我从来不需要使用 ($nn,x) 寻址模式。 什么是它有用的例子？

Question

I've used ($nn),y plenty of times, it's pretty much the 6502's bread-and-butter method of iterating through arrays. But I've never found a use for ($nn,x) . The only time I've ever thought about using it is when x = 0 . It seems like any problem that can be solved with STA ($nn,x) can just as easily be done with hardcoded pointers like in this example with writing to the Konami VRC6 sound hardware:

LDX #2
loop:
LDA (musicptr),y
STA $9000,x  ;pulse 1 channel regs
STA $A000,x  ;pulse 2 channel regs
STA $B000,x  ;sawtooth channel regs
DEX
BNE loop

Versus:

LDA #$00
STA $10
STA $12
STA $14
LDA #$90
STA $11
LDA #$A0
STA $13
LDA #$B0
STA $15

LDX #$00
loop:
LDA (musicptr),y
STA ($10,x)  ;first pass: STA $9000. second pass: STA $A000. third pass: STA $B000
INX
INX
CPX #6
BNE loop

I know that not every tool needs to be used, but I just can't think of a purpose for this addressing mode at all.

Answer 1

Checking out some well-known source code of the era, from a system where the 6502 was used to build an actual OS with hardware abstraction, the BBC MOS uses ($nn,x) in only two instances:

• OSWORD 5 and 6 , which read and write bytes to IO addresses; and
• to write teletext characters to the screen .

So:

1. this probably confirms that the addressing mode isn't especially useful — just three usages in 16kb of code, with two of those just being the read/write versions of the same operation; and
2. it demonstrates the main use case: reading or writing to somewhere via a table of pointers.

Despite your assertion as a comment above:

• IO space cannot just be made into a table of bytes indexed by ($nn),y because it has fixed locations; and
• video memory cannot just be made into a table of bytes indexed by ($nn),y because it has fixed locations.

Answer 2

Since there is no indirect mode without indexing in the original 6502, one use is to set x to 0 to emulate such a mode.

ldx #0
lda ($zp,x)

Yes you can do it with the y register and lda ($zp),y , but maybe y has got a value in it that you want to keep.

Other than that, I've got nothing.