On Tue, Aug 13, 2019 at 10:18 AM Bob Supnik <b...@supnik.org> wrote: > As was pointed out, the existing PDP-8 CPU is basically a PDP-8/E or > -8/A.
It's a mongrel. SIMH's FPP (floating point processor) peripheral emulates the version for the 8/a, but memory is limited to 32 kWords as in all models before the 8/a, which added a mode to allow 128 kWords of core or semiconductor RAM. The 32 kWord limit is hard-coded in pdp8_defs.h as MAXMEMSIZE. I'd expect at least that limit to be lifted if you're going to call the SIMH simulator a PDP-8/a simulator. I think it'd be a lot of work to give the current CPU emulator model-specific modes down to a level necessary to get full branch coverage in software that probes for model quirks and changes behavior accordingly. I'm personally aware of multiple pieces of commonly-available PDP-8 software that does this, and there was doubtless a lot more that I'm not aware of, because the economic lifetime of the PDP-8 was roughly 2 decades long! Closer to a quarter century, if you count the PDP-5. it's clear that the DECtape controllers should > have been abstracted to a library ten years ago, but doing so would be a > major PITA, now that there are six (at least) distinct implementations > (PDP1, PDP18b, PDP11, PDP8 TC, PDP8 TD, KA10). I think it'd be more work than it's worth to make a single abstraction layer; I think you'd want at least two versions. The reason there are two DECtape controllers in the current PDP-8 simulator is that one is block-by-block (TC) and the other is line-by-line (TD), where "line" is 3 bits! That means there is a device driver in TD-aware PDP-8 software that has to assemble incoming data 3 bits at a time! We have a mode in the PiDP-8/I software that lets us build the OS/8 disk packs in a way that works with either driver. The OS/8 TD8K driver for the TD8E DECtape is so-named because it requires an 8 kWord PDP-8 because the code complexity in the driver requires that it be split over two "fields" (4 kWords) of core, which is unusual for an OS/8 device driver. We have this mode so you can use the PiDP-8/I software stack to build disk images for real hardware, where switching modes is much more consequential than simply changing a "C" to a "D" in a SIMH boot script. This feature of the PiDP-8/I software is scriptable, so that for any question OS/8's "BUILD" program can ask, you can script an answer to it to get a new custom OS/8 medium build: https://tangentsoft.com/pidp8i/doc/trunk/doc/os8-run.md
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