PC Processors
The original IBM PC used an Intel 8088 processor with 16-bit registers,
handling eight-bit instructions. Follow-ups used the 8086, with the same
instruction set but faster 16-bit memory, and the production-engineered
80186. The (80)286 was used in old 'AT' PCs of the mid 1980s. They were
hardly 'Advanced Technology' even then, but the US computer illiterati
bought what they were told and Intel and Microsoft profitted accordingly.
Compared with the 68000, with its 24 bit addressing and 32 bit registers,
the 8086 is feeble. The '286 catches up on address range, but not registers.
Intel's first real 32 bit chip was the '386, introduced in 1985.
By then they had a lot of backward-compatibility to worry about.
The 8088 was derived from Intel's eight-bit1974-vintage 8080, and has many
inhereited flaws, deliberately included so that programs could be
automatically translated from 8080 to 8088 machine code. This was good
news in the early days, since slow software was better than none at all,
but led to restrictions which still dog PC users today.
A total of a megabyte of memory could be accessed. The PC design assumed a
maximum of 640K for RAM, leaving 384K to be shared among screens, firmware
and hardware. All this was divided into 64K hunks or 'segments' to match
the addressing limitations of the 8080.
For compatibility reasons, certain parts of any PC system must be in the
first 640K, causing bottlenecks. Other parts avoid 32 bit instructions,
losing speed and locking the current task.The 64K segment limit means that
even multi-megabyte PCs barf at 64K of code in Microsoft's QuickBASIC; when
a 64K table inside Windows is full, the system effectively runs out of
memory however many megs there are spare.
Intel's 286 was faster and had 24 bit addressing, for up to 16 Mb, with
simple memory management, but retained the restrictions of its
predecessors. The 16Mb boundary also limits cheaper Apple Macs and Amigas -
witness fuss about PCMCIA compatibility on the A1200, with its 24 bit
addressing, the 16 Mb Zorro 2 limit, and anguished calls for '32 bit clean'
code on the Mac.
The 386 was Intel's breakthrough, introduced as a a full 32-bit DX model,
followed by SX versions for 16-bit memory. It brought Intel's instruction set close to
68000 standards, with improved memory addressing. All the 8088 instructions
and modes were retained for compatibility.
Speeds doubled with the introduction of the 486, the first Intel processor
with an on-chip cache, shared between instructions and data. Motorola chips
from the 68030 onwards have twin caches which can be accessed simultaneously,
but this faster 'Harvard architecture' did not arrive on PCs till the current
Pentium models. Intel concentrated on boosting the speed of their chips,
eventually pushing the 486 to four times its original pace.
486DX versions have a built-in floating point co-processor. A 486SX lacks
floating point (like Commodore's 68LC040) while Cyrix's 486SLC is a hybrid
with a 16 bit bus like a 386SX, but 486 internal operations.
Intel have no monopoly on PC processors, and rival chips soon appeared when
US Courts ruled that numbers like '386' could not be Trade Marked. Now
AMD's 586 and the IBM/Cyrix 686 claim to deliver Pentium performance at
budget prices. Compatibility is not guaranteed.
|
Compatibility
Backwards compatibility is generally assumed by PC vendors, but not
guaranteed. The general attitude is "if you find a problem, buy your way
out of it", as my Executive Producer at Central ITV used to say, long ago.
There's no definitive 'PC Hardware Reference'. Just because it doesn't work
on your emulator, doesn't mean it WILL work on your friend's PC. Or vice
versa. PCs are called compatible to disguise the fact that they're not.
Much of the 'art' of the PC is learning cryptic switches, like /D:X,
without which Windows 3.11 gets stuck on PC-Task, and some real PCs. These
options and acronyms must be squirrelled away in obscurely named files in
just the right order to keep a PC going. There's no standard installer as
there is on the Amiga.
'AUTOEXEC.BAT' is the equivalent of the Amiga's 'startup-sequence'. PC file
names are up to eleven characters long, without spaces or small letters,
with a compulsory dot before the last three characters.
Arbitraryness rules. Messages are opaque, with several sets of numbered
errors; 'parse error 3', in the latest MS-DOS, seems to mean you've used a
forward slash where Microsoft perversely expect a backward one!
|
|
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.
|
