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Assembly Coding Optimization

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Halifax:
wow very nice

EDIT: I just gained (with this post here) post number 666!

Iambian:
Size and speed optimization:
If you make an unconditional CALL prior to a RET, you can replace that CALL with a JP instruction (since you're then going to be using that calling routine's RET). This won't work if the calling routine doesn't exit out using a RET, but that's up to you to decide. But if it works, that's one byte saved by not having to use a RET.

If the calling routine is local enough, you can save another byte by using the JR instruction instead.
i.e.
c1-->CODE ec1 CALL Someroutine
 RET
;more code
Someroutine:
 XOR A
 RETc2ec2
can be condensed to this:
c1-->CODE ec1 JR Someroutine
;more code
SomeRoutine:
 XOR A
 RETc2ec2
If you can rearrange the code to make "SomeRoutine" appear right after the calling routine, you can save two more bytes by omitting JR altogether.

In this respect, it may do you some good to rearrange your ASM code to take advantage of this kind of optimization. Of course, you're going to have to be wary about errors resulting in the use of JR.

Oh, and as a response to the previous post that made a size optimization using a "LD A,(DE)", it's not exactly the same as using two NOPs. The "LD A,(DE)" is faster by one clockcycle. I understand that this is negligible, but I just wanted to point that out.

Jon:
yeah good point :) 7

Iambian:
It might also be worth it to mention, in case you're doing interrupts, that if you wanted to call the TI-OS's interrupt service routine (RST 38h) on a conditional jump, you could save some memory by using a trick of the Z80's instruction set.

Instead of, say, "CALL C,0038h" or "JP C,0038h", you could do "JR C,$FF". Conditions will vary, all of which will do the same thing.

This works because the "$FF" is an offset to make a relative jump one byte behind the already-executed instruction ( JR C,$+1). The part of the argument for JR, "$FF", is the opcode for RST 38h. In that sense, you're combining two instructions in one.

For what you'd use this for, I'd have no idea. Perhaps someone that wanted to call the interrupt service routine while the interrupts were off? Perhaps it might be a way to keep romcalls like _getCSC and _getKey working while the interrupts are gone.

Jon:
That's brilliant man.  And it's pure luck that the opcode for rst 38h is the signed 8-bit value for -1. That's friggin' awesome, kudos! :)
Although, wouldn't the command be jr c,$-1 ? I believe tasm will interpret jr c,$ff to mean jr c,$00ff, and hence give you a range of relative branch error.

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