This article was originally written by Simon N Goodwin for Amiga Format magazine, and is used by permission of the joint copyright holders. It should not be copied any further without written permission. The free emulators mentioned were included on the accompanying CD with the magazine, and are generally available - often in updated form - from this site. The text is as originally submitted to the magazine - which means there are some differences, mostly extra text which was cut to fit into the pages available - with the addition of hyperlinks to new or updated information which has since become available. This approach has been taken to preserve the structure of the original series, while making new information readily available. Simon welcomes comments from readers, care of Tomas Amsrud, who has generously prepared the material for publication on the Internet.

Bridgeboards

The first Amiga PC emulator was the Sidecar add-on for Amiga 1000, produced by Commodore and based on an 8088 processor. This introduced the concept of 'bridgeboards' - plug-in co-processors with shared RAM for communication with the main Amiga system.

Big-box Amigas have sixteen bit ISA (Industry Standard Architecture) PC sockets alongside their native Zorro expansions. A bridgeboard links the two so Amiga or PC cards can fill the remaining spaces.

Commodore bridgeboards used 8088, 286 and 386SX processors. The ultimate model is the Golden Gate 486, from Vortex in Germany. Don't confuse this with the US Golden Gate interface card, supported by PC Task, which has no processor and simply allows Amiga programs to read and write ISA cards.

The bridgeboard concept allows memory on PC or Amiga to be accessible either way, but in practice it's quicker to keep the two distinct, except on an old 16 bit Amiga 1500 or 2000. For speed, put PC memory on the bridgeboard. Golden Gate can use Zorro 2 RAM, but on most A3000 and A4000 systems this means precious chip RAM. That will be slow by the time it's dodged AGA graphics, disk and sound data, split into 16 bit words and been doled out over Zorro 2.

Printer and serial data can be diverted to Amiga ports or add-on cards. Golden Gate software also supports an ISA serial or parallel port. ISA graphics cards are cheap and powerful. Get one with at least 512K RAM - and custom software drivers to switch the 64K segments around - to make reasonable use of a 14 inch multisync monitor.

Bridgeboards are no longer made, but they're worth looking out for on the second hand market, expecially if your Amiga does not have a state-of-the art processor to make software PC emulation viable. Eight-bit bridgeboards are incompatible with Zorro 3 and pathetically slow, but later 286 ones may still outrun software emulators. These include the AT-Once for Amiga 500, as well as Commodore 286 bridgeboards. Sometimes the CPU chip on a bridgeboard can be swapped or boosted. Aminet has notes, for would-be upgraders with nerves of silicon.

Graphics modes

PCs can have hundreds of graphics modes. Like Amiga OCS, ECS and AGA modes, these come in sets with three letter acronyms, where each new set incorporates all the earlier ones. The most common are grouped by chipset and resolution in this order: CGA (colour, just!) followed by EGA (sub-ST), VGA (sub-AGA), XGA, SVGA and then off into custom graphics card territory. Each acronym adds more modes, colours and resolutions.

The emulators - like most real PCs - concentrate on commonly-used modes so programs that use odd or customised ones give weird results. Fashion favours 800 by 600 pixels in 256 colours, with 1024 by 768 pixels close behind. These are high resolutions by Amiga or TV standards.

The release PCX 1.1 supports 320 by 200 block graphics in 256 colours, and higher resolution 2, 4 and 16 colour modes, like the Vortex software. These are the most Amiga-like of the scores of PC modes. The low resolution 256 colour mode is fine for Doom but not much else, and it's the only one currently useable with graphics cards! PCX supports both CGA graphics palettes, while PC-Task only uses black, white, cyan and purple.

PC-Task 4 handles modes flexibly, but needs careful configuration and can be slow. It lets you assign an Amiga, Picasso96 or CyberGraphix mode, chosen from the familiar screen mode requester, to each of two dozen PC modes. Each screen can be updated synchronously or periodically, and optionally diverted to the Workbench.

