In what sense would the Atari 3200 have been a 10-bit machine?

Question

The Atari 3200 was a canceled project that would have produced a compatible successor to the 2600. Little seems to be known about it; about the best reference I have been able to find is https://forums.atariage.com/topic/77312-the-atari-3200-system-x/

And then came Project Sylvia, also known as Super-Stella and the Atari 3200. It retained compatibility with the 2600 while offering new possibilities through enhanced sound, graphics and an improved 10-bit processor. It also looked a lot like sandwich toaster, but that's not what killed it. When game designers were presented with the new machine to begin experimental development they found it was simply too difficult to program -- and remember, these people were accustomed to 2600 development which as good as required them to program the TV's electron beam directly!

An intriguing summary; as OP observes, just what could the machine have required of programmers, to look difficult compared to the 2600?

But the particularly puzzling thing is the mention of a 10-bit processor.

Mind you, I do think there would have been merit in expanding the byte to 10 bits. That would have led to 20-bit systems, that could address a megabyte of memory while being significantly cheaper than a 32-bit machine.

But that seems very unlikely to be what is meant here. A backward-compatible game console is not going to get to change anything that fundamental.

So just what did the reference to a 10-bit processor mean?

Answer 1

TL;DR: Don't Listen to the Internet's Mumbling.

The '10 Bit Processor' is a nerd myth without any foundation.

Your guess is as good as mine what it is based on. (*1).

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What is Really Told

Sylvia, intended as middle product line between Stella (2600) and the Personal Arcade Machine (PAM) was intended to be a Super-Stella. Positioned above the Atari 400/800 but without all parts that make it a home computer. It was to be based on

• a full 6502 to enable large cartridges without banking,
• full 8 KiB of RAM to relief memory stress,
• an enhanced TIA called STIA to improve colour range and resolution and
• an ANTIC based DMA processor dubbed FRANTIC to enable better graphics

During development the RAM got reduced to 2 KiB for the first known design. At that time the intended product name was 3200, while PAM was handled as 'System X'. Not much later the whole Sylvia project was scrapped, while was PAM cut down to be essentially a repackaged Atari 400, renamed 5200 and introduced as the new top end console.

Read the Book

Whenever it is about Atari, Curt Vendel's incredible Atari Inc - Business Is Fun must be the first source to be checked - and a must item every book shelf. On pages 626ff the development history of the 2600 is described. P. 628 mentions above facts, with more spread out over the following pages. The book is not only useful due facts but even more because of the stories around giving a great insight into the reasoning leading to the history we know. Page 663 shows a partial image of he Sylvia schemaics, although labled misleading as 'Super TIA Processor'.

The incredible Atarimueum.com (*3) has of course a Sylvia page as well. It not only describes the 10 Bit as well as a popular myth, but also gives highly detailed picture of that schematic, clearly showing Sylvia being a pure 8 bit 6502 based design.

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*1 - One common explanation (also mentioned in a comment by Marc Brenier) is due introduction of PAM and Sylvia being intended to squeeze Mattel's Intelivision out of the market. It used a General Instruments CP1610 CPU (*2) which interprets only 10 bit out of every 16 bit instruction word. In turn the Intelivision has a fitting 10 bit wide ROM, instead of 16 bit wide, considerably reducing ship cost by 40% by not storing the unused 6 bit at all. This in turn mean that any 16 bit value had to be loaded from two 16 bit words with only 10 valid bits each. Odd and remarkable, so something that may have spured the fantasy of interested Nerds. Not to mention that calling the opponent 10 bit instead of the 16 they claimed does fit the picture of fierce competition, doesn't it?

*2 - Further history includes GI spawning off it's semiconductor department as Microchip Technology, a company up to today most famous for one of their CPUs. You know which :)

*3 - After the loss of Curt Vendel they felt out of maintenance, but the Wayback Machine is still of help.