[BBC-Micro] OT PC architecture

Rick Murray rick at rs432.net
Thu Feb 18 01:38:28 GMT 2010

On 17/02/2010 01:31, Phill Harvey-Smith wrote:

> This is not to do with XMS/EMS, but more to do with the segmented
> addressing having a segment size of 64K.

True, but its all part'n'parcel. There were numerous programs that 
needed more than 64K (but not that much more) that asked for a memory 
manager to be loaded. I guess messing with memory management in extended 
memory was easier than messing with memory models for the entire 

Both have to be easier than expanded memory - that 380-oddK chunk 
between the 640K that DOS 'sees' and the actual first 1Mb chunk; which 
was nicely carved up into 64K pieces and paged in. Like BBC ROMs (to 
make a really lame attempt at on-topicness ;^) ).

 > and the main reason for that limitation has nothing to do with MSDOS,

However, had the IBM not used the x86 processors...

I wonder how Unixen worked pre-386? I know a lot of modern Linux-likes 
won't run on earlier hardware, however the concept predates the 
introduction of the 386. Was any sort of Unix released for earlier x86s? 
I ask because it seemed in numerous walks of life, MS-DOS suffered from 
the 64K/640K limitations (ever had to start a game from a special boot 
disc as loading your regular device drivers only left half a megabyte 
free?); so did these Unixen suffer similar problems or did they work 
around it? [or not exist at all?]

> I dunno I seem to remember running Win95 on a 386 DX40 and it not being
> too bad by 1995 standards :)

Windows95 will RUN of a 386 (I got it started on my 386 co-pro) but it 
isn't happy. If we ignore the emulated device support (Win98 and the 
RiscPC 486 co-pro are not a good match, all sorts of weird stuff happens 
like patterns and lines instead of icons), the physical side is largely 
enhanced by having a better cache. Pipelining is difficult in the x86 
because the instruction set is icky (but in its defence it dates back to 
the '70s when the 6502/Z80 were all that), however the setup in the 486 
improved things.

In my room are five PCs (old one, mom's one, Aiko, Ayleigh, Azumi). [1]

In my room are four RISC OS machines (seven if you count the two Bush 
boxes and the A7000 board. Plus my Zen. And the printer. And the PVR [2]...
ARM wins. Nerrr!

Best wishes,


1 - plus 3 ST20s (satellite boxes), plus unknown processors in the 
routers, *six* 6502-class (2 Beebs, 2 Elks, E01S, SJ Bridge). Heck, I 
have more 6502s than x86!

2 - don't even try to count the number of processors in my room... to 
give an example, the PVR TMS320DM320 contains an ARM core. Fair enough. 
But the DSP is also a type of processor. As is the DCT unit. And the 
video unit, but to mention the other hardware assist. The video 
encoder/slicer contains a processor inside it. As does the audio chip. 
There's probably a rudimentary processor inside the telly. Three DVD 
players, several MP3 players... G*d almighty, it is starting to sound 
like a cheap imitation of a William Gibson novel.

...I think it is safe to say the record player does NOT have a processor 
in it. ;^)

Rick Murray, eeePC901 & ADSL WiFI'd into it, all ETLAs!
BBC B: DNFS, 2 x 5.25" floppies, EPROM prog, Acorn TTX
E01S FileStore, A3000/A5000/RiscPC/various PCs/blahblah...

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