Hacker News Reader

Comment by forinti | Hacker News Reader

There were Dutch computers with weird combinations like 27 bit registers and 15 bit address lines made by Electrologica.

View on HN

A lot of older computers were "bit-serial", i.e. the ALU processed "bytes" bit by bit. This, along with how every part used to make computers was also 1bit (because VLSI wasn't a thing yet).

MIT Whirlwind is in some ways ancestor to all modern computers by switching to bit-parallel architecture with multiple bits being handled at a time.

36bit word length became common for scientific applications because it was apparently considered enough to do equivalent of 10 digit precision in fixed precision arithmetic, thus common pattern of 36 among American scientific computer lines.

Later common availability of 4bit components like 74xxx families meant that multiplies of 4 bits started being common.

4bits also allow to handle calculations in BCD form, which had obvious use cases in businesses, which is partially why S/360 which attempted to consolidate IBMs scientific and business lines went with 32bit word length - 8 bit was comfortably 2 BCD digits and also would fit one character of then-upcoming ASCII standard (the reason EBCDIC survives in S/360 is that ASCII was not yet ready and IBM decided to extend their existing codes instead of taking unfinished standard that could change - and they had to hardcode it in some devices back then)

The final nail in 36bit and longer word lengths was floating point becoming fast enough - and arguably infighting between VAX and PDP-10 groups at DEC which among other things triggered existence of BSD Sockets API and their horrible legacy on networking.

The first generation computers, using vacuum tubes, were indeed usually bit-serial. Some had parallel ALUs, or sometimes just parallel multipliers units, but an entirely parallel computer designed with vacuum tubes would have been very power hungry and unreliable.

Some notorious exceptions are the ENIAC and Whirlwind 1 which indeed were fully parallel.

As for 36-bit machines, they were also popular because of LISP, as a cons cell was 2x18 bits. I think this influenced the design of the PDP-6 and 10.

And yeah, I also wished DEC didn't kill the PDP-10 for the VAX, but that's another story.

36bits for lisp was specific to PDP-6/10 not anything else.

And Lisp in turn took its "memory word is a cons cell because you can fit two addresses in one word" from a 36bit IBM scientific computer with 15 bit addressing.

The Symbolics 3600 series were also 36bit machines. Later the Symbolics Ivory microprocessor then was a 40bit cpu.

yes, but in both cases the motivation was to have 32bit fixnums and addresses and thus word size was extended with extra bits for tags, unlike the predecessor design of CADR which used 32bit bus and had to stuff tags and fixnum/addresses together - resulting in 24bit fixnum/address space, or 3 8bit characters stuffed into each word, plus 4 bits of tags. TI Explorer later updated CADR/LMI design to use 25bit fixnum/address and 3bit mandatory tags.

For people who don't know:

Tag space included both CDR-coding (compression mechanism for linked lists), datatype tags, as well as formed portion of the instructions if the word contained instructions - on 3600 and Ivory each word could contain 1 or 2 instructions.

Bit serial made a lot of sense because the memory was often serial (drum memory or acoustic delay lines use in 1960s calculators).

>And yeah, I also wished DEC didn't kill the PDP-10 for the VAX, but that's another story.

The DEC-20 was 6x faster than VAX-11/780 on at least one benchmark (that I wrote when I had access to both in the 80s..). DEC-20 was a PDP-10 implemented in ECL, it was quite fast.

Ah, yes, there were even ternary computers, using three states (1, -1 and 0) instead of two.

In the UK we had Dekatrons that could come in a variety of different bases:

> While most dekatrons are decimal counters, models were also made to count in base-5 and base-12 for specific applications.

Source: https://en.m.wikipedia.org/wiki/Dekatron

I’ve personally played on the Harwell Dekatron, it’s a fun machine

https://www.tnmoc.org/first-generation-gallery

Aka Dijkstra's baby. Those were the predecessor of our modern CPUs in many ways, so many concepts were pioneered on those. There is even a direct line between that machine via THE and Multics to Unix.

Reply on news.ycombinator.com