The ST was Atari's standard home computer in the early 90s, around the same time as the Amiga. ST and Amiga owners were often in fierce competition arguing over which computer was the better (while 32bit Acorn users just sat back with a smile). Whilst the Amiga was often seen as a graphics and games machine, the ST was popular with musicians, because it had a built in MIDI port, but it still had it's fair share of games and professional packages, and was quite a neat little home computer. Although the ST wasn't particularly fast by todays standards, it uses the MC68000 CPU, which is notoriously slow to emulate on an Acorn (since it is 'wrong-endian' to the ARM) which means that all emulation has a built in handicap to speed.
Sam
Ellis has written STem from scratch in C++ and ARM code. The emulator is
very impressive, and can run many things quite comfortably, but unfortunately
very slowly, Pacmania for example works perfectly (if a little slowly).
Unsurprisingly the emulator needs a high-end Acorn machine. The emulator
also needs a TOS image to be able to run it, this is a copy of the original
system ROMs that were supplied with an Atari ST. Unfortunately, it is illegal
to possess a copy of this unless you own the original ST, however, there
are web pages where you can download TOS images from. You should check
which version of the TOS ROMs you need, as some do not work on STem yet,
the latest UK version that does work, is 2.06.
The latest versions of the emulator (2.00 and later) have fixed a problem that was present in earlier versions with the mouse emulation, which now makes the emulator much more usable. Basic support for serial and parallel port emulation is now present. The emulator now supports Magic Shadow Archive (MSA) format image files, as well as the ability to pause and resume emulation and take screenshots of the emulation.
At present, although the graphics emulation is very good but now although sound emulation included (since the author used a YM2149 sound chip emulation module taken with permission from Andreas Stroiczek's !CPC) the emulation is too slow for it to sound very good, though some game music is apparently recognisable.
STem v2.00 (8/12/98)
Speed Rating: ARM
2 
ARM 250
ARM 3
ARM 6 
ARM
7 StrongARM
STem page
Coming Soon...
Sam is making large improvements to STem over this Summer, and is hoping to release v2.0 sometime this Autumn. Large amounts of the code has been changed, and there are many improvements. In particular, the mouse control problem, which was caused by poor emulation of timer C has been fixed, Magic Shadow Archive (MSA) support has been added, possibly some serial port support, and best of all, sound!
There has also been for a while now, an ST sound emulator, by the bITmASTER, it emulates the YM2149 Soundchip from the ST and can play various sounds from the ST on your Acorn.
bITmASTERs
ST sound emulation page
STSound
(ZIP archive)
Also, there's a Java ST emulator in development.
Technical details
CPU - Motorola 68000, clocked at approx 8 MHz
Display - 640x400 in monochrome, 640x200 in 4 colours, 320x200
in 16 colours
Sound - Uses the Yamaha 2149 sound chip 3 tone generators
+ 1 noise generator, and MIDI IN and OUT ports.
RAM - 512Kb is most common (max 4Mb)
VRAM - Not seperate, always uses 32Kb of system RAM
ROM - 192Kb as standard 256Kb in more modern machines, or
as an upgrade
reference - Sam Ellis, author of STem
Software:
Little Green
Desktop
Spotty
Dog's Atari ST page
A
whole host of Atari links
Dave's
Arcade Classics
TOS ROM images:
Little
Green Desktop TOS images - loads of different TOS ROMS, you should
use the UK one, version 2.06!