At 10:46 AM 4/9/2007, Mark Hahn wrote:
I believe the issue was the location in memory of reserved space, stuck at
640K is the location of the video buffer. this is a hardware issue, not sw.
the cpu could address up to 1M (+64k, iirc.)
8086/8088 can address 1M (20 bits).. 16 bit segment register*16+ 16
bit offset..
There is a trick, though, because the processor put out which of the
4 memory spaces was being accessed on the high order address lines
during some of the later clocks (T2, etc.), so you could use that to
have different physical memory spaces (each 1 M long) addressed for
Alt Data, Code, Stack, and Data.
FFFF:10 is actually the same as 0:0 and, as I recall, wouldn't result
in a trap if you used addresses that "wrapped around" (the address
generator didn't feed the carry bit to the trap generator).
That is, you could use FF00:1000 as a legal address
But, after all, who'd ever use more than 64K of code, stack, or data
space for a single task... you were just going to port your 8080 code
over anyway, right?
When a more virtual memory scheme came in(286), and the segment
register became more of a pointer to a table, things did change a
bit, and it might have generated a trap if you wrapped around
physical memory. At least you had a larger virtual address space (2G)
that mapped into 16M of physical address space.
James Lux, P.E.
Spacecraft Radio Frequency Subsystems Group
Flight Communications Systems Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875
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