How much did IBM save by limiting the PC to 4.77 MHz?

Question

My understanding is the CPU clock speed on the Intel 8088 in the IBM PC was selected as 4.77 MHz to simplify the design of the system. This despite the 8088 coming in two versions - 5 MHz and 8 MHz. Since later PC's routinely operated at higher clock speeds and typically up to the CPU's spec, I assume some cost savings was realized on the original PC.

What specific parts were able to be eliminated from the PC design by limiting the CPU clock rate? Can anyone approximate the cost savings by removing those parts?

Answer 1

Worshipping at the altar of color clock

Back in that day, everything was built around the NTSC color clock frequency of 3.579545 MHz. Everything from the Atari VCS to the C64 made ample use of it, because some iteration of your product would inevitably need to talk to a commodity color NTSC display operating at that frequency.

This will come up; so FYI the way you divide by 3.5 is by starting with a crystal at 4x the color clock (14.31818) and dividing by 14.

• The Atari VCS/2600 used 1.19 MHz -- color clock / 3 -- crystal / 12
• The Atari 400/800 used 1.79 MHz -- color clock / 2 -- crystal / 8
• Most of the rest used 1.022 MHz -- color clock / 3.5 -- crystal / 14

By "rest" I mean Apple II family, Commodore Pet, Vic, C64. Note that most of these systems actually overclocked the CPU. I can vouch that Apple did not use a 2 MHz version of the 6502 chip; they used the 1 MHz version.

The 8088 master clock operates at 4x RAM cycle speed

Mind you, all these systems above have one system clock per memory cycle. The Intel philosophy was quite different, with a fast master clock, but memory fetches only occurring at 1/4 of this speed.

So the IBM PC clock was chosen, still worshipping the color clock, at 4.77 MHz -- crystal / 3.

Wow, that sounds fantastic, right? IBM, big gorilla of the computer biz, comes out with a processor seemingly an order of magnitude faster than those clunky 1.02-1.79 MHz 6502 systems. Yeah, hold on. It has an 8-bit bus, and memory operations happened at 1/4 this frequency, at 1.19 MHz -- crystal / 12.

The IBM PC had the memory throughput of an Atari 2600.

That's not so awesome. Nor was it intended to be. IBM's market strategy was not to be "Amiga", outshining the competition in raw performance. Much the opposite, they wanted to avoid at all costs cannibalizing sales of their larger and much, much more expensive minicomputer line e.g. Series/1. The purpose of the IBM PC was to put I B M on a conservatively designed system -- and let IBM's unbelievable reputation and market power sell it. That can't be overstated - they had market dominance of (larger-than-Apple-II) computers like Google has dominance in search. My dad was an IBM salesman, and there was an adage universally known:

Nobody ever got fired for buying IBM.

Seriously. It was just like that. If your mainframe or minicomputer project went sideways for any of a variety of reasons, scuttlebutt is "this wouldn't have happened if you had bought IBM". (even if that had nothing to do with it). So the business desktop market was a vacuum, avoiding perfectly useful offerings by Tandy, Commodore and Apple, holding its breath for 3 magic letters.

All that to say, performance was not Job One.

Why would IBM give any care at all about color clock?

After all, it grew up as a business machine; many systems actually used the Monochrome Display Adapter (which had much higher resolution than NTSC); many monitors were RGBI for which color clocks did not matter, and IBM's dabble in the home market with the PCjr was a fiasco.

True, but nobody knew that, at the time.

IBM did not know what it would take for the PC to adopt. But they certainly knew the home computer market was proven. So they wanted to "keep their options open" for a side-step into the home market, where they MUST by necessity play nice with NTSC. Costs must be kept low in such a system - which means doing what everyone else does: sharing main RAM with the video card, which in turn requires memory and therefore CPU clock to be in syncopation with NTSC color clock.

As such, choosing 1.19 MHz memory clock was a conservative, safe choice; as made by the Atari VCS/2600.

