Comment by ajross

13 days ago

Exactly. Like the Apple ][, the original Mac framebuffer was set up with alternating accesses, relying on the framebuffer reads to manage DRAM refresh.

It looks like DRAM was set up on a 6-CPU-cycle period, as 512 bits (32 16-bit bus accesses) x 342 lines x 60 Hz x 6 cycles x 2 gives 7.87968 MHz, which is just slightly faster than the nominal 7.83 MHz, the remaining .6% presumably being spent during vblank.

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ajross

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meatmanek  13 days ago

But why 342 and tune the clock speed down instead of keeping the clock speed at 8MHz and having floor(8e6/2/6/60/32) = 347 lines?

I suspect kmill is right: https://news.ycombinator.com/item?id=44110611 -- 512x342 is very close to 3:2 aspect ratio, whereas 347 would give you an awkward 1.476:1 aspect ratio.

  • ajross  13 days ago

    That doesn't sound right. The tube the mac was displaying on was much closer to a TV-style 4:3 ratio anyway, there were significant blank spaces at the top and bottom.

    If I was placing bets, it was another hardware limitation. Maybe 342 put them right at some particular DRAM timing limit for the chips they were signing contracts for. Or maybe more likely, the ~21.5 kHz scan rate was a hard limit from the tube supplier (that was much faster than TVs could do) and they had a firm 60 Hz requirement from Jobs or whoever.

  • kragen  12 days ago

    You could reduce the gain on the horizontal deflection drive coil by 2% to get back to 3:2. In fact, I doubt that it was precise to within 2%.