[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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