1 reply to this topic

[Propane13]

I've got a good inkling for how the stack works, but I wanted to confirm something.
When I do the following in code:

LDA #$FF
TXS

Is the stack now initialized to 0x01ff?
I just want to verify the result of the following code:

LDA #$FF
TXS
JSR Place
Place:
LDA #$FF
STA $FF
RTS

I want to make sure this wouldn't corrupt my return address, due to a stack-push from an RTS.

Conversely, I would assume that this code *would* break:

LDA #$FF
TXS
JSR Place
Place:
LDA #$FF
STA $01FF
RTS

Can someone confirm?

Below is some documentation from the 7800 dev guide I found for reference, but it isn't *too* concrete in its explanation of the stack.

There are two (2) 6116 2Kx8 RAM chips on the 7800 PC board.
Together they occupy addresses x '1800' to x'27FF'. They are
also partly accessible (shadowed) at addresses x '0040' - x
'00ff' and x '0140' - x '00FF' to extend zero page (quick access)
RAM and first page (stack) RAM. Refer to the memory map appendix
for further information.


Memory map:
_________________________________
0000 | |
| TIA Registers |
|_________________________________| 001F
0020 | |
| MARIA Registers |
|_________________________________| 003F
0040 | |
| RAM |
| (6116 Block Zero) |
|_________________________________| 00FF
0100 | |
| Shadow of Page 0 |
| (TIA and MARIA) |
|_________________________________| 013F
0140 | |
| RAM |
| (6116 Block One) |
|_________________________________| 01FF

Many thanks to anyone who can confirm this!
-John

P.S. Sorry for posting a lot here recently, but I'm working on a project, and am trying to dive back in.

[Robert M]

Propane13, on Thu Sep 13, 2007 11:58 AM, said:

I've got a good inkling for how the stack works, but I wanted to confirm something.
When I do the following in code:

LDA #$FF
TXS

Is the stack now initialized to 0x01ff?
I just want to verify the result of the following code:

LDA #$FF
TXS
JSR Place
Place:
LDA #$FF
STA $FF
RTS

I want to make sure this wouldn't corrupt my return address, due to a stack-push from an RTS.

Conversely, I would assume that this code *would* break:

LDA #$FF
TXS
JSR Place
Place:
LDA #$FF
STA $01FF
RTS

Can someone confirm?

Your code will not work as you expect.

TXS transfers the contents of the X register to the SP register. If you want to set the stack to point at $01FF, then you must do this:

	 LDX #$FF
	 TSX

If the SP is set to $01FF, then a JSR instruction will place the hi-byte of the return address at $01FF and the lo-byte at $01FE. The resulting SP value will be $FD. In the Atari 2600, memory pages 0 and 1 are shadowed, so a write to $00FF is the same as a write to $01FF. I am not sure if this is true on the 7800 in 7800 mod, but it must be true while the 7800 is in 2600 compatibility mode. If the 7800 memory map overlaps both page zero and page 1 like a 2600, then both of your test code examples will fail. Unless the hi-byte of the label Place: is $FF In which case you will write $FF on top of the existing $FF.

The RTS instruction performs only 2 read operations. It does not write anything to memory. It adds 2 to the SP, and the PC is read from the stack. The values loaded into the PC are still in memory until you overwrite them.