Comment by kens | Hacker News Reader

Comment by kens | Hacker News Reader

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The Datapoint 2200, the source of the 8008 instruction set, is an interesting machine. The CPU was built from TTL chips. To decode instructions, they used decimal BCD decoder chips, specifically the 7442. But they'd use them as octal decoder chips, only using 8 outputs.

The Datapoint 2200 documentation gave the opcodes in octal, so they were clearly thinking in octal. The 8008 documentation, however, didn't use octal or hexadecimal. The opcodes were given in binary, but grouped in 3 bits, octal style, e.g. 10 111 010. (They didn't specify opcodes in octal or hex!) I think the 8008 was right at the time where octal was on the way out and hexadecimal was taking over. (The 8008 assembler manual uses both octal and hexadecimal, but hexadecimal primarily.)

The Intel 8080 still specified the instruction set in binary, not octal or hexadecimal. The 8085 had opcodes in binary in a 1983 manual, but now split with a line into 4-bit chunks (i.e. hexadecimal-style). And then an appendix gave the opcodes in hexadecimal.

(Just some random history.)

One thing I forgot to mention: the 6502 microprocessor also uses groups of 3 bits in its instructions. However, they group them in the "wrong" way, aaabbbcc, so looking at the instructions in octal doesn't help you at all.

Also, after using the Xerox Alto, which uses 16-bit words, I realized that octal is terrible. The problem is that if you're looking at two bytes in a word, the values make no sense in octal. For example, the characters "AB" form the hex word 0x4142, while "BA" forms 0x4241; the two letters are clear. But in octal, "AB" is 0o40502 and "BA" is 0o41101; the two letters turn into unrecognizable numbers.

With 12-bit, 18-bit, or 36-bit words octal is pretty great. It just sucks with 8-bit bytes being grouped into 16-bit or 32-bit words.

That's just a consequence of us sticking to 8-bit bytes (and derivative word sizes), no? Octal would have made a lot more sense if it was, say, 12-bit.

“Us sticking to 8 bit bytes” is a consequence of having preferred BCD to octal in the past, so the causation is reversed (“12-bit words would have made a lot more sense if it was, say, octal.”) [Edited: actually 12 bit words would make sense in either case, as it's three BCD digits or four octal digits]

The Intel 4004 used four bits to manipulate a single BCD digit. The 8086 had BCD instructions. There were many reasons for preferring BCD when designing computer architectures, though my favourite which was already becoming less relevant at 8086-time was that it meant a full column on a punchcard wouldn’t be “all holes” and reduced the likelihood of the cards tearing.

I'm thinking of earlier times, before microprocessors in general. 6-bit bytes were a thing for a while - fairly logical, too, given that it was just enough bits to encode the entirety of ITA2 without needing any control codes to switch between character banks.

I implemented an 8086 emulator using the 1981 "iAPX 86, 88 USER'S MANUAL". It specifies the opcodes in bit patterns, so add is specified as "0 0 0 0 0 0 d w | mod reg r/m", where d is direction (mem to reg or reg to mem) and w is width (byte or word). Since this kind of pattern is used across many instructions it makes the code fairly easy to comprehend (to me at least).

Extracting the alu function from bits 4-6 means that you can group together the implementation of add, or, adc, sbb, and, sub, xor, to and from memory, for bytes and words into one function.

The code's not as fast as the "one code block per instruction" approach of something like DosBOX but at least it doesn't cause me dread to look at.

In case anyone is curious like I was and wants to look at the documentation, Bitsavers has some[1], including the octal opcode reference[2].

[1] http://bitsavers.org/pdf/datapoint/2200/

[2] http://bitsavers.org/pdf/datapoint/2200/2200_Programmers_Man...

Do you know if Federico Faggin copied the logic design of the 2200 or implemented the ISA using his own design?

His own. There would have been no way to fit the logic design of the 2200 on a chip, both because of lack of metal layers, and because the 2200 was designed to be frugal in how many chips it used, using off-the-shelf chips, which is very different from being frugal in the amount of transistors you use when you have freedom to lay out each one by hand.

The implementation of the 8008 is completely different from the 2200 (as is Texas Instruments' forgotten TMX 1795 implementation). It would be extremely inefficient to copy the TTL implementation, since that depended on what chips were available. But the biggest difference is that the Datapoint 2200 was a serial machine that used serial shift-register memory while the 8008 had a "normal" 8-bit datapath.

Thanks - even in 1972 we had multiple radically different implementations of the same ISA!

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