Cross Emulation

PC emulators for Apple Macs and Unix boxes can run on Amigas extended with Shapeshifter or NetBSD. SoftWindows and SoftPC, from Insignia Solutions, are leading commercial PC emulators on Macs. SoftPC will not run on a 68060 but SoftWindows is compatible.

Like PC-Task 3 in Turbo mode, these emulators trade memory for speed. SoftWindows needs over 34 Mb to emulate an 8 Mb PC. It's still an interpreter, so it cannot match the speed of PC-Task 4 in dynamic mode, but crucial parts of the Windows user-interface are re-coded for the Mac, reducing the amount of clumsy Intel code that must be interpreted.

PC emulators for Unix are outclassed by the new Amiga emulators. Most are better suited to MS-DOS than Windows emulation. DOSEMU, XDOS and PCEM are among the best for MS-DOS, while BOCHS and WINE are promising Unix Windows emulators.

New software PC emulators for Amigas

Two new emulators arrived early this year, first as demos, then in early release form. I tested PCX 1.1 and the corresponding demo 2.1, and PC Task 4.00 and 4.10. Both claim to translate, rather than interpret, Intel code as they go along, for unprecedented performance. Both claim compatibility with Microsoft Windows. But is it reasonable to expect a useful PC for fity quid? Either way, what DO you get?

PCX 1.1

PCX, from Microcode Solutions (formerly Utilities Unlimited) requires a 68020 or faster, at least three megabytes each of fast memory and hard disk space, plus Workbench 2.04 or later. To get started you also need a PC operating system - generally this means MS-DOS.

PCX emulates the instruction set of an Intel Pentium processor, with some gaps in memory management. The emulator comes in two versions, with and without support for floating point instructions. You need an Amiga with a co-processor - either a 68881 or 68882 add-on, or a full 68040 or 68060 - to run the floating point version.

PCX comes in a CD case with one floppy disk inside. There's no printed documentation, apart from the CD inlay which lists requirements, features (some now, some later) and promotes forthcoming emulators.

The feature list is misleading, claiming that PCX emulates a Cirus 5428 graphics chip, capable of 'VGA/SVGA graphics' with 'support for all usable graphics boards'. Its only mode that works via a board, as opposed to native Amiga graphics, is PC MODE 13 - fine for low-resolution DOOM, but not a lot else.

PC-Task 4.1

PC-Task 4 also needs a 68020 or better, but can just be squeezed into 2 Mb of RAM. It emulates a 486SX processor, very slowly if you run it from chip RAM, so RAM expansion is highly desirable.

PC-Task 4 also comes on one 880K disk, but this time you get a 50 page A5 printed manual. There's much duplication - one cautionary paragraph appears seven times - but it's still a lot better than the sketchy PCX AmigaGuide. A lot of the wording is identical to that for PC-Task 3 - indeed the cover has a small piece of paper saying '4.0' stuck over the heading 'version 3' - but the text is accurate apart from one reference to Appendix D which should say Appendix E.

Much of the manual discusses accessing PC files from the Amiga. There's a brief but useful explanation of MS-DOS commands, a discussion of 'common problems' and useful extra information in a 'readme' file.

PC-Task 4 supports 23 graphics modes, including 16 and 256 colour modes in resolutions of 800 by 600, 1024 by 768 and 1280 by 1024 pixels; you'll need a graphics card to see those properly. It can also handle 256 colour modes with 640 pixels per line and 400 or 480 lines, which are compatible with AGA. Both emulators are configured from neat GadTools-based windows.

PC Drives

The emulators support up to two floppy and two hard drives. Floppy emulation is rather slow and it may take a couple of tries before a disk change is recognised. The 'hard drives' may be partitions or files on an Amiga drive. Hard files can be interchanged between PC Task and PCX, but not the faster partitions.

