Emulators for Qdos
Qdos is popular with programmers and has more than its fair share of
emulators. Many are out-classed by Amiga-specific programs, but some
deserve attention as they mimic machines which cannot otherwise be emulated
on an Amiga.
Xtricator
Xtricator is a Sinclair ZX81 emulator from the Netherlands. It runs well on
Amiga Qdos. You can adjust the priority of the screen and Z80 interpreter
to get it working at a sensible speed on most Amiga systems from A1200
upwards. On a 68060 system it ran almost 26 times faster than the real
thing, albeit at considerably more expense...
It runs just right on A4000/030s if you reduce the main Xtricator task
priority to half that of the XTR_IO screen-update task, and gobble up some
CPU time by boosting the Qdos screen:
BLIT_OFF : ACE_ON : ACE_PRIORITY 8,1
Sinclair's ZX81 was the first home computer mass-marketed at a price below
£100, and sold millions worldwide. It was very slow, as the processor was
used to generate the display, and had just 1K of memory, but it's fondly
regarded by many for whom it was the first step into home computing.
Xtricator is the only ZX81 emulator that runs on the Amiga, and it requires
Amiga Qdos - but that's no problem as both work well and are freely
distributable.
Xtricator has enormous advantages over the real thing, like fast disk
loading and saving, multi-tasking and support for up to 48K RAM. CBI, an
extended BASIC toolkit, is built in, and it even runs 'hires' games which
re-programmed the ZX81 display on the fly to get around the limitations of
simple character graphics. A substantial library of ZX81 programs is
available in Xtricator format from the author Carlo Delhez, on registration
of the shareware version.
Spectator
Spectator is a Sinclair Spectrum emulator from the same stable. It's slower
than Amiga ZX emulators, but more compatible as it emulates the later
Spectrum 128 and Interface 1 (serial port and microdrive expansion) as well
as the basic 48K Spectrum. It supports three file formats - the compressed
Z80 format, used by many Spectrum emulators, its own microdrive image format,
since adopted by Gerton Lunter's Z80 for PCs, and SPT tape format. Files
are readily available on CD and the net, particularly as Z80 snapshots.
The main weakness of Spectator is its sloth, particularly in 128K mode, and
lack of emulation of Spectrum 128 sound, which does not translate well to
Qdos BEEPs. If that's a priority you're better off with the Amiga-specific
ZXAM, which emulates Spectrum 128 sound well, but doesn't support more than
48K of RAM yet.
The shareware version of Spectator on the AF CD runs about 2.5 times the
speed of the real thing on a 68060 Amiga, but substantially slower on an
A4000/030. You can boost it by switching from the CPU-intensive eight
colour display to a four colour blitter version, with these commands:
ACE_OFF : BLIT_ON
On a 68060, in 48K mode, the problem is an excess of speed, rather than a dearth.
I dragged it down to 100 per cent Spectrum speed with these commands:
MODE 8 : ACE_ON : BLIT_OFF : ACE_PRIORITY 16,1
Bizarrely, Spectator 1.52 thinks it's running on a machine with a German
keymap, transposing the letters Z and Y. You can get around that,
perversely, by selecting a German map for Amiga Qdos with the command
KEYDT - which means everything gets swapped twice, back to the UK format.
Use KEYUK to swap back when you're in SuperBASIC.
ZM/HT
ZM/HT is currently the only emulator that TRANSLATES programs as it runs,
reading Z80 code and generating corresponding 68000 routines. This means
that it's potentially much the fastest Z80 emulator; Spectrum program
speeds increase as ZM/HT compiles them into native 68000 code, to the point
where a 16 MHz 68000 can out-run a real Spectrum! Given plenty of disk
space you can save compiled code along with the original, for a quick start
when you reload.
ZM/HT avoids the overhead of decoding and looking up each Z80 instruction
whenever it is encountered. At first it's slow because it must generate new
68000 routines as well as execute them, but later it can run pure 68000
code, only returning to the Z80 interpreter when it runs out of space for
68000 code or hits an instruction that it has not yet translated.
Put that way ZM/HT sounds simple, but it has to be fiendishly clever to
cope with self-modifying programs, which may poke their own code at any
time. It keeps a table with an entry for every byte of emulated memory,
marking routines for re-compilation when they are modified. It's complicated
but if it can be done in Qdos, an Amiga version is surely possible.
This is probably the only way we'll see fast PC emulation on current
Amigas. In practice it's a technique more suited to eight bit than 16 bit
emulation, because it demands several bytes of emulation memory for every
byte emulated. The compiled code is much longer than the Z80 original, as
68000 code is relatively verbose and optimised for speed, rather than size.
But the speed is so impressive, compared with a processor interpreter, that
compiling emulators are surely the way of the future.
The rest
There are many more emulators for Qdos, both commercial and freely
distributable. CPMulator, Solution, Success and Conqueror emulate business
bogey-systems CP/M and MSDOS. Other Spectrum emulators include William
James' Speculator 93, interesting as it's the parent of Amiga Speculator,
with full 68000 source code. ZM/1, ZM/2 and ZM/3 come from Ergon of Italy,
authors of ZM/HT; they are feature-packed but relatively slow Z80
interpreters, rather than compilers. |
QLs were manufactured by Thorn EMI in the UK and Samsung in Korea, for
export to Germany and the USA. The ROM contains a block-structured
extensible language, SuperBASIC, designed by Jan Jones, and Tony Tebby's
Qdos, a multi-tasking, device independent operating system modelled on
Unix. London's Psion contributed a business software suite which was
advanced for the time, though slow running from microdrive tapes.
