Atari 2600 stack question

callan · 5th August 2012, 11:48 PM

I've been mystified for some time by the architecture of the Atari 2600.

It's using a 6507 processor, which is a 6502, with some address lines, IRQ, NMI, SO and a few other pins sawn off - giving it 8K address space. a 6532 RIOT chip provides IO and 128 bytes of RAM, from $0080 through 00ff (the lower 128 bytes is where the TIO (Video chip) registers reside.

Now, on a 6502, the stack is stored in page 1 of memory ($0100-$01FF) - so by all rights there is no stack present for JSR, or other operands that use the stack (like PHP)

This contradicts with my citing of 2600 code, which (naturally) needs JSR to call subroutines. Therefore there must be a stack - HOW??

The only way I can think that the processor can have it's stack in page 1 is if the RAM in the RIOT chip is not properly decoded, and is imaged at $0180-$01FF.

Does anyone here know if this is the case? - or am I barking up the wrong tree?

Callan

InsaneFurball · 6th August 2012, 11:43 AM

I.... I.... wut?!

*Leaves thread*

power · 6th August 2012, 11:45 AM

you're overthinking it - Atari 2600 = Space Invaders and River Raid.

yeah no idea, maybe ask those programming types in the programming section.

spondy · 6th August 2012, 12:28 PM

I think they'd stack quite well once you remove the cartridges.

flugle · 6th August 2012, 1:03 PM

Mate I'd head over to AtariAge and ask there, there's some incredibly knowledgeable people over there.

callan · 8th August 2012, 6:26 PM

Quote:

Originally Posted by flugle

Mate I'd head over to AtariAge and ask there, there's some incredibly knowledgeable people over there.

I registered there the other day.
Actually, I'm getting close to working it out. It has to do with the RAM not being "ram" as we know it, but as part of a 6532 RIOT chip (Ram/IO) chip - and how it has some "kinda" chip-select pins that do double-duty as address lines. There is some cross-wiring between the 6507 and 6532 address lines, and these pins, that probably do the trick. I suspect it was a not uncommon trick for using the 6532, but haven't found out any examples in old MOS documentation.

Good suggestion, tho - if I can't nut it out, I'll drop a question there.

Callan

dingaling · 19th August 2012, 6:45 PM

Good on ya Callan, your the da man! we need more ppl like you!

atari 2600 my first console growing up, fond memories!

Heywood · 22nd August 2012, 10:19 AM

Callan,

Did you resolve how this works? I was under the impression that the RIOT memory was seen as both Page 0 and Page 1 to the 6507

Since the stackpointer starts at 7F and decrements as stuff gets shoved onto the stack, as long as you don't push too much* on the stack, you shouldn't be clobbering your real data.

*Too much in the scheme of 128 bytes is actually quite small

5th August 2012, 11:48 PM	#1
callan Member Join Date: Aug 2001 Location: melbourne Posts: 2,724	Atari 2600 stack question I've been mystified for some time by the architecture of the Atari 2600. It's using a 6507 processor, which is a 6502, with some address lines, IRQ, NMI, SO and a few other pins sawn off - giving it 8K address space. a 6532 RIOT chip provides IO and 128 bytes of RAM, from $0080 through 00ff (the lower 128 bytes is where the TIO (Video chip) registers reside. Now, on a 6502, the stack is stored in page 1 of memory ($0100-$01FF) - so by all rights there is no stack present for JSR, or other operands that use the stack (like PHP) This contradicts with my citing of 2600 code, which (naturally) needs JSR to call subroutines. Therefore there must be a stack - HOW?? The only way I can think that the processor can have it's stack in page 1 is if the RAM in the RIOT chip is not properly decoded, and is imaged at $0180-$01FF. Does anyone here know if this is the case? - or am I barking up the wrong tree? Callan __________________ Wanted: Commodore 1571, and/or 1541/II disk drives

6th August 2012, 11:43 AM	#2
InsaneFurball Member Join Date: Sep 2005 Location: Launceston TAS Posts: 551	I.... I.... wut?! Leaves thread __________________ Intel i5 2500k @ 4ghz \|\| NZXT Havik 140 \|\| Asrock z68 Entreme3 Gen 3 \|\| Leadtek 3gb GTX580 \|\| 8GB G.Skill Ripjaws WD 1TB SATA \|\| WD 300GB SATA \|\| Thermaltake HX750watt \|\| Coolermaster HAF 922 ZALMAN Owners Club\|\|OCAU MetaL Club Member #1982\|\|Logitech 'G' Owners Club Member #98

6th August 2012, 11:45 AM	#3
power Member Join Date: Apr 2002 Location: brisbane Posts: 27,114	you're overthinking it - Atari 2600 = Space Invaders and River Raid. yeah no idea, maybe ask those programming types in the programming section. __________________ this is who we are.

6th August 2012, 12:28 PM	#4
spondy Member Join Date: Jan 2008 Location: Ballajura, WA, 6066 Posts: 196	I think they'd stack quite well once you remove the cartridges. __________________ Successful Trades: Statts, solid.egg, Comma, murcielago, TX3, mindstormr, scodye, MetalPhreak, snoopy, gs.mrwolf, downforce, kemitri, mcfee03, OZeight, Scum, terrastrife, 123realtalk, Fedevor

22nd August 2012, 10:19 AM	#8
Heywood Member Join Date: Dec 2001 Posts: 443	Callan, Did you resolve how this works? I was under the impression that the RIOT memory was seen as both Page 0 and Page 1 to the 6507 Since the stackpointer starts at 7F and decrements as stuff gets shoved onto the stack, as long as you don't push too much* on the stack, you shouldn't be clobbering your real data. Too much in the scheme of 128 bytes is actually quite small Last edited by Heywood; 22nd August 2012 at 3:57 PM. Reason: Originally had FF instead of 7F - It may be the same thing is the the MSB of the address byte is ignored...*

6th August 2012, 1:03 PM	#5
flugle Member Join Date: Jun 2001 Location: Newcastle, NSW Posts: 795	Mate I'd head over to AtariAge and ask there, there's some incredibly knowledgeable people over there.

19th August 2012, 6:45 PM	#7
dingaling Member Join Date: Mar 2006 Location: Launceston Posts: 13	Good on ya Callan, your the da man! we need more ppl like you! atari 2600 my first console growing up, fond memories!