Optimising Assembly Like an 80's Hacker

Forget about fancy algorithms and data structures. If you want respect as an 80's hacker, follow these simple tips.

Never get caught setting a register to zero without using xor:

Z80 Code

ld a,0           ; bad, 2 bytes / 7 cycles

xor a            ; good, 1 byte / 4 cycles

8088 Code

mov ax,0         ; bad, 3 bytes / 4 cycles

xor ax,ax        ; good, 2 bytes / 3 cycles

Never set two 8 bit register independently. Code readability is not required:

Z80 Code

ld b,10          ; bad, 4 bytes / 14 cycles
ld c,32

ld bc,10*256+32  ; good, 3 bytes / 11 cycles

8088 Code

mov ch,10        ; bad, 4 bytes / 8 cycles
mov cl,32

mov cx,10*256+32 ; good, 3 bytes / 4 cycles

Never compare to zero:

Z80 Code

cp 0             ; bad, 2 bytes / 7 cycles

or a             ; good, 1 byte / 4 cycles

8088 Code

cmp ax,0         ; bad, 3 bytes / 4 cycles

test ax,ax       ; good, 2 bytes / 3 cycles

Remember, you don't need to worry about code alignment, order of instructions or processor penalties. Follow these simple tips and your super-optimised bubble sort will demand the utmost respect!