[Coco] 6502 to 6809 translation

[John Kent]

On 2/08/2011 8:40 AM, William Astle wrote:
>
> You're missing a detail in the original question. Let's make the 
> following assumptions, codified as source code:
>
>     org $2000
> data_addr    fdb $8000
>
>     org $2020
>     fdb $8272
>
>     org $4000
>     ldy #$20
> * sequence 1
>     lda [data_addr,y]
> * sequence 2
>     lda data_addr,y
> * sequence 3
>     ldd data_addr
>     lda d,y
> * end options
>     org $8020
>     fcb $42
>
>     org $8272
>     fcb $32
>
> Sequence 1 will result in A containing $32 because it will read the 
> address at $2020 and use that as the address to load A from.
>
> Sequence 2 will result in A containing $82 because it will fetch A 
> from $2020.
>
> Sequence 3 will result in A containing $42, which is what the original 
> 6502 instruction would do (assuming it were implemented on the 6809).
>
> Essentially, the 6502 is loading a 16 bit base address from the 
> address of data_addr and adding the value in Y to that address to form 
> the effective address which is used for loading A. Using 6809 
> nomenclature, that would be "LDA [data_addr],y", which obviously does 
> not exist. Such weird (to us) gyrations are required because the 6502 
> has no 16 bit registers and on the 6502, the address of "data_addr" 
> would have to be in the direct page.
>
>
For  lda b,y  according to my VHDL 6809, B is sign extended so you are 
right for an offset from 128 to 255 from a 16 bit base address you will 
need to use D as the base address rather than B as a replacement for Y 
so your sequence 3 I would say is correct. The thing is that D is made 
up of A & B so when you load A you destroy the base address in D but I 
guess you can reload it again later anyway.

LDA data_addr, y  "data_addr" is a constant offset not a variable 
address. I think the 6502 used direct page memory for the indirect 
indexed addressing although it's a while since I looked at the 6502.

John.

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