🚀 go-pugleaf

c2a192@ugrad.cs.ubc.ca (Kazimir Kylheku) writes:
>In article <809039446.1190@ksmith.com>, Keith Smith <keith@ksmith.com> wrote:
>>Tom L. Davis <davis@adrs1.dseg.ti.com> wrote:
>>>IBM's 640K error.
>>Bill Gates 640K error.  All that 640K limitation in MS-DOS was due to
>>software not hardware.  Bill admitted he made a mistake when he said no
>>one would ever need 640K of memory in a PC.
>Are you sure about that? THe 640K limit stems from the fact that AT-bus
>devices have memory-mapped I/O in the upper 360K, as well as ROMs, etc.
>Wasn't it IBM who put the video memory of the Monochrome Display Adapter
>of the PC at location 0xB0000?  That's at the 720K barrier.  EGA and VGA
>frame buffers start at 0xA0000, the 640K barrier.
>I may have forgotten much of the PC history (good riddance), but I don't
>see how Gates could have influenced the placement of the hardware memory
>apertures.  Did he have some influence in the matter?  I'd definitely
>point the finger at IBM in this case.  Gates can't be blamed for
>_everything_, now!

You have the facts right but the conclusions wrong.
IBM specified the place to put those adapters (BTW after Intel specifying the
8080 to start at 1M, not 0), but MS-DOS (and thus MS) is to blame that any
memory between and beyond those adapters aren't accessible (the between-part
finally changed in DOS 5, *provided* you had a 386 or better).
Look at it this way: on the *same* PC, with the *same* adapters, OS/2, NT, SCO
Unix, Linux etc. all are able to address all available RAM. Only DOS can't.
Now ask yourself again: is it the hardware (specification) or the software?

>In any case, the 640K "barrier" isn't a problem anymore, since operating
>systems can simply map virtual memory pages around the I/O areas. Everyone
>in Linuxland sees a flat address space, with the kernel mapped somewhere
>in the the upper GB, protected from user processes by a segment limit.

And now you notice it yourself, contradicting yourself.
The ability to map *virtual* memory pages around the I/O areas implies that
the OS software can *physically* address all available RAM, whether or not it
is below the adapters (640K) or not. Only DOS misses this capability, and it
is just because it was designed that way. No runtime allocating of memory
structures and such. It's called 'assumptions'.
Most assumptions turn out to be wrong (I wonder if there's a lesson to be
learnt here about the assumed de-facto market dominance of a certain just-
released product????).
-- 
Illya Vaes  (ivaes@hr.ns.nl)
Holland Railconsult BV, Signalling CAD
Postbus 2855, 3500 GW Utrecht
Tel +31.30.358586, Fax 357212           "Do...or do not, there is no try" - Yoda