Could the 7800's 6502C CPU run at 7.16mhz or 14.32mhz ?

[Math You]

I was just reading about how the Atari 7800's graphics chip runs at 7.16mhz and was wondering if (in theory) the 6502C cpu could also run at the same speed or maybe twice the speed?

 

Just curious

[GroovyBee]

I'm not sure if anybody has tested the upper speed of SALLY. You could probably replace SALLY with a 65C816 at a higher clock rate but you'd need a legacy mode for the original games. I think it would be much more interesting to replace the CPU with something completely different like a 68008. You could also go completely mad and hook MARIA up to an embedded ARM chip. For added madness you could run a 6502 emulator on the ARM too . However, if you change the CPU getting new games developed is always the most difficult part. Without new games it would be hard to sell such a modification.

[The Usotsuki]

Maybe a 65C02 with the glue logic from the 400?

[Math You]

if you change the CPU getting new games developed is always the most difficult part. Without new games it would be hard to sell such a modification.

 

True. I guess the speed would have to be switchable to maintain compatibility with existing software.

 

To be honest, all this talk of the new XM adapter has got me thinking about what the future has in store for the 7800. It would be interesting to see if the XM project could evolve into a complete 7800 console with a few new features. A faster processor would allow more programs to use the 7800's 320 x 200 mode or display more sprites.

 

Another possibility would be to allow TIA and Maria to work simultaneously to either create more on-screen colours or possibly 2 separate graphics screens, one over the other.

[GroovyBee]

mpatibility with existing software.

To be honest, all this talk of the new XM adapter has got me thinking about what the future has in store for the 7800. It would be interesting to see if the XM project could evolve into a complete 7800 console with a few new features. A faster processor would allow more programs to use the 7800's 320 x 200 mode or display more sprites.

 

The bottleneck in the 320 modes is MARIA and not the CPU. It only has so many video cycles to process the Display List (DL) data and only a limited number of colours can be on screen in each mode as standard too. However, you could use programmer tricks to change colours on a per scan line basis. So you could have a very colourful display with some limitations.

 

Another possibility would be to allow TIA and Maria to work simultaneously to either create more on-screen colours or possibly 2 separate graphics screens, one over the other.

 

Nice idea but unfortunately that won't work because TIA requires the CPU to be running 100% of the time to generate its display while MARIA needs the CPU to be halted to generate its display. So its either one or the other but not both at the same time.

[kskunk]

I was just reading about how the Atari 7800's graphics chip runs at 7.16mhz and was wondering if (in theory) the 6502C cpu could also run at the same speed or maybe twice the speed?

A big problem is the memory speed. The 6502 executes a LOT more memory accesses than most other CPUs. A 7.16MHz 6502 runs memory 4 times faster than a 7.16MHz 68000. This means the cartridge ROM and on-board RAM would need to be many times faster, which is not possible with the type of parts and bus used in the 7800.

 

If you wanted a much faster processor, you would need to build it into the expansion module along with its own local memory. This type of design would be more like Harmony's DPC+/ARM mode, where the 7800's 6502 would cooperate with a completely separate computer running with its own memory and peripherals.

 

- KS

[GroovyBee]

A big problem is the memory speed. The 6502 executes a LOT more memory accesses than most other CPUs. A 7.16MHz 6502 runs memory 4 times faster than a 7.16MHz 68000. This means the cartridge ROM and on-board RAM would need to be many times faster, which is not possible with the type of parts and bus used in the 7800.

 

When MARIA makes an access to the Display List List (DLL) or Display List (DL) data it requires two of the 7.16MHz video clocks (that is why they are held in its RAM). If it pulls a byte of graphics data from ROM it requires 3 MARIA clock cycles per access. If you had access to SALLY's RDY pin you could extend the read bus cycles as required (depending on what type of memory was being accessed). However RDY cannot be asserted during writes so any RAM would need write cycle timing to match the CPU.

[Math You]

A big problem is the memory speed. The 6502 executes a LOT more memory accesses than most other CPUs. A 7.16MHz 6502 runs memory 4 times faster than a 7.16MHz 68000. This means the cartridge ROM and on-board RAM would need to be many times faster, which is not possible with the type of parts and bus used in the 7800.

 

When MARIA makes an access to the Display List List (DLL) or Display List (DL) data it requires two of the 7.16MHz video clocks (that is why they are held in its RAM). If it pulls a byte of graphics data from ROM it requires 3 MARIA clock cycles per access. If you had access to SALLY's RDY pin you could extend the read bus cycles as required (depending on what type of memory was being accessed). However RDY cannot be asserted during writes so any RAM would need write cycle timing to match the CPU.

