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PsiXZX Page
  Updated Mon Dec 30 22:03:55 2002 Site Map Site Home page Matt's home page Angela's home page Natasha's home page Hannah's home page Our Wedding Pages Personal Online CV Family history Projects Latest News The Crypt (Old, Dead Projects) Miscellaneous QtGantt HTML Macro System SigChange Home Page Sync Psion EPOC Overview Mixed Language Programming GDB Debug Stub hForth Port Disassembler Psion SIBO All SIBO projects are in The Crypt Linux Toshiba 40xx & Linux Network-RS232 Bridge Psion Protocol Analyser Serial Line Analyser Work on plptools Psion .img tools Happy Badger Linux Linux Psion Link Ideas Amateur Radio Amateur Radio Software Popularity  accesses so far.. Matt J. Gumbley - http://www.gumbley.demon.co.ukPsiXZX Psion Series 3 Spectrum Emulator
Table Of COntents
1 Introduction 2 Availability 3 Limitations 4 Why it's so slow 5 Why the keyboard handling is so bad 6 Wot no colour? 7 Why won't it load Jet Set Willy? 8 So emulation on a Series 3 is a no-go, then? 9 Hasn't the Gameboy got a Z80?
1 Introduction
PsiXZX is a port of the XZX ZX Spectrum Emulator to the EPOC 16 platform.
There's not a lot of information available yet, and nothing you can really
try out, I'm afraid...
2 Availability
It isn't available yet. Erik Kunze, the main author of the XZX emulator has
asked me to merge the Psion port with the UNIX/X11 source tree. Since I ported
this as a very quick hack to see if it was feasible, I hacked away at his
source with great relish. There's little of the base XZX code left intact..
it'll take a while to comply with his wishes (see comments about whether I can
be bothered continuing with this)...
3 Limitations
This is an extremely slow emulator, even running under the EPOC 16 Emulators on
high-speed PCs. I'm working to improve matters by optimising it, but it'll
never run in real-time on a real Psion.
It emulates a 48KB standard ZX Spectrum, with no additional interfaces. There
is no support for sound, and the attribute (colour) handling isn't implemented.
Keyboard handling needs some more work, and there's no snapshot loader yet.
Spectrum BASIC seems to work OK though, if you're prepared for a wait. Timing
isn't as accurate as the XZX emulator which this program uses as its base.
The Spectrum screen is 256x192 pixels - taller than a Psion screen. I haven't
yet added any screen scrolling routines to enable the screen to be viewed on a
real Psion; I simply changed the emulator's configuration to give me a larger
screen.
There's a lot of work still to do. Although several people have expressed an
interest in seeing this run on a real Psion, I'm not entirely sure it's
worth the effort. It does run extremely slowly.
4 Why it's so slow
The Z80 emulator in XZX uses a large switch() statement to process each Z80
opcode. The extended opcodes are handled by nested switch() statements. Thus,
the resulting emulator main loop is VERY large. Several UNIX compilers cannot
compile it, and Topspeed C won't compile it either. Therefore, I've had to
break the switch() statement down into separate functions which handle each
opcode, and a function-pointer table. This imposes an additional lookup and
jump/return per opcode, which slows things down somewhat.
The Z80 has a 64KB memory map, with the Spectrum's 16KB ROM mapped in at
the start of memory, followed by the screen RAM, then the user RAM. Since
EPOC 16 only allows a single 64KB code and 64KB data segment per process,
an additional segment has had to be allocated as the emulated computer's
memory. Accessing this memory has to be done using EPOC system calls.
This slows down each memory access considerably. There is some optimisation
to be done using assembly language inline access, if possible.
The screen bitmap is held in another separate segment, and is accessed in the
same manner. When a write to Spectrum screen memory is performed, the write is
done to the main memory segment, and then processed, and done to the Psion
screen bitmap. Since the bitmap is in a separate segment, the EPOC OS calls
have to be done, slowing things down even more...
5 Why the keyboard handling is so bad
When a key is pressed on a real Spectrum, certain bits of an I/O port are
lowered. When the key is released, these bits are raised. So, simply map
key presses and releases onto bits of this I/O port. Except that (AFAIK)
you can't detect when a key has been released in EPOC 16. Also, the Spectrum
OS performs key rollover, and so a key has to be pressed for a certain
duration before it is accepted. So, a system of counters has been added.
When a key is pressed, the appropriate bit(s) in the port are lowered,
and a counter set to a value. When the port is read, the keypress is
registered, and the counter decremented. When the counter reaches zero, the
appropriate bit is raised, simulating a key release. It works, but it's a
kludge, and games probably won't like it.
6 Wot no colour?
I couldn't be bothered :-)
The screen is used as a black-and-white image, the grey plane of the screen
bitmap is not yet used.
7 Why won't it load Jet Set Willy?
Because I haven't added the snapshot loader yet. You could always write a clone
in Spectrum BASIC. And type it in every time you want to play it :-)
8 So emulation on a Series 3 is a no-go, then?
I wouldn't say that... it does work, but we're only using an NEC V30H at
7.68MHz so don't expect anything fast. Emulation is a very time consuming
process. The Z80 is quite a complex processor, IMHO. Simpler processors (such
as the 6502) would be easier to emulate. This doesn't mean that a Commodore 64
would be simple to emulate - that machine had some quite complex hardware. The
Spectrum's appeal lies in the incredible simplicity of the hardware. It's
relatively simple to emulate. I could rewrite PsiXZX's core in assembler, but
a) I'd really rather have root canal work done with a Bosch hammer drill and b)
it might not gain much speed. Certain parts can be improved, and if these don't
make it worthwhile, I'll abandon the project.
If I do anything more with emulation on the '3, I'd look at porting an
Apple //e emulator, since I've got a real //e, they're rather nice
machines, and I've got plenty of technical info to work from. Anyone
else interested? (I just want to play Fooblitzky again...)
9 Hasn't the Gameboy got a Z80?
I believe so. I haven't looked for any technical information on it. It may
be possible to adapt PsiXZX into PsiGameboy, yes. Mail me what tech.info.
you have....
Matt J. Gumbley - matt <at> gumbley <dot> demon <dot> co<dot> uk
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