6502.org

Hey! First post here! - I'm an ex-EE student from Denmark, and have been into building vintage computer systems for some years now, though non-6502 based. Messed around with the 8085 and 6809 for a while, even have ½ a cosmac ELF somewhere, that I never finished. Latest I've been working on a small z80 system, but I've never really looked into the 65xx family.

I got very lucky at a flea market, and picked up a bunch of IC tubes with some real retro gold: about 100 chips, mostly CPU's, gold plated / purple/brown ceramic, mostly. - 8086's, 8088's, AND: 6503's -
Now this is a strange processor. Can't find much info on it, though the datasheet seems to indicate it's almost a 6502 in a 25pin package, cutting away some address space and less important pins.
Now- does anyone know this processor, computers using it, or have used it themselves?

I think I'll make this my first "6502" project, as I just decided it's enough in the ballpark. I have a decent amount of 65xx family support chips as well, like the 6532 RAM/IO/timer and 6522 interface adapter (also from my recent haul).

I hope this is thread is newbie enough for this category, as I've never worked with the family before. If not: I'll move.

-Thanks in advance!

--Christoffer

It's an old NMOS 6502 but with only 4K of addressable memory space, and no SYNC or RDY or phase 1 or SO\ pins. You can see the pinout on page 10 of the 12-page .pdf data sheet on this site, at http://6502.org/documents/datasheets/mo ... v_1985.pdf .

The RIOT ICs are very rare, so handle accordingly. The 65c02 and 65c22 are being made today, guaranteed to meet the timing specs at 14MHz. They'll usually top out at about 25MHz. For a list of differences between the NMOS 6502 and the CMOS 65c02, see http://wilsonminesco.com/NMOS-CMOSdif/ . My 6502 primer is at http://wilsonminesco.com/6502primer/ (for building your own 6502 computer).

Welcome, Christoffer. Sounds like you should be able to make a small system around that CPU. Remember, start small, and good luck!

Welcome, Christoffer

This Rockwell datasheet covers 6503, if that adds anything to what Garth already posted:cheers,
Jeff

Hey, I've read your primer! - Had great use of your general construction technique info in the past, thanks!

And thanks all for datasheets. The 6503 would be almost entirely software compatible with the 6502, right?

I think I'll mold the first iteration of the system around the 6503, the RIOT, providing both IO and ram, and 8k eprom.
That should be sufficient to blink some LED's.

- Christoffer

It is a 6502-- it just doesn't have all the connections to the outside world.

Oh, sorry. Missed the point that it was literally the same die in there. I guess that'd make it sorta software compatible, yeah.

--Christoffer

It also has the same errata as the 6502.

Okay, this I hadn't anticipated: Most of the 6502 interrupt/reset vectors is outside the address space.
Would it just be the lowest 12 bits of the same addresses?

--Christoffer

Yes, almost certainly the address space just wraps around. So it'll be at the top of the memory space, no matter how you look at it.

Yes -- and the high 4 bits become "don't care." Since those 4 don't reach the outside world, they can't be decoded or heeded in any way.

That means LDA $0000 produces the same result as LDA $1000, same as LDA $2000 etc... there are 16 different addresses that all amount to the same thing.

Same with any address, including the Reset Vector for example. Addressing $FFFC produces the same result as $0FFC, same as $1FFC, same as $2FFC etc

Fantastic- thanks!

--Christoffer

Jeff's explanation also points out in a roundabout way that the 6503's usefulness in a general purpose computing application is limited. 4K doesn't give you a great deal of elbow room for code and data, especially when you consider that space for I/O devices will eat into that 4K.

Plenty of room for a 256 byte monitor and a 1k interpreter!

Yeah, the 6503 does seem rather limited that way, with very few advantages, the main being board space, I assume.
Probably why you don't hear much about it. I suspect the memory capabilities are adequate for a small "trainer" like system.

I'll admit the main choice over the 6502 was that I have about 35 of the 6503's, and no 6502.