Zero Page indexed indirect adressing

[Grevle]

Is the Zero Page type of indexed indirect adressing uniqe to the Zero page ?

 

Put a value into adress $43 for the low byte and then a value into adress $44 for the high byte then use with a LDA ($43),Y or STA ($43),Y  to acces the resulting adress from locations $43 and $44.

 

Is this uniqe for the zero page or is it possible to use similar adressing mode in the other memory pages ? 

 

 

 

[Alfred]

Zero page only.

[Grevle]

15 minutes ago, Alfred said:

Zero page only

OK. Im looking in the book "Mapping the Atari" and the free user ram in the Zero page seems to be small, Only a few bytes avaible there ? Whats the most common user adresses to use in the Zero Page ?

Edited November 24, 2020 by Grevle

[Rybags]

Depends on usage case - at first it looks like not many locations are available but Basic uses a lot, so they're free if Basic not in use.  But other language processors will generally use much of the top half of z-page.

 

The floating point area is good for temporary use.  If you know what you're doing the lower half (OS) has numerous locations you can temporarily use.

[Wrathchild]

2 hours ago, Grevle said:

Is the Zero Page type of indexed indirect adressing uniqe to the Zero page ?

Not entirely, the JMP instruction can take an indirect address from anywhere in memory, e.g.

 

$C01D: 6C 00 02          JMP (VDSLST)

[TGB1718]

If your using BASIC then really $CB to $CF are the only ones safe to use and as 

@Rybags said there are many others you can use if you know what functions your not

using (Mapping the Atari) is a good source of these.

[drac030]

6 hours ago, Grevle said:

Whats the most common user adresses to use in the Zero Page ?

"User addresses" on the ZP are $80-$FF.

 

If your program uses the Floating Point package, addresses $D4-$FF are in use.

 

If your program has to coexist with BASIC, addresses $80-$D3 are used by the BASIC interpreter.

 

[Grevle]

On 11/24/2020 at 9:40 AM, Wrathchild said:

Not entirely, the JMP instruction can take an indirect address from anywhere in memory, e.g.

The same for JSR ?.. I believe so.

 

I was trying something like this. 

 

1600 = VAR ; EQUATE for changeable varible

LDX 35000

STX VAR

; later in the program

JMP VAR ; So this is a try to reach the adress stored in VAR but my guess is it doesnt work this vay

 

Gonna have to do something like this I think.

 

LDX #184 ; low byte

STX $43

LDX #136 ; high byte

STX $44

 

So then, Possible to JMP or JSR $43 to reach adress 35000 in this way ? Point for this is having a changeable variable to control program flow , when its needed in certain routines etc....

 

 

 

 

Edited November 27, 2020 by Grevle

[Wrathchild]

No, just JMP. So setup would be:

 

vec35000 = $42

LDA #$4C
LDY #<35000
LDX #>35000
STA vec35000
STY vec35000+1
STX vec35000+2

 

and calling would be:

JSR vec35000

Edited November 27, 2020 by Wrathchild
$4C for direct JMP, not $6C for indirect

[fenrock]

A useful resource to see what modes are available is http://www.6502.org/tutorials/6502opcodes.html#JSR

 

From the link above, you can see JSR only has Absolute addressing, but JMP has Indirect too.