68020 Data registers as addresses? (VASM?)

[Auscoder]

Mentioned here : http://jvaltane.kapsi.fi/amiga/howto...0x0issues.html

Data registers can be used as addresses. (d0) is 3 cycles slower than (a0), and it only takes 2 cycles to move a data register to an address register, but this can help in situations where there is not a free address register.

Though I have tried using a data register as an address in vasm with -m68020, and essentially the code compiles but is not working.

		move.l #x,d0		                ; Allocation bytes
		move.l #MEMF_CHIP,d1              	; CHIP Ram
		jsr    _LVOAllocMem(a6)		        ; exec -198

                move.l d0,a0
                move.l #0,0(a0) ; WORKS
                move.l #0,0(d0) ; FAILS

I guess two questions :

Is it true a data register can be used as address register on 68020+ (with penalty)?
If so, is it able to be used with said addressing mode?

Thank you!

[a/b]

Yes, 020+ new adressing modes can handle that. But it doesn't quite work the same way.
Basically, the full addressing mode is ([offset,ax or pc,index*scale],displacement) and the individual parts can be suppressed. So you do that to everything except the index, which is either ax or dx, so dx in your case, and you get ([dx]).
Now what does this do? It reads a 32-bit address from memory pointed by dx (everything inside square brackets), and then adds displacement to it and it reads/writes data from/to that displaced 32-bit address. So it's kind of a double index, or in plain m68k it would be like:

  move.l offset(ax,rx*scale),tmp_areg
  move.l #0,displacement(tmp_areg)

[meynaf]

If 0(d0) fails, try (d0.l) instead.

[alkis]

works if you invoke vasm with -m68020

vasmm68k_mot -m68020 -Fhunk -o foo foo.asm

[phx]

Meynaf makes an important point. d0 is the index register! So when you write 0(d0) it is really (0,za0,d0.w). The address register is assumed as a0 and suppressed. The size of the index register always defaults to Word(!). Which means the MSW from your pointer in d0 will be disregarded.

[Auscoder]

Thank you all,

Absolutely clear now.

Thanks a/b for the explanation on the full addressing mode. Working successfully now with suppressed or full syntax.

!! As noted by phx and meynaf d0.l is absolutely key.