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Comment by mark-r | Hacker News Reader

One trick I used a lot with the Z80 is the conditional return instructions. Instead of doing a jump, structure your functions so you could return early on one of the condition flags. The jump instructions were 3 bytes, the return instructions were only 1.

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And it could also save time. The Z-80 conditional return takes 5 cycles (or "T-States" in Zilog terminology) to execute if the condition is not met and 11 cycles otherwise. Quite worthwhile in testing for uncommon cases. Consider this routine to print a message that is terminated by nul or newline:

   PMESS  LD   A,(HL)     ; get character from message
          INC  HL         ; move message pointer to next character
          CP   10         ; newline?
          RET  Z          ; return if so
          OR   A          ; zero?
          RET  Z          ; done if so
          CALL PCHAR      ; print character
          JP   PMESS      ; keep looping

If instead of "RET Z" we had to do a conditional jump to a return it would be 10 cycles for each test instead of 5.

          CP   10
          JP   Z,DONE     ; 10 cycles, jump taken or not
          ...
          JP   PMESS
   DONE:  RET             ; 10 cycles, BTW

The conditional return just happens to be cheaper if not taken because it skips the work of popping the return address off the stack. Though purely an outcome of the implementation you can treat it as sort of a branch prediction.

Incidentally, the Z-80 also has relative branches (JR) that differ in execution time whether they are taken or not. The branch offset is a single byte so JR is only 2 bytes compared to JP's 3. A JR is 7 cycles if not taken, 12 otherwise. Again, we can treat it as a hard-coded branch prediction that predicts the branch is not taken. If space or distance to target is not a problem, a JR is faster if taken less than 60% of the time.

> The jump instructions were 3 bytes

Absolute jumps ("JP") were 3 bytes, relative jumps ("JR") were two, whether conditional or not -- but, that's still longer than the 1-byte return instruction...; Though, absolute jumps were the fastest - they did not need to access the stack and adjust SP like "ret", and did not need to do addition like "jr".

I used to know the opcodes by heart, but I last wrote Z80 code in 1987, and last wrote x86 machine code in 1997, and nowadays I don't remember all of the binary representations, and sometimes I get the binary representations of the two mixed. I'm getting old ...

It was only useful when stack wasn’t involved I presume?

Not sure about Z80 but stack is a bit of a luxury on 6502 ...

The Z-80 stack pointer (SP) is a full 16 bit register. The stack can be anywhere in memory and be as large as needed.

The 6502 has only an 8 bit stack pointer which points into page 1 only (addresses 0x100 .. 0x1FF).

The Z80 stack was only limited by your memory size.

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