Secret Opcodes of the 8 Bit Processors

Undocumented instructions were common on early processors. A few would crash the computer (HCF - halt and catch fire) while others had strange but occasionally useful behaviour. Any self-respecting programmer would make use of these to squeeze out the last few cycles of performance.

The effect of undocumented opcodes would vary between different versions of some processors, no doubt leading to the classic excuse “it worked on my machine”. Here are a few examples I've found useful.

Secrets of the Z80

Zilog's Z80 was used in a number of popular 8 bit computers including the Sinclair Spectrum, Amstrad CPC, TRS-80 and MSX. There are a number of undocumented opcodes with the CB, DD, ED and FD prefix.

• CB30-CB37 - SLL reg shifts a register left, setting bit 0.
• DD - when used as a prefix to instructions which use H or L, either the high or low 8 bits of IX are used.
• FD - as DD, but the high or low 8 bits of IY will be used.
• ED70 - IN (C) reads from i/o port C, setting the flags and discarding the result.
• ED71 - OUT (C),0 outputs a zero to port C.

Secrets of the 8086/8088

Intel's 8088 was used in the original IBM PC and has spawned an entire family of processors.

• D6 - SALC sets the AL register to either 00 or FF depending on the carry flag. SALC was finally documented with the introduction of the Pentium Pro 27 years later.
• 0F - POP CS pops the CS register from the stack. Only works on 8086 processors.
• 0F05 - LOADALL loads all registers from memory location 0800. Only works on 80286 processors.

Which processors have you programmed and did you find any undocumented opcodes useful?

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!