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Apple "mistake? video 512 x 342 vs. 512 x 384


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Why does Apple state that the SE/30 supports 512 x 382 video? https://support.apple.com/kb/sp195?locale=de_DE

(underneath it does say that the number of lines on the physical screen is 342) Every other source (and the Mac itself) says that the resolution is 512 x 342 with no mention of 384.

 

I'm really curious about why 384 is used here at all. Is it just that claiming the extra lines was a good spec for sales to quote, in the same way other manufacturers might go on about 4096 colors even though that might be only in one screen mode you would never normally see? Is there some way for the SE/30 to generate 512 x 384 on an external screen? But if that is true how does an external screen connect with no hardware?

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Seems that all B/W compact Macs have that; compare for example the Macintosh 128k page.

 

That table is labeled as being about “Video Memory” which I think is only really relevant for models that have multiple display options. For external displays, 512×384 is an option but 512×342 isn’t. It’s probably just a standard table that they didn’t bother to specialize for compact Macs.

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I know 512 x 384 is a handy number and exactly half the XGA standard resolution but can't understand why it wasn't used. If the memory is there why not? It was expensive in 1989! Supposedly the stock monitor is just fine at higher and grayscale resolution if you score a Micro Exceed, so why a smaller 512 x 342 display when the mainboard could have displayed 384 lines? Is this just carrying over from earlier Macs where this was the economical memory increment, but other technical issues made 342 lines the maximum?

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It's not a memory issue. It's a timing and synchronisation issue. Since the video and 68000 are synched and share the same memory, The video subsystem could only read memory at specific times (and stopped the processor while doing so IIRC). 

 

It's more of an "how many lines can I read in one second while stopping the processor" issue than a memory one.

There are articles around there that explain this "issue" better than I can. 

 

Also, XGA wasn't born yet (1984 vs 1990) and 512x384 never was in any VGA spec.

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You're probably correct there though I'm not sure if the screen redraws involved a CPU interrupt or were simply executed during natural processor wait states/in between clocks. But there were tons of compromises back in the early days for stuff like that, such as the IBM PC's use of the 4.77MHz processor clock to allow it to interface to cheap and pre-existing NTSC TV circuits. Same with Macs using 15.67MHz CPUs instead of a round 16MHz: it was done to match the dot clock of later displays and simplify the circuits required. Lots of weird choices go into saving a buck or ten.

 

As for why they kept it for so long? Mostly because they were cheap (the SE/30 really should have had a grayscale screen) but also compatibility: the Plus logic board was an optional upgrade to the 512 and original Mac, and the SE/30 board was available as an upgrade to the SE. so they had to remain completely hardware compatible (except for the exchange of the rear bucket), and some really old programs do not play nice if they're not run at 512x342@1bpp. Same reason the PB 5x0 series had an option for running their displays at 640x400 instead of their full 640x480: compatibility with older PB-specific software (though I couldn't tell you what software required this).

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Yes, I don’t know if it was a technical limitation that kept B/W compacts at 512×342 and more—that’s the hardware and software they already had, so why change things?

 

As for 640×400, this was available on desktop Macs too. Machines with 512k of VRAM (such as the LC III) can display 16-bit color at 640×400, are limited to 8-bit color at 640×480. The same consideration may have driven the choice of resolution for the Powerbook 100; 32k VRAM will get you 640×400 at 1-bit but not 640×480.

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Regarding the initial question, it seems pretty clear this is just confusing use of a standard “here’s a list of various Apple video resolutions” table, including the later Color Classic 512x384, which was retroactively applied to earlier Macs, thus the “Yes this model supports 512x384” with a footnote saying “actually 512x342”.  The early Macs couldn’t support more than 342 scanlines and their memory buffers used to hold the screen bitmap were exactly sized for 512x342 and not a single one more .... every byte was precious!

 

I don’t know the logic behind the initial choice of 342, but 512 specifically made sense because it’s a power of 2, which allows one to construct a byte offset into a B&W screen buffer with a bitshift and a bitwise OR, no multiplication or even addition, and no wasted bits:

 

    byte offset of byte containing pixel (x, y) = (y * 512 + x)/8 = y*32 + x/8 = (y << 6) | (x >> 3)

 

This would have been useful for making QuickDraw “quick” on a slow processor.

 

Edited by Crutch
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14 hours ago, sfiera said:

As for 640×400, this was available on desktop Macs too. Machines with 512k of VRAM (such as the LC III) can display 16-bit color at 640×400, are limited to 8-bit color at 640×480. The same consideration may have driven the choice of resolution for the Powerbook 100; 32k VRAM will get you 640×400 at 1-bit but not 640×480.

Now that's interesting, didn't know about 640x400 in the LCIII, LEM doesn't say so, but did that carry over to the 475/605 in "all resolutions" as opposed to recommended?

 

Somewhere in my research I ran across 640x400 at some odd refresh rate as an early VGA standard. WAG is that it was an early LCD resolution which likely makes sense in PowerBook implementations. Was 640x400 the most common early PC Laptop resolution?

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I should maybe add: I don't have any personal experience with this mode. It was just something I found in the LC III Developer Note (p14, “VRAM”) while researching video modes and parameters. The LC 475 Developer Note does not mention it, nor the Technical Specifications pages for the LC 475 or LC II. I guess it was really specific to the LC III.

 

Timing-wise, the LC III’s 640×400 is identical to letterboxed 640×480 (pp21-22).

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