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According to https://www.folklore.org/StoryView.py?project=Macintosh&story=Joining_Apple_Computer

The Apple II displayed white text on a black background. I argued that to do graphics properly we had to switch to a white background like paper. It works fine to invert text when printing, but it would not work for a photo to be printed in negative. The Lisa hardware team complained the screen would flicker too much, and they would need faster refresh with more expensive RAM to prevent smearing when scrolling. Steve listened to all the pros and cons then sided with a white background for the sake of graphics.

What exactly was the refresh rate of the Lisa and Macintosh? Was it faster than the usual 60 Hz?

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TL;DR:

No, not really, the Mac's refresh rate is 60.14 Hz.

I'm missing the exact data on the Lisa, but it's as well close to (or at) 60 Hz (*1).


How Come?

The Mac timing is a bit odd and built around the Video timing.

  • Base clock is 7.83 MHz (*2)
  • Line frequency is 22.255 kHz.
  • Thus a video line is exactly 352 clock cycles
  • A 68k needs 4 cycles per read/write
  • This gives 88 memory access slots
  • One line is 512 pixel or 32 (16 bit) words
  • CPU and video access RAM alternating during the first 64 access slots
  • CPU gets 24 exclusive slots during horizontal retrace.
  • CPU gets as well all slots during vertical retrace
  • There are ~32,560 access slots in frame (Refresh is 60.14 Hz)
  • There are 342 lines per screen
  • Resulting in 342 x 32 = 10,944 video reads
  • This leaves about 2/3rd for the CPU
  • If the CPU would utilize memory at all times, the effective speed would be close to 5.2 MHz
  • Since the 68k isn't, and it's only stopped when there is a memory access, effective speed is more like 6 MHz

*1 - Just flipped thru the whole Lisa 1 and 2 repair manuals (as well as MacXL modifications), but couldn't find any numbers. There is a reference to 22.9 kHz line frequency, which means it'll end up at 60 Hz +/- 3 Hz, but information to close that gap.

*2 - Not 8 MHz as it's usually claimed.

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  • 1
    @Tommy Well, I may mix up something here. It's true that the SE (which the Classic got it's video from) is about 20% faster. Then again, I have a hard time to imagine any other division that would allow the CPU to work at all during display - which in turn would cost way more than 20%. I guess we need better documentation. – Raffzahn Dec 19 '19 at 22:21
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    annoyingly, I know there was one sentence I spotted in Apple documentation once that led me to my current belief, but since I can't find it again that may or may not be true. But I had the feeling that the video goes every other slot, but the CPU can use only one out of every of those two gaps. For reasons unspecified. Yeah, though, definitely consider that belief highly questionable in the absence of documentation. – Tommy Dec 19 '19 at 22:24
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    ... so, yes, I think we're describing the same scheme. Which is, possibly, to alternate between four unobstructed cycles for a video access then four for the CPU. Even though if you keep the address buses separate you can do it in two cycles each, with RAM of the era. Notably the Atari ST (the main other 68000 I know anything about, sorry to keep citing it) offers 2bpp colour at 640 pixels across with only minimal CPU blocking so that obviously is getting something of the order of double the bandwidth. – Tommy Dec 20 '19 at 1:28
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    @Tommy Sure. The whole, CPU access cycle is 4. Just interleaving these 4 is the most simple way to share the bus, unless additional logic is spend, which Atari did. Now, I just flipped thru Apples guide to Mac Hardware, and it seams to reveal how the SE added performance: by using 32 bit access for video: "One longword video access is made for each four RAM access cycles" (p.195). So here the CPU gets three 16 bit access cycles during a scan line for each video access (of 32 bit). This fits well the mentioned (CPU) memory bandwith of 2.56 vs. 3.22 MB/S on p.194. – Raffzahn Dec 20 '19 at 8:45
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    So it seams as if the SE works much like the Atari ST - made possible by having the RAM behind the BBE custom chip, which integrates all RAM access as well as the video shifters. With a system like that they could have cranked up the CPU to twice the speed without much additional cost - I guess a political decision ... as usual. – Raffzahn Dec 20 '19 at 8:50
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I wrote an emulator earlier the year, so I can answer on the Macintosh.

The processor's clock rate is 7,833,600 Hz; the video subsystem is completely synchronous and completes each line in 352 processor cycles, outputting a total of 370 lines per frame.

Therefore each frame is 130,240 cycles long.

So the original Macintosh produces a touch less than 60.15 frames per second. So it's as much 60Hz as almost any other machine you care to mention, though nowhere near NTSC timings otherwise.

(Additionally, for the curious: two pixels are output per processor clock, 512 are pixels, and the equivalent of an additional 192 are spent on blanking and retrace. Of the 370 line period that completes a frame, 342 contain pixels and the other 28 are blanking and retrace. So the actual output resolution is 512x342).

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