Some new text adventure stuff for 2.11BSD

Adam Thornton athornton at
Sat Jan 4 12:19:26 CST 2020

So, it's _mostly_ a 64K problem.  All z-machine games have, in the file
header, a "high memory" address above which it's safe to page data in and
out as needed, because it's all read-only.  Trinity's header claims what
smells like an implausibly high value for a z4 game to me: 0xf771.  There's
no way it needs _that_ close to the 64K limit for writable store.  Its
"restart address" is 0x9310, though, which is also quite large, so maybe it
does do a lot more with RAM than most z-machine games?

The pure/impure and caching system is actually quite simple.  Everything
below the "high memory" mark stays in RAM, everything above is divided into
512-byte pages which are paged in and out of the disk file as you need enough to cover dynamic memory, plus at least one
page, plus whatever your program requires.  Cache replacement is simple
LRU.  This works quite well in practice.

I'm already running with split I/D, which did help quite a lot, but not
enough to let me run something that claims to need all but 2K of the
potentially-writeable memory map.

ZEMU is much, MUCH tighter than ZIP, but of course it's in MACRO-11
assembly, not C-compiled-with-the-2.11BSD-C-Compiler, which isn't all that
tight a compiler by modern standards, and ZIP wasn't written to be
optimized for memory consumption anyway.

I tried compiling ZEMU with m11 on 2.11BSD, but there are zillions of
errors, and I certainly won't have time to figure it out (and write an
OS-dependent piece) by next Saturday.  However, I may put ZEMU and a bunch
of games on the RSX-11 system on the PiDP-11 instead.  But first I gotta
replace the spinner in my MAME cabinet.


On Sat, Jan 4, 2020 at 10:54 AM Ethan Dicks <ethan.dicks at> wrote:

> On Sat, Jan 4, 2020 at 12:12 AM Adam Thornton via cctech
> <cctech at> wrote:
> > I'm having a party on Saturday January 11 (and if any of you are in
> Tucson...
> I'm a bit far off but it sounds cool.
> > So I kind of wanted to put a general-purpose Z-machine interpreter on my
> > PiDP-11, so that people could play Infocom (and community) games on a
> real
> > terminal.
> Fun!
> > Turns out there wasn't really one, so I ported the venerable ZIP
> >
> >
> Thanks for this.
> > My biggest disappointment is that the memory map of Trinity, my favorite
> > Infocom game, is weird and even though it's only a V5 game, I can't
> > allocate enough memory to start it.
> Is this just a 64K segment problem?  Perhaps there needs to be some
> examination and optimization of the impure and pure storage and
> caching design?
> One can play larger games with 128K of total RAM on microcomputers,
> but most of those engines are written in assembler not C so the engine
> is tightly coupled to the processor architecture.
> -ethan

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