There's a couple more interesting pages on Ed Fries' site:

According to a forum post in 2013 by Stiletto at ,
the rom in the Gran Trak 10 is a Mostek MK28000 PROM:

In case this comes up AGAIN, here is the preliminary datasheet for the Atari Le Mans PROM.

It's been identified through past Internet research as a Mostek MK28000 PROM (the same as in Tank / Tank II.)
313K mk28000p.pdf -

Also here are the databooks which contain it. - 06-Aug-2013 14:47 17M - 06-Aug-2013 18:07 38M

(these links are dead but it's there at bitsavers under Components)


Sometimes you can't figure out how things work unless you build a model. I couldn't quite see how the horizontal counter system worked in Lemans.

The manual lays it out, but that seems confusing.

[Linked Image from]

So let's take the schematic and make a logisim model:

[Linked Image from]

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and what you see is that 512H isn't actually 512H but represents more of the "right half of the screen" since it gets set after the 0x1c2 clock and the counter gets reset but leaving the 512H bit making 0x200.
(In other words, the counter jumps from 0x1c2 to 0x200).
Once it hits 0x3c2 then it gets reset again back to 0x000, and then does HSYNC for 32 clocks.

This explains why the 7th row byte disappears in the racetrack. 512/64 = 8 for 8 bytes, but 0x1c2 / 64 = 7.03 so it jumps from 0x1c2 to 0x200 which skips that seventh byte.