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| Authored by: Anonymous on Monday, April 15 2013 @ 08:13 PM EDT |
> I'm not sure how far it was IBM's design, and how much it was DOS casting
that fact into stone.
The 640Kb 'limit' was that of the IBM PC because of the memory mapped devices.
MS-DOS and CP/M-86 could use all the 1Mb memory address space on machines that
did not have the IBM design. eg S100 bus systems, Sharp MZ-5600 (I have one
here).
> I think the original PC was limited to 1Mb of address space, and DOS was
written to assume all that "interesting" stuff lay between 640K and
1M. So when PCs with a bigger address space came along, they couldn't move it
without
breaking DOS.
The 'original PC', ie 'Personal Computer' was the Apple II in 1978. The
'original _IBM_ PC', the Model A, could only support 256Kb memory due to
limitations in the planar (mother board). The Model B (I have one here,
identifiable by a blue inkstamped B in a circle on the back of the box) had a
revised planar with address lines for 1Mbyte and different slot spacing.
Most MS/PC-DOS versions were limited to the 1 Mbyte address range because they
ran in real (8086) mode even on 80286. There was a 80286 mode MS-DOS which was
versions 4.0 and 4.1 - not to be confused with the much later 4.00 and 4.01.
These were known as 'European DOS' and were restricted release by Siemens and
ICL (I worked for ICL at the time). I think Wang may have used it too.
MS-DOS 3.x programs ran in real mode in the first 1Mb space but it was possible
to write 'background' programs that ran in 80286 mode in the rest of memory, to
give some form of multi-tasking. It was killed by MS-DOS 5.0 (not to be confused
with the much later MS-DOS 5) which became OS/2.
> Because CP/M did everything via the low-memory jump table, using a tactic
like that would have made it easy to move those buffers etc out of the way.
MS-DOS programs did have a 'low-memory jump table', the PSP, which is in the
first 256bytes of the data segment. This has the same functionality as the first
256bytes of CP/M including the FCBs. It was (or maybe even is) possible to use
CP/M style calls to get to BDOS or BIOS.
The problem with the buffers at 640Kb - 1Mbyte was not with MS-DOS but was
entirely with user programs that bypassed the available MS-DOS and BIOS routine
but did direct screen writes to the CGA and MDA buffers. MS-DOS would happily
run on 1Mbytes machines, Lotus would not*.
In theory MS-DOS could run on 80286 machines in protected mode, but too many
programs used 'pointer calculations' that broke this.
* Actually there were 'fixes' to make Lotus run on non-IBM machines, this
involved patching the loaded program before it ran, it was very version
specific.
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| Authored by: JamesK on Monday, April 15 2013 @ 09:22 PM EDT |
{
I'm not sure how far it was IBM's design, and how much it was DOS casting that
fact into stone.
}
IBM put the hardware, video, etc. at a fixed location, which then got in the way
of using memory beyond 640K. One work around was an expansion memory board that
used memory mapping to move chunks of memory to within the usable address range.
The eventual standard was referred to as "LIM" (Lotus, Intel,
Microsoft IIRC). There were also tricks such as HIMEM, Extended memory and
Expanded memory, which could be used on the CPUs that came after the 8088.
---
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