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Why the original Macintosh had a screen resolution of 512×324

512pixels.net

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Re: Why the original Macintosh had a screen resolution of 512×324

#11

Post by ChuckMcM »

bscphil wrote: post #8
jerbear4328 wrote: post #4

Earlier quoted context omitted.

It looks like it's just the HN submitted title which is wrong (currently "Why the Original Macintosh Had a Screen Resolution of 512×324"). The article's title is "Why the Original Macintosh Had a Screen Resolution of 512×342", and "324" doesn't appear anywhere on the page.

Looks like someone is reading Hacker News comments and editing the page - archive.org captured the page probably mid-edit, and it says "324" in one place: https://web.archive.org/web/20250527202300/https://512pixels...

Oh that's priceless. Real time HN feedback loops.

Re: Why the original Macintosh had a screen resolution of 512×324

#12

Post by nssnsjsjsjs »

bryanlarsen wrote: post #5
hyperhello wrote: post #3

The article really didn’t explain why they picked that number.

For efficient graphics routines on a 32 bit machine, it's important that the scan line direction (aka horizontal for normally mounted CRT's) be a factor of 32, preferably one that's a power of 2. The article mentions the desire for square pixels. So presumably they chose the horizontal resolution first and then chose the vertical resolution that gave them square pixels for a 512 pixel horizontal resolution.

It was 32bit?!

Re: Why the original Macintosh had a screen resolution of 512×324

#13

Post by 90s_dev »

johnklos wrote: post #2

The title is incorrect, because b&w Macs have 512×342 resolution, not 512x324. It wouldn't've been too crazy had Apple went with 64K x 4 chips, so they'd've just needed four of them to get 128 KB at a full 16 bits wide. 512x342 was 16.7% of 128 KB of memory, as opposed to 18.75% with 512x384. Not much of a difference. But having square pixels is nice.

> wouldn't've

Really, John? You really had to make me parse that word?

Re: Why the original Macintosh had a screen resolution of 512×324

#14

Post by webstrand »

90s_dev wrote: post #13
johnklos wrote: post #2

The title is incorrect, because b&w Macs have 512×342 resolution, not 512x324. It wouldn't've been too crazy had Apple went with 64K x 4 chips, so they'd've just needed four of them to get 128 KB at a full 16 bits wide. 512x342 was 16.7% of 128 KB of memory, as opposed to 18.75% with 512x384. Not much of a difference. But having square pixels is nice.

> wouldn't've Really, John? You really had to make me parse that word?

It's a great word, I use it all the time.

Re: Why the original Macintosh had a screen resolution of 512×324

#15

Post by tom_ »

nssnsjsjsjs wrote: post #12
bryanlarsen wrote: post #5

Earlier quoted context omitted.

For efficient graphics routines on a 32 bit machine, it's important that the scan line direction (aka horizontal for normally mounted CRT's) be a factor of 32, preferably one that's a power of 2. The article mentions the desire for square pixels. So presumably they chose the horizontal resolution first and then chose the vertical resolution that gave them square pixels for a 512 pixel horizontal resolution.

It was 32bit?!

The 68000 is 16 bit internally, and can access memory only 16 bits at a time, but the instruction set was designed with future iterations in mind, and most instructions can operate on 32 bit quantities - with a performance penalty. (Because in essence it has to do the work in 2 stages.)

Whether this is enough to make it count as actually 32 bits is one for the philosophers.

Re: Why the original Macintosh had a screen resolution of 512×324

#16

Post by mayoff »

nssnsjsjsjs wrote: post #12
bryanlarsen wrote: post #5

Earlier quoted context omitted.

For efficient graphics routines on a 32 bit machine, it's important that the scan line direction (aka horizontal for normally mounted CRT's) be a factor of 32, preferably one that's a power of 2. The article mentions the desire for square pixels. So presumably they chose the horizontal resolution first and then chose the vertical resolution that gave them square pixels for a 512 pixel horizontal resolution.

It was 32bit?!

The data and address registers of the 68000 were 32 bits wide.

Re: Why the original Macintosh had a screen resolution of 512×324

#17

Post by kzrdude »

nssnsjsjsjs wrote: post #12
bryanlarsen wrote: post #5

Earlier quoted context omitted.

For efficient graphics routines on a 32 bit machine, it's important that the scan line direction (aka horizontal for normally mounted CRT's) be a factor of 32, preferably one that's a power of 2. The article mentions the desire for square pixels. So presumably they chose the horizontal resolution first and then chose the vertical resolution that gave them square pixels for a 512 pixel horizontal resolution.

It was 32bit?!

That reminds me of this old system settings panel https://lowendmac.com/2015/32-bit-addressing-on-older-macs/

I remember the "enable 32-bit addressing" part (but it's not pictured..)

Re: Why the original Macintosh had a screen resolution of 512×324

#18

Post by rsynnott »

tom_ wrote: post #15
nssnsjsjsjs wrote: post #12

Earlier quoted context omitted.

It was 32bit?!

The 68000 is 16 bit internally, and can access memory only 16 bits at a time, but the instruction set was designed with future iterations in mind, and most instructions can operate on 32 bit quantities - with a performance penalty. (Because in essence it has to do the work in 2 stages.) Whether this is enough to make it count as actually 32 bits is one for the philosophers.

The 386SX was a similar story, and is normally thought of as basically 32bit. I think the perception difference may be down to timing; the 386SX came out _after_ the DX (with 32 bit data bus), so was thought of as a cheap 32bit chip, vs the 68000 which started off life with a 16bit data bus.

(Fun fact: there was also the 68008, which was a 68k with an 8 bit bus!)

Re: Why the original Macintosh had a screen resolution of 512×324

#19

Post by perching_aix »

Regardless of whether we go with 512x324, 512x342, or 512x384, the claim of 72 PPI (exact) and 9" of diagonal size (exact) are not simultaneously possible.

Extremely nitpicky thing I know, but this kinda stuff really bugs me, could somebody please clarify what was the real size (and/or PPI) here?

For reference:

512x324 @ 72 PPI = 8.42" (or 214 mm) (rounded)

512x342 @ 72 PPI = 8.55" (or 217 mm) (rounded)

512x384 @ 72 PPI = 8.89" (or 226 mm) (rounded)

The first two don't even yield an integer result for the number of diagonal pixels, let alone yield an integer multiple of 72. Or would there be bars around the screen, or how would this work?

Re: Why the original Macintosh had a screen resolution of 512×324

#20

Post by pavlov »

perching_aix wrote: post #19

Regardless of whether we go with 512x324, 512x342, or 512x384, the claim of 72 PPI (exact) and 9" of diagonal size (exact) are not simultaneously possible. Extremely nitpicky thing I know, but this kinda stuff really bugs me, could somebody please clarify what was the real size (and/or PPI) here? For reference: 512x324 @ 72 PPI = 8.42" (or 214 mm) (rounded) 512x342 @ 72 PPI = 8.55" (or 217 mm) (rounded) 512x384 @ 72…

For CRTs, the diagonal measurement was of the physical tube. The actual viewable area was smaller. Part of the tube’s edges were covered by plastic, and there was always also some margin that wasn’t used for picture so it was just black.

It was a 9” tube with 3:2 aspect ratio. Your calculation of a 8.5” image at 72 dpi sounds right.

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