The choice of 4.77 MHz instead of 5 MHz only cost 4.5% performance. You can argue that they could have architected the system for 4.77 MHz and then just used a faster crystal in the 5150 PC (planning to downsize the crystal to 4.77 in the future imaginary consumer PC). However, in that era, the prevailing view was if you change hardware, it breaks existing software, which as a rule were written to run on bare metal and software routines as-shipped. (think Apple's Sweet16 and FP routines, which were removed to fit AppleSoft BASIC). So they wouldn't want to risk it for 4.5% performance.

They did end up doing that "future imaginary consumer PC" and making use of those concepts, but far past the time of its relevance (technical or market). Maybe they were thinking "nobody ever got kicked out of bed for buying IBM".

Answer 2

For many kinds of parts, there's a substantial gap between specified maximum/minimum timings and typical timings. The 8088 specification requires that the clock be high for a minimum of 69 ns every cycle and low for a minimum of 118 ns. Dividing a 14.3818 Mhz clock by three yields a high time of 69.5 ns and a low time of 139.1 ns, satisfying both requirements for up to 20 ns of timing uncertainty on the divide circuit, provided that one ensures that it is biased toward lengthening its output.

If one were to divide a 15 Mhz clock by three while meeting specifications, the natural high time would be 66.7 ns, which would fall slightly short of the timing spec and the low time would be 133.3 ns, which would only tolerate about 15 ns of timing uncertainty. There are ways one can slightly stretch a signal, but using a 14.3818 Mhz signal that can tolerate anywhere from 0 to 20 ns of stretching is cleaner than using a design that would require stretching the signal by at least 2.5 ns but not more than 15 ns.

Answer 3

Quartz used in colour TVs such as 3.579545MHz or 14.31818MHz used to be much cheaper than other frequencies.

It was important for home computers and game consoles (8/16 bits), which used the same quartz for video and the CPU, keeping everything synchronous.

It was far less siginificant for rather expensive computers such as IBM PC which didn't need to produce NTSC compatible video (although CGA was NTSC).

Edit : this post : http://dosmandrivel.blogspot.com/2009/03/ibm-pc-design-antics.html tells that the reason was to save a quartz for CGA.

Answer 4

The earlier answers correctly say that IBM chose to divide the 14.3818 MHz NTSC color clock by 3. This decision had nothing to do with CGA graphics, however (whose TV output was asynchronous anyway). It was, in fact, carried over from an earlier, now almost-forgotten, IBM personal computer that came with its own built-in five-inch monochrome screen, and did not use NTSC color signals at all. The fact that one optional expansion card for the IBM PC later happened to allow optional NTSC color output was a coincidence. The bus had already been designed for a different system entirely.

It was in fact a holdover from the same engineers’ previous computer, the IBM System/23 Datamaster. They were released only a month apart, the System/23 is therefore almost forgotten, and the delays due to its ROM BASIC led IBM to contract out the BASIC for the PC to Microsoft instead. The 8-bit bus of the IBM PC (model 5150) was an update of the System/23 Datamaster’s bus, which was one reason IBM used the 8088 with its 8-bit bus rather than the 8086 with a 16-bit bus. As you can see on page 38 of the IBM 5322 Service Manual, the “IO Card Sockets” of the System/23 is very similar to the bus we today know as 8-bit ISA, except for some small differences in how the interrupt pins work between the Intel 8085 and 8088 CPUs. These machines were designed by many of the same people, the PC was something of a rush job immediately afterward, and both ran the same expansion bus at the same speed.

Instead of the custom chips that its predecessor, the IBM 5110, had used, the System/24 used the 8-bit Intel 8085 as its CPU along with several other Intel support chips that were later used in the IBM PC model 5150. The 5110 contained very similar display and peripherals, but it had used a custom 16-bit CPU called the Palm, and its oscillator ran at 15.1 MHz.. Its internal buses were also completely different. The System/23 might have switched to NTSC color clock oscillators, despite not using NTSC or color at all, as part of switching to cheaper off-the-shelf components.