Re: [TUHS] A New History of Modern Computing - my thoughts

[Rich Morin <rdm@cfcl.com>]

> On Nov 30, 2021, at 11:27, Ralph Corderoy <ralph@inputplus.co.uk> wrote:
> 
> ... The point of that bit of history is they were not chip designers, but
> knew electronics and programming.  Wilson designed the ARM's instruction
> set and it was a delight to code: very orthogonal, and every instruction
> had four-bits of condition-flag test, e.g. Carry Set, and a bit to
> indicate if this instruction should set the condition flags.  Thus
> several instructions in a row could test the condition flags set by an
> instruction a few earlier and unaltered since; this cut the need for
> quite a few branches. ...

I wrote a fair amount of PDP-11 assembler, back in the early 70's (about 10K LOC).  I was particularly happy with a routine that moved a cell between a pair of doubly-linked, circular linked lists (a "free" list and a "busy" list).

The routine only had to modify six pointers, which isn't a hard problem.  The cute part was that it was able to do so using (IIRC) only eight or nine instructions.  The PDP-11's auto-increment mode obviated the need for separate index modification code.

I had previously written a fair amount of code for a Varian 620i, which had an AQX architecture.  The 620i wasn't _hard_ to program, but it _was_ a bit tedious. The PDP-11, by comparison, was a programmer's delight.

Which brings me to a historical notion:

The DG Nova (https://en.wikipedia.org/wiki/Data_General_Nova) came out in 1969, just a bit before the PDP-11 (https://en.wikipedia.org/wiki/PDP-11).  My impression, when I (later on) looked at the Nova ISA, was that they had moved in the right direction from the AQX approach, but not quite far enough.  DEC, IMNSHO, got it right.  (ducking).

-r