3 ms·

Fun fact[1]: the x86 instruction encoding (pre Pentium or so) is itself octal in organization. This is immediately apparent in the 2+3+3-bit structure of the Mo

Fun fact[1]: the x86 instruction encoding (pre Pentium or so) is itself octal in organization. This is immediately apparent in the 2+3+3-bit structure of the ModR/M and SIB bytes, but looking at the opcode table that way is also helpful. The manuals stubbornly describe it in hex, though—you have to go way, way back to the Datapoint 2200 “programmable terminal” to find a manual that actually talks in octal, and that wasn’t even made by Intel! (The 8008 was commissioned from Intel by the original manifacturer as a “2200 on a chip”.) Weird Intel history aside, everybody used octal in the old days. I don’t actually know why, but I suspect this goes back to how early binary computers used 36-bit words (later 18- or 12-bit ones) because that’s how many bits you need to represent 10 decimal digits (and a sign), the standard for (electro)mechanical arithmometers which (among other devices) they were trying to displace. So three-bit groupings made more sense than four-bit ones. Besides, as far as instruction encodings go, a 8-way mux seems like a reasonable size if a 16-way one is too expensive. (Octal on the 16-bit PDP-11 is probably a holdover from the 12-bit PDP-8? Looking at the encoding table, it does seem to be using three- and six-bit groups a lot.) [1] https://news.ycombinator.com/item?id=30409889 https://news.ycombinator.com/item?id=30409889

gumby 4y ago
> but I suspect this goes back to how early binary computers used 36-bit words (later 18- or 12-bit ones) Actually the PDP-1 was an 18 bit machine; its successor the PDP-6 had 36 bit words (and 18 bit addresses -- yes it was explicitly designed to be a Lisp machine in 1963). Other DEC machines like the PDP-7 (on which Unix was developed) were 18-bit machines (Multics used an 18-bit non-DEC architechture). Probably the most popular minicomputer ever, the PDP-8, used 12 bits, as did a bunch of DEC industrial control machines.
- eesmith 4y ago
  There's a story about Grace Hopper, who learned octal for the BINAC. She later had problems balancing her checkbook before she realized she was doing so in octal. That lead her to conclude it would be better to teach the computer to handle decimal than to force everyone to use octal. A lot of the old systems used 6-bit character codes (https://en.wikipedia.org/wiki/Six-bit_character_code https://en.wikipedia.org/wiki/Six-bit_character_code ), including BCD six-bit codes, making 6*n word sizes more appropriate.
  - gumby 4y ago
    My dad used to give me maths problems when I was little but tell me to solve them in specific bases (typically decimal, octal or hex, but not always). Doing long division by hand in hex gives you a feel for how the numbers relate. The anecdote about decimal makes sense — she was the key to the design of COBOL. I used quite a few machines with six bit characters into the mid 1980s.

gumby 4y ago

> but I suspect this goes back to how early binary computers used 36-bit words (later 18- or 12-bit ones) Actually the PDP-1 was an 18 bit machine; its successor the PDP-6 had 36 bit words (and 18 bit addresses -- yes it was explicitly designed to be a Lisp machine in 1963). Other DEC machines like the PDP-7 (on which Unix was developed) were 18-bit machines (Multics used an 18-bit non-DEC architechture). Probably the most popular minicomputer ever, the PDP-8, used 12 bits, as did a bunch of DEC industrial control machines.

eesmith 4y ago
There's a story about Grace Hopper, who learned octal for the BINAC. She later had problems balancing her checkbook before she realized she was doing so in octal. That lead her to conclude it would be better to teach the computer to handle decimal than to force everyone to use octal. A lot of the old systems used 6-bit character codes (https://en.wikipedia.org/wiki/Six-bit_character_code https://en.wikipedia.org/wiki/Six-bit_character_code ), including BCD six-bit codes, making 6*n word sizes more appropriate.
- gumby 4y ago
  My dad used to give me maths problems when I was little but tell me to solve them in specific bases (typically decimal, octal or hex, but not always). Doing long division by hand in hex gives you a feel for how the numbers relate. The anecdote about decimal makes sense — she was the key to the design of COBOL. I used quite a few machines with six bit characters into the mid 1980s.