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Atari invented the '816? (Joe Decuir, VCF East 2019)
Posted: Thu May 16, 2019 10:50 am
by BigEd
An interesting and perhaps controversial aside by Joe Decuir in this recent presentation
Speaking about the initial design of the Atari 8 bit computers (released in '79 as the 400 and 800) in about 1977, at about the 40min mark of the video above:
How are we gonna build this? Well, we had money, we were generating money from the 2600 stuff... We thought about building a 16-bit version of the 6502 called the 6509, we specced it all out, we got people to bid on it and then we decided: we're building three custom chips - why put even more schedule risk into it? So we dropped that - but that became the 65816 which wound up in several Apple products. Nice processor.
(Of course this 6509 has no particular relationship with the much later 8-bit chip made by Commodore which used that number. Although of course this anecdote shows that there was a currency of thinking about memory banking schemes and how to integrate them.)
Re: Atari invented the '816? (Joe Decuir)
Posted: Thu May 16, 2019 11:54 am
by BigEd
Many other interesting things in that presentation, of course (in particular about the sea-change which occurred when the FCC relaxed their regulations for RF interference, and the hazards of falling the wrong side of that line.) If you have a subscription, you can read a similar written account in
Encore: Atari's Second System, where a similar aside appears:
The Atari VCS used a modified MOS 6502 CPU, and the same CPU was chosen for the Atari PCS. During the development process, Atari considered a 16-b extension of the design, tentatively called a “6509.” It would have a 16-b accumulator, two 16-b index registers, a 16-b stack pointer, and a zero page pointer. After consideration, Atari decided not to risk the development schedule any further, since it was already committed to more than one ASIC for I/O. However, the “6509” design became the “65816,” used in later Apple II-family models.
There's also a freely-available article
Champions In Our Midst about the 2600 design. Particularly interesting for the background to the selection of the 6502.
Re: Atari invented the '816? (Joe Decuir, VCF East 2019)
Posted: Thu May 16, 2019 3:42 pm
by whartung
We thought about building a 16-bit version of the 6502 called the 6509, we specced it all out, we got people to bid on it ... - but that became the 65816 which wound up in several Apple products.
So is he saying that his group designed, or had significant input in to the design of the 65816?
Because if you wanted to make a 16 Bit 6502, I don't think the 65816 is what you'd get, as, especially in their use case (no established software base outside of the 2600, which was a non-starter), compatibility with the 6502 isn't even necessary. So I'm curious how much of what they spec'd out actually got in to the 65816.
Re: Atari invented the '816? (Joe Decuir, VCF East 2019)
Posted: Thu May 16, 2019 4:06 pm
by BigEd
Who knows what exactly is meant by "became" - but it seems plausible to me that a collection of experienced people could put together a good specification which could have been used as a foundation by Bill Mensch. Or by people at Apple - I'm not crystal clear on how much input Apple had to WDC's eventual offering of the '816.
(By "good" specification I mean one which is realistic to implement, to verify, to program, and to build into systems. There's a whole lot more to processor design than filling out an opcode table!)