PCX automagically makes 'mount files', allowing the Amiga system full access to the PC files. This is trickier with PC-Task, but well documented. Either way, you must avoid reading from one system while writing on the other, or the PC and Amiga may get out of sync, with disasterous consequences.

Both emulators come with rudimentary CD ROM handlers to access PC CDs in Amiga drives from the emulator. You need Microsoft's freeware MSCDEX to decode the normal CD file systems. PCX includes an ASPI handler, for the PC equivalent of Amiga SCSI direct, and a text screen accelerator.

PCX claims support for eight bit digitised sound, emulating a Sound Blaster card. Sound Blaster synthesis is promised but not yet implemented. PC-Task only emulates the pathetic one-bit IBM BEEPer, but that's enough to alert you when a disk change is required.

Speed

The difficulty of PC emulation is that each Intel instruction does a very small amount of work. PCs have high MIPs (Millions of Instructions Per Second, or Meaningless Indicator of Performance) statistics because of their simplistic instructions. A 68000 can do in one step what might take three or more on an Intel processor.

This is little consolation when you must interpret Intel code. The Amiga must execute several Motorola instructions to fetch and decypher each Intel one, perhaps a single byte. It takes a lot of effort to interpret PC code. The answer is to generate Motorola code on the fly - PCX's CPU Transcription, Dynamic Execution in PC-Task - and try to re-use bits of ready-made code when the program loops. As yet, this boosts PC-Task much more than it does PCX.

Transcription reduces the overhead of repeatedly fetching and decoding Intel instructions, making the emulator substantially faster. The simpler and more repetitive the original, the greater the benefit, as decoding effort otherwise predominates. The buffer size used for temporarily generated code may make emulation faster or slower, depending on the program and hence the ratio between time spent book-keeping versus running transcribed code.

Code generation matters little when the emulated instructions are complicated like division, floating point or string operations. If programs are self-modifying - often the case for 3D games on the PC - code generation may become counterproductive, as code is repeatedly generated and thrown away as the ground shifts under it.

Ideally we'd translate programs en masse, and eliminate redundant steps only needed to get around quirks of the Intel architecture. This would be great, if it could be done, but so far it's defeated the best efforts of great programmers.

Such 'cross-compilers' exist for RISC workstation and VAX minicomputer code, but even they don't work all the time, requiring manual intervention. Intel code is very tricky to translate; despite the enormous commercial potential of an Intel translator, so far we're stuck with interpreters and code transcribers.

Comparisons

You can't say for certain which is fastest of PCX and PC-Task - it depends on the program you try. Carefully-chosen benchmarks can 'prove' that PC-Task is anything up to five times faster than PCX, or vice versa! Overall, both are slow by PC standards, although usable if you choose your applications with discretion. Modern games are least likely to work. PC -Task has the edge on integer processing, while PCX gives faster Amiga displays.

In general PC-Task 4 out-runs PCX in MS-DOS, although less than crude benchmarks would suggest. If your programs make heavy use of floating point arithmetic PCX has a big advantage; PC-Task must interpret software FP emulation, making it many times slower.

Despite fast processing, PC-Task 4 seems relatively slow and erratic, refreshing the display intermittently, swapping back and forth between processor and screen emulation. PC-Task has the most modes; PCX has fewer, but better. PCX has less rough edges, like the tendency of PC-Task to misplace the cursor.

Conclusions

PC emulation is an extremely difficult thing to get right, and both PCX and PC-Task 4 have a good stab at it. Compared with a real PC or a second-hand bridgeboard, the price is reasonable - but you get what you pay for.

You need a fast 68040 and preferably a 68060 to run Windows at a bearable speed. Lesser Amigas can run most MS-DOS programs at a useful if slow pace. PC-Task makes the best of slow Amigas. PCX excels if you're into floating point maths, but has some catching up to do in support for graphics cards.

Both these emulators are grand hacks in the guru tradition, hand crafted in 68020 assembly code. Each crashed or locked up at times, just like real PCs, but they're quite usable, in view of their complexity and the moving target.