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'[ot] Extended memory access on ISA bus'
Totally off topic, but I figure there are some experts here that
I have not found on pc newsgroups.
I'm working with Data Light's ROM-DOS, putting a BIOS extension
at 0 d000:0 that refers to data at 8 0000:0. My hardware seems to be
working with the stuff in conventional memory, but neither view-xm nor
rom-dos can find the stuff in extended memory.
In a PAL (16R8), I'm generating chip selects to drive the EPROM
and a 74hc245 bus transceiver based on latching LA23..LA20 and A19..A16
on the negative edge of ALE. For the chip enable to go true, AEN also
has to be low (not in a DMA cycle).
Anyone have more experience with the ISA bus? I've got a logic
analyzer coming in this morning to dig into it. One question that
remains to me, however, is how does the motherboard memory map decoding
know where to go to find memory? Stuff in extended memory COULD be in
SIMMS, or could be on the ISA bus. Does it go looking during POST and
map stuff accordingly?
Bill (WL) Boulton
Don't you have to set the processor in protected mode to access the
extended memory? Not available by direct addressing in real mode.
Here is a book that has some stuff that may be useful.
PC Architecture and Assembler Language
by Barry Kauler
In Australia it's from Karda Prints in Edgewater, WA and in the USA from
Annabooks in San Diego, CA.
At 02:14 PM 6/01/98 EST, you wrote:
Harold M Hallikainen
On Wed, 7 Jan 1998 06:30:06 -0500 "Bill (WL) Boulton"
<WINSHOP.COM.AU> writes: bill-794
>Don't you have to set the processor in protected mode to access the
>extended memory? Not available by direct addressing in real mode.
Yes you do, but you can use INT 15 to do it from real mode
programs. To test my hardware, I've been using a program (from the net)
called view-xm . It uses a "load all" instruction to access extended
memory in real mode.
The problem I was having had to do with which read strobe I was
using. I didn't realize there was a difference between the strobes on
the 8 and 16 bit portions of the bus. The strobe on the 8 bit portion is
only active when accessing conventional memory. It is documented, but I
missed it. A logic analyzer found it!
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