[TUHS] Re: Clever code

[Douglas McIlroy <douglas.mcilroy@dartmouth.edu>]

Apropos of accessing rotating storage, John Kelly used to describe the
Packard-Bell 250, which had a delay-line memory, as a machine where
addresses refer to time rather than space.

The PB 250 had two instruction-sequencing modes. In one mode, each
instruction included the address of its successor. In the other mode,
whatever popped out the delay line when the current instruction
completed would be executed next.

Doug

On Tue, Dec 13, 2022 at 6:48 AM John P. Linderman <jpl.jpl@gmail.com> wrote:
>
> There was a story that old hands would torment newcomers to the IBM 650
> by tinkering with the optimizer to make it as slow as possible (and, with
> rotating drums, that could be VERY slow). Then they'd look at the
> newcomer's code, make a trivial change, run it with the real optimizer,
> and get dazzling improvements.
>
> I also recall punched card bootstrap programs for the IBM 7094 that
> would load column binary when run column binary, and load row binary
> when run row binary. -- jpl
>
> On Mon, Dec 12, 2022 at 10:53 PM Dave Horsfall <dave@horsfall.org> wrote:
>>
>> On Tue, 13 Dec 2022, Rudi Blom wrote:
>>
>> > I vaguely remember having read here about 'clever code' which took into
>> > account the time a magnetic drum needed to rotate in order to optimise
>> > access.
>>
>> Sounds like you're referring to SOAP (Symbolic Optimal Assembly Program)
>> on the IBM 650; the programmer wrote the code "straight down" and SOAP
>> reordered it for rotational latency.
>>
>> > Similarly I can imagine that with resource restraints you sometimes need to
>> > be clever in order to get your program to fit. Of course, any such
>> > cleverness needs extra documentation.
>>
>> Try writing a bootstrap program in 512 bytes :-)  Self-modifying code was
>> the order of the day...
>>
>> > I only ever programmed in user space but even then without lots of comment
>> > in my code I may already start wondering what I did after only a few months
>> > past.
>>
>> You could be clever in kernel space too, such as taking advantage of
>> the DATIP/DATO cycles on DEC's Unibus when updating a memory word i.e.
>> read/modify/write.
>>
>> -- Dave