Fixes for MACHINE_XT build

[640-KB]

Fixes a few small issues for IBM PC/XT build:

1. Standard PPI equipment switch is 1 to read SW5-8 (opposite of FE2010A).
2. Some option ROMs can trash IMR (including IBM Xebec FD controller). Preserve and restore after ROM scan.
3. MDA can only be video mode 7. Some "badly behaved" programs may attempt to change to a video mode other than 7 with MDA installed (causing what appears to be a crash).

[640-KB]

saves 1 byte :)

[skiselev]

That's great stuff! Getting the XT version working was on my "to do" list already :)

I am wondering if you've actually tried running the code on a real XT machine?

[640-KB]

That's great stuff! Getting the XT version working was on my "to do" list already :)

Yes, saw your post on vcfed about it and thought I'd give it a try and see!

I am wondering if you've actually tried running the code on a real XT machine?

I did. I think there are still some things, potentially related to DRAM refresh. I'll keep plugging and get back to you.