Denial

Mike wrote: ↑Fri Nov 15, 2024 7:51 am About the only address mode that goes unused is (ZP,X) - and only in more recent times I have found use for that address mode, not just for the trivial case of X=0, but also when X<>0.

I second this. Wanna slap myself around all Sunday for X < 128.

pixel wrote:I second this.

Using (ZP,X) with X<>0 comes around with reserving a larger part of the ZP as data stack and using X as data stack pointer. Especially ($00,X) can be used to work on memory data with a set of pointers contained in said data stack. INC ZP,X and DEC ZP,X are available to conveniently increment or decrement these pointers - $00,X and $01,X address the respective low- and high-byte of the pointers. Other offsets can be used to probe deeper into parameter lists: as indirect-ZP wraps, the "base address" actually works as signed byte offset on the data stack pointer in X!

If one dispenses with BASIC as built-in language and only uses KERNAL routines, the addresses $00..$8F are free for that purpose.

A rather benign use of that is found in MINIMON, where I use INC $00,X and INC $01,X to increment two pointers in ZP by the same amount (this is necessary in the T command when copying backwards, then the pointers to source and target block are moved to their respective end). Instead of:
... which uses 30 bytes (size resides outside ZP), I do:
... which just needs 26 bytes.

Another use in MINIGRAFIK, with X either 0 or 1, lets the current pixel co-ordinates in $FD/$FE run into the direction of the endpoint in $F9/$FA, or stop when both are equal (and signal that condition to the caller in the Z flag):
Forth anyone?

Mike wrote: ↑Sun Nov 17, 2024 8:06 am Forth anyone?

https://github.com/SvenMichaelKlose/tun ... src/lib/zp

That's cute. But combine pointers and stacks with indexed indirect X...

pixel wrote:

Code: Select all

.proc zpw_dec_x
    dec 0,x
    lda 0,x
    cmp #$ff
    bne :+
    dec 1,x
:   rts
.endproc

One instruction shorter:

Mike wrote: ↑Sun Nov 17, 2024 8:40 am One instruction shorter:

Code: Select all

.proc zpw_dec_x
    lda 0,x
    bne :+
    dec 1,x
:   dec 0,x
    rts
.endproc

m)