Reply 20 of 30, by idspispopd
wrote:wrote:I can't see how you either disabled all RAM above 640k or only installed 640k in the 586 so there was no conflict with existing memory.
That's a good point. All I did was to not enable shadow memory.. The remaining memory was treated as extended memory which begins beyond 1MiB.
Memory between 640k and 1MB is usually not enabled by default, except for ROM shadowing. This is especially true for A000-BFFF which is reserved for video cards.
Since that question came up, just in case this is not common knowledge: A000-AFFF: EGA/VGA graphics modes. B000-B7FF: MDA/Hercules half mode (MDA only has 4kB so in that case only B000-B0FF). B800-BFFF: CGA (graphics and text modes - a true CGA card has 16 kB so actually B800-BBFF), EGA/VGA text modes, second Hercules page in full mode.
BTW, i doubt that a character ROM will make any difference. The character is not accessible by the CPU (not mapped on the ISA bus), it is only used by the video controller.
Perhaps you could try accessing the Hercules memory yourself? (Write, read again and compare.) debug.com should to the trick, but the CLI interface takes a moment to get used to.
I remember I had a memory monitor as a tiny TSR program (typed-in from a computer magazine). For some reason it wasn't compatible with the XT we used (machine hung when TSR got activated), but it worked on lates machines (286+). I can't find a trace on the web, though, and I don't know if I still have it somewhere.