Re: EPZ directive

[dempson@actrix.gen.nz (David Empson)]

John B. Matthews <nospam@nospam.com> wrote:

> In article <44913484$0$877$ba4acef3@news.orange.fr>,
>  "Guillaume Tello" <houten.van@wanadoo.fr> wrote:
> 
> > hello,
> > 
> > i'm trying to learn 6502 asm. here is a part I don't understand:
> > 
> > B    EPZ $6    ; low part of adr
> > BH    EPZ $7    ; high part of adr
> > ...
> >     LDA (B,X)
> > 
> > looking at the first two lines, I can think that the adr is $706.
> > but the assembly code for LDA (B,X) is A1 06 meaning that only low part of
> > the adress is used, pointing to page zero.
> > 
> You have stumbled upon the (little used) indexed indirect address mode.
> In your example, the second byte of the instruction and the value in the
> X register are added to to form a one-byte, zero-page address. The 
> contents of that address and the one following become the 16-bit 
> effective address from which the accumulator is loaded.

Right so far.

> Given
> 
> C  EPZ $8
> CH EPZ $9
> 
> If X is zero, then LDA (B,X) acts like LDA $706.
> If X is one,  then LDA (B,X) acts like LDA $807.
> If X is two,  then LDA (B,X) acts like LDA $908.
> etc.

No it doesn't.  EPZ doesn't cause anything to be stored in zero page
locations. It just defines a symbol.

If X is zero, then LDA (B,X) acts like the 65C02 instruction LDA ($06).
If X is one,  then LDA (B,X) acts like the 65C02 instruction LDA ($07).
If X is two,  then LDA (B,X) acts like the 65C02 instruction LDA ($08).
etc.

The actual locations accessed will depend on the values stored in the
zero page locations.

Assuming the following data is stored:

06: 11
07: 22
08: 33
09: 44

then:

If X is zero, then LDA (B,X) acts like LDA $2211
If X is one,  then LDA (B,X) acts like LDA $3322
If X is two,  then LDA (B,X) acts like LDA $4433
etc.

-- 
David Empson
dempson@actrix.gen.nz