 

Sounds complicated

 

What if a new 7800 had a Sally and Maria chip that could both run at 2 or 3 times their current speed? Would it work or would the extra speed affect the video output.

[GroovyBee]

Sounds complicated

 

Its not complicated if you have electronics and software skills. It just takes time and money to develop.

 

What if a new 7800 had a Sally and Maria chip that could both run at 2 or 3 times their current speed? Would it work or would the extra speed affect the video output.

 

If everything is developed new (not using legacy parts) it makes the designers life much easier because you can specify everything upfront. When you design things for products that came out 25+ years ago your hands are pretty much tied.

[kskunk]

What if a new 7800 had a Sally and Maria chip that could both run at 2 or 3 times their current speed? Would it work or...

The answer also depends on what you mean by "would it work". For the library of 7800 games, a hypothetical overclock would mostly break them, at best showing no benefit at all. Only more more modern games are designed to scale up or down with CPU power, which is why overclocking can make a difference. Not so on older systems.

 

So, your overclocked 7800 wouldn't help existing games. But, like GroovyBee points out, you would probably need to use all new components and modern technology. This is not a mod, it's a new console from scratch. Somebody could write a special game to show off your new faster console, but it's not really a 7800 anymore...

 

Because of the design of the 7800, your best hope to add more power is probably by adding a faster processor that plugs into the cartridge port. A lot of systems have done this, starting with the 2600's 'Graduate', all the way to the Sega CD or 32X.

 

- KS

[Rex Dart]

I'm not sure if anybody has tested the upper speed of SALLY. You could probably replace SALLY with a 65C816 at a higher clock rate but you'd need a legacy mode for the original games. I think it would be much more interesting to replace the CPU with something completely different like a 68008. You could also go completely mad and hook MARIA up to an embedded ARM chip. For added madness you could run a 6502 emulator on the ARM too . However, if you change the CPU getting new games developed is always the most difficult part. Without new games it would be hard to sell such a modification.

 

Oooh what if you replaced the whole shebang with an x86 setup & ran it all in software emulation? Exotic!

[GroovyBee]

Oooh what if you replaced the whole shebang with an x86 setup & ran it all in software emulation? Exotic!

 

You'd be better off with a Raspberry Pi in there and a custom USB dongle that would get you access to the console switches and joysticks. It'd work out much cheaper .

[Math You]

Perhaps it would be best to leave everything just the way it is. Too many changes would only ruin the retro experience

 

The reason I asked was to see if an enhanced 7800 could be made, rather than a mod for existing 7800 consoles.

 

I get the impression that programmers can achieve all kinds of things that the original designers didn't think possible, the only problem being that enhancements such as displaying higher resolutions or more colours aren't really practical in games as there isn't enough power to perform hardware tricks and play games at the same time. Even the 7800's 320 x 200 resolution seems to be too much for it's humble processor.

Edited March 14, 2012 by Math You

[GroovyBee]

I get the impression that programmers can achieve all kinds of things that the original designers didn't think possible, the only problem being that enhancements such as displaying higher resolutions or more colours aren't really practical in games as there isn't enough power to perform hardware tricks and play games at the same time. Even the 7800's 320 x 200 resolution seems to be too much for it's humble processor.

 

Its possible to make an enhanced MARIA that is both backwards compatible with the legacy games and offers enhanced graphics modes. For NTSC machines you'd need to desolder the original MARIA from the motherboard, which not something that everybody can do. On PAL consoles its much easier to install a modification because MARIA is in a socket.

 

You can make quite colourful 320x200 games but it all depends on the type of game and what programmer tricks you use.

[carmel_andrews]

i think the 65816 might be an option, since if i recall correctly it featured or had a fallback 6502 mode (for compatibility reasons i guess)

[The Usotsuki]

You'd have to adapt it into a 65802 type thing though. And the 6502 compatibility mode is really 65SC02-compatible.

[carmel_andrews]

^{so, not exactly 6502 compatible then.....oops}

[The Usotsuki]

Well, as compatible as a 65SC02 is. Wouldn't be so useful in Atariland where the illops are assumed to exist, but it was plenty fine on the Apple ][ line where the 65SC02 was actually quite common, being that the //c and all //es after 1985 used them.

[31336haxx0r]

I wonder what happens when clocking MARIA twice the normal speed. Would it net 640x400 @ 31.25 kHz horizontal/ 100 Hz vertical frequency? Then you could could connect to a VGA monitor (after decoding composite to RGB which wouldn't be so difficult).