy doesn't have NEG but it has CPL which flips all the bits. inc a ;returns %11110001 in a</lang>
16 Bit
NEG and CPL only work on the accumulator A.
The following can be written to work with BC, DE, HL, IX, or IY.
<lang z80>xor a ;ld a,0
sub c
ld c,a
sbc a ;loads &FF into A if "sub c" set the carry (borrow) flag. Otherwise, a remains zero.
sub b
ld b,a</lang>
C
<lang C>int a = 3; a = -a;</lang>