Columns of text in early microcomputers

Question

As discussed in some previous questions here, 80-column text was established by IBM as the standard for business computing as early as the sixties, but monitors capable of displaying that resolution legibly were still somewhat expensive in the seventies, so early microcomputers that used TV sets, or monitors with off-the-shelf TV tubes, such as the trinity of 1977, settled for a lower resolution: 40 columns for the Apple II and Commodore PET, 64 columns for the TRS-80.

As I understand it, a black-and-white TV tube with no color burst signal and the RF stage skipped, while it could not quite reach 80 columns, could handle 64 columns well enough, which raises the question of why Apple and Commodore didn't try for that, or at least for something a bit beyond 40. 64x25 = 1600 bytes, not prohibitive even by the standards of the time.

I get the impression memory bandwidth was a significant limitation, such that when the Apple II later added an 80-column option, the engineering of such involved the careful juxtaposition of two memory banks to provide data at the required rate.

Was memory bandwidth the reason for Apple and Commodore sticking with 40 columns? If so, how did Tandy achieve 64 columns? Is it purely a coincidence that the 64-column machine is the only one of the trinity to use the Z80?

(I'm assuming the Vic-20 later supplying only 22 columns, is not so much due to technical limitations as to the use of a video chip designed more for games than general purpose computing.)

Answer 1

Don't underestimate the cost of additional RAM for display memory!

The TRS-80 used seven, not eight, 1x1024 static RAM chips for its display memory. They left off the eighth chip to reduce cost. Without it, the machine was limited to uppercase-only display -- one bit per character cell to specify whether that cell displayed a character or a 3x2 graphic tile, and the other bits to specify the character or the state of each graphic block.

It was common to solder an eighth chip piggy-back onto one of the existing display chips. Once you connected that extra data bit to the bus, you could display lowercase -- the character ROM already had lowercase characters and special symbols in its map, it just needed the data to drive it.

At the time these machines were introduced, I think a static 1K chip was still a few bucks. That's a significant chunk in a product like this. It would also have required more board space; in those days of medium-scale TTL DIPs, dual-layer boards, and coarse design rules, each additional part took up quite a bit of room. Bigger board, more expense.

Now, imagine doubling the amount of display RAM, and requiring a more expensive higher-bandwidth monitor. In the late 1970s, it just didn't make economic sense.

Answer 2

This answer has the Commodore 64, the Amstrad and the Sinclair ZX Spectrum and QL series in mind.

Note these home computers did not have a real "text" mode (as opposed to the more professional systems like IBM MDA, for example).

This means text needs to be displayed as graphics, and graphics needs resolution. A reasonably readable text font needs at least 5-6 pixels horizontal (business use would have required 8), and 8-10 pixels vertical size (including blank pixels to separate the characters from each other). Multiply that by 80 characters and you end up with resolutions >400 pixels horizontal, >200 vertical, which means at least 10k (better 16) of video memory, even with monochrome text, even more if you want more readable fonts, way more if you want at least four colors for your text. The Sinclair QL spent 32k of its scarce 128k for the video frame buffer, enabling it to display 85 characters by 25 lines in four colors.

With memory sizes from 16k to 48, maybe 64k on a typical home computer, 16k are not easily expendable.

That was the one reason - Another one is, once you got the pixels in the frame buffer, you need to get them out onto the screen - once every video frame, you need to clock out all the pixel data to the monitor. While this is done, the video circuitry needs exclusive access to the framebuffer and, in principle, halts the CPU (on simpler designs like most home computers, on more complex designs, the lockout of the CPU may be restricted to the framebuffer and it would still be able to work on other memory).

This means that the higher the screen resolution gets on such simple designs, the more the CPU needs to wait for access to the memory because it is locked out by the video circuitry (so-called "contended memory"). Some designs get across this using dual-ported memory, or dedicated memory chips for the frame buffer, but that is expensive.

Displays that work with a pure text mode storing characters in the frame buffer do not have this problems - Their video memory is much smaller (it doesn't need to store the pixels of a character, but rather only one byte for it's ASCII code), also their video output needs way less memory bandwidth. But again, home computers wanted graphics and a switchable graphics/text mode again uses circuity and thus costs money.

In the end, most 8 bit home computers restricted themselves to resolutions not much larger than 600 x 200 for these reasons.

Answer 3

As I understand it, a black-and-white TV tube with no color burst signal and the RF stage skipped, while it could not quite reach 80 columns, could handle 64 columns well enough,

It can do 80 as well, but that's already borderline.

which raises the question of why Apple and Commodore didn't try for that, or at least for something a bit beyond 40. 64x25 = 1600 bytes, not prohibitive even by the standards of the time.

As Stephen Kitt already mentioned, 1 KiB is a lot of memory, especially considering that the early machines like PET and Apple II were meant to work with as little as 4 KiB total RAM. The 'mere' 600 bytes needed for 64x25 would have been a rather deep cut.

Where Commodore and Apple stayed with a 'traditional 24/25 line display, Tandy did go a different way by making the screen 'binary' compatible with a 64x16 resolution - thus staying within the 1 KiB needed for display memory but showing longer lines.

I get the impression memory bandwidth was a significant limitation, such that when the Apple II later added an 80-column option, the engineering of such involved the careful juxtaposition of two memory banks to provide data at the required rate.

Not in any way. Simple math tells that a 1 MHz machine with video a video access window on every clock can deliver 1 MB/s. A 40x24 screen at 60 Hz needs less than 60 kB/s, that's less 6%. There's much room for a larger display, even when including other constraints like frame structure.

The problem with the Apple II was that its timing is built around the video timing. The CPU doesn't get a monotone clock rate, but a varying one to make it fit. While this was a quite clever design, it's somewhat inflexible when data needs to be accessed at double the rate. Ofc, they could have went along and redesigned the whole timing - or doubled the machines speed (which would have been nice). The least intrusive way to double accessed data within the existing timing was adding just another KiB and make video access 16 bits wide.

Was memory bandwidth the reason for Apple and Commodore sticking with 40 columns?

No. On the Apple side it's rather clear that WOZ intended the use of a TV set with (or without) intermediate RF. Thus 64 would be out of scope and 40 seams a nice thing.

For the PET, I can only guess that they tried to stay on the safe side (*1) - and, maybe more important wanted to keep 24 lines. With 24 lines 42 would have been possible - but that's really odd, isn't it?

We often forget here the other early machine, the Atari 400/800. Here, like with the Apple, a standard TV set was intended as main display, so 40x24 is a logical choice.

If so, how did Tandy achieve 64 columns? Is it purely a coincidence that the 64-column machine is the only one of the trinity to use the Z80?

As soon as one is willing to diverge from the 24/25 lines paradigm, 64x16 is a quite reasonable choice. Especially when it comes with an 'internal' screen

(I'm assuming the Vic-20 later supplying only 22 columns, is not so much due to technical limitations as to the use of a video chip designed more for games than general purpose computing.)

I think that's a good assumption.

Much like the TI 99/4 was designed around the use of a TV set. Here the standard mode was only 32x24 (a two color 40x24 was also available).

The C64 later on benefited from the 40x24 decision for the PET.

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*1 - I would like to think that they did keep an eye on the use of a TV during design - and before the decision was made to produce an all-in-one unit.

Answer 4

The Apple II is the only one of the 1977 trinity to have memory mapped bitmap graphics: 280x192. With a terribly small font 5x3, you could achieve 70 columns by 32 rows using the "hires" graphics mode. Thank goodness there was a plethora of 80 column text cards that came out in the early 80's for the Apple II. The late Apple //e (1983) model had 80 columns builtin, but that was long after the trinity.