> On Dec 4, 2016, at 10:12 PM, Bob Supnik <[email protected]> wrote:
>
> Josh Dersch at the Living Computer Museum found a snippet of real DECtape
> code (that runs on a real 11/40) which fails on the simulator.
> ...
> This leads me to think that there's a second principle to bear in mind when
> simulating older machines. The first is "economy of gates". In early systems,
> gates were precious, and the hardware tended to implement no more than the
> minimum functionality required. Error checks were a luxury and were often
> omitted.
This makes me wonder about the fuzzy line between quirky features and
sort-of-bugs. The code snippet you mentioned sounds like it falls on the side
of the "quirky", and it sounds right for the simulator to implement that. On
the other hand, there's one I recently read about a machine in which a
subroutine call instruction would fail with the stack pointer equal to -0, but
when the stack pointer was +0 it would produce an address error only for some
of the addressing modes. "The schematics ... confirm this; the reason is
unknown" says the article. Implementing that sort of corner case is obviously
doable, but not necessarily all that useful.
paul
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