Van: Paul Koning via cctalk<mailto:cctalk@classiccmp.org>
Verzonden: woensdag 21 februari 2018 20:37
Aan: Guy Sotomayor Jr<mailto:g...@shiresoft.com>
CC: General Discussion: On-Topic and Off-Topic
Posts<mailto:cctalk@classiccmp.org>
Onderwerp: Re: Writing emulators (was Re: VCF PNW 2018: Pictures!)
> However, it is my belief (and I think others have also stated) that assuming
> infinitely fast I/O (e.g. no delays what so ever) can cause issues because in
> many cases the SW expects to be able to do some work between the time that
> the I/O is started and when it completes.
True, that is unfortunately a fairly common type of software bug. And because
it is, emulators have to work around those bugs. I make it a point to call it
a bug, though, because I don't want anyone to get the impression that OS
programmers who wrote such things were doing the right thing.
paul
Yeah, I found that out when I was working on the PDP8/e emulation running on a
6809. OS/8 does that as well. After issueing the disk I/O it executes a few
more instructions, because it “knows” that the requested disk data cannot yet
have been loaded into memory. I solved that problem with a counter that can be
preset to some TBD value. The value defines the number of extra emulated
instructions before it jumps to the (now) loaded data from disk – at least,
that is how I remember it doing over 10 years ago. I have an extensive webpage
on pdp8 emulation on 6809. I succeeded in finishing it: booting OS/8 and
running spacewr on it!
Don’t ask how “fast” it ran …
