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

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

#21

Post by silvestrov »

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.

No, this is very wrong. The 68000 is 32 bit internally and it has 32 bit registers: https://en.wikipedia.org/wiki/Motorola_68000

Externally it had 16 bits for databus and 24 bits for addresses. That is why we later got the 32 bit clean ROMs as Apple used the upper unused 8 address bits for flags.

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

#23

Post by [deleted] »

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…

[deleted]

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

#24

Post by perching_aix »

Reason077 wrote: post #22

> “To minimize CRT flicker, Apple worked to achieve a vertical refresh rate of 60 Hz” … a limitation that many Macs, and even some iPhones, are still stuck with over 40 years later!

It's always surprising for me to see people regard 60 Hz CRT as "flicker-free", or "minimal flicker", etc. Whenever I saw a CRT running at 60 Hz, I'd be immediately be able to tell. Always used at minimum 75 Hz but preferably 85 Hz at home (early 2000s, Windows).

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

#25

Post by bryanlarsen »

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.

According to https://wiki.neogeodev.org/index.php?title=68k_instructions_... the 32 bit register add on the 68000 is faster (6 cycles) than the 16 & 8 bit register add (8 cycles).

Most 32 bit operations are slower than 16 bit operations because the external data bus is only 16 bits and most operations use the external data bus. But simple internal ops are faster at 32 bits, so that seems to indicate the 68000 is 32 bit internally.

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

#26

Post by Suppafly »

perching_aix wrote: post #24
Reason077 wrote: post #22

> “To minimize CRT flicker, Apple worked to achieve a vertical refresh rate of 60 Hz” … a limitation that many Macs, and even some iPhones, are still stuck with over 40 years later!

It's always surprising for me to see people regard 60 Hz CRT as "flicker-free", or "minimal flicker", etc. Whenever I saw a CRT running at 60 Hz, I'd be immediately be able to tell. Always used at minimum 75 Hz but preferably 85 Hz at home (early 2000s, Windows).

>Whenever I saw a CRT running at 60 Hz, I'd be immediately be able to tell. Always used at minimum 75 Hz but preferably 85 Hz at home (early 2000s, Windows).

Same, I remember installing some program that would let you quickly change the display settings on basically every computer I ever interacted with. It was especially bad if the crt was in a room with fluorescent lighting.

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

#27

Post by Suppafly »

pavlov wrote: post #20
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.

>For CRTs, the diagonal measurement was of the physical tube. The actual viewable area was smaller.

That's also why TVs and monitors of that era always seemed smaller than advertised. I remember having to explain that to a lot of people.

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

#28

Post by bluGill »

perching_aix wrote: post #24
Reason077 wrote: post #22

> “To minimize CRT flicker, Apple worked to achieve a vertical refresh rate of 60 Hz” … a limitation that many Macs, and even some iPhones, are still stuck with over 40 years later!

It's always surprising for me to see people regard 60 Hz CRT as "flicker-free", or "minimal flicker", etc. Whenever I saw a CRT running at 60 Hz, I'd be immediately be able to tell. Always used at minimum 75 Hz but preferably 85 Hz at home (early 2000s, Windows).

Have you ever seen something running at 30 Hz? Or even 15? The difference in flicker between 30 and 60 is much much larger than the difference between 60 and 120! Yeah 60 isn't flicker free, any finite number is not (there is probably quantum limits), but realistically you reach a point where you can't really tell. For most purposes 60Hz is close enough, though you can still tell.

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

#29

Post by pavon »

The article didn't nail down an exact reason. Here is my guess. The quote from Andy Hertzfeld suggests the limiting factor was the memory bandwidth not the memory volume:

> The most important decision was admitting that the software would never fit into 64K of memory and going with a full 16-bit memory bus, requiring 16 RAM chips instead of 8. The extra memory bandwidth allowed him to double the display resolution, going to dimensions of 512 by 342 instead of 384 by 256

If you look at the specs for the machine, you see that during an active scan line, the video is using exactly half of the available memory bandwidth, with the CPU able to use the other half (during horizontal and vertical blanking periods the CPU can use the entire memory bandwidth)[1]. That dictated the scanline duration.

If the computer had any more scan lines, something would have had to give, as every nanosecond was already accounted for[2]. The refresh rate would have to be lower, or the blanking periods would have had to been shorter, or the memory bandwidth would have to be higher, or the memory bandwidth would have had to be divided unevenly between the CPU and video which was probably harder to implement. I don't know which of those things they would have been able to adjust and which were hard requirements of the hardware they could find, but I'm guessing that they couldn't do 384 scan lines given the memory bandwidth of the RAM chips, and the blanking times of the CRT they selected, if they wanted to hit 60Hz.

[1]https://archive.org/details/Guide_to_the_Macintosh_Family_Ha...

[2]https://archive.org/details/Guide_to_the_Macintosh_Family_Ha...

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

#30

Post by detourdog »

edwinjm wrote: post #6
hyperhello wrote: post #3

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

The article says: In short, there’s no easy answer to explain why early compact Macs ran at a screen resolution of 512×342. Rather, Apple was doing what it does best: designing a product with the right trade-offs for performance, ease of use, and cost.

It was noticeably better than anything else I had ever seen.
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