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Comment by rahen | Hacker News Reader

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.

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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.

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