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.
6502 Assembly
8-Bit
<lang 6502asm>LDA #%01010101 EOR #255 CLC ADC #1 ;result: #%10101011</lang>
16-bit
<lang 6502asm>myVar equ $20
LDA #3 STA myVar LDA #0 STA myVar+1 ;equivalent C: uint16_t myVar = 3;
negate: LDA myVar+1 EOR #255 STA myVar+1
LDA myVar EOR #255 STA myVar CLC ADC #1 STA myVar
- this handles the case if we started with something where the low byte was zero.
LDA myVar+1 ADC #0 STA myVar+1</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>