[sebhc] H89 roms

Lee Hart leeahart at earthlink.net
Wed Dec 14 19:38:22 CST 2005

Barry Watzman wrote:
> Agreed it can be done, but you not only need to do the flipping, you need
> to do the copying, because a normal CP/M call while the rom was "in" would
> crash the system.

Yes. The proceedure from within CP/M is to:

1. Disable interrupts
2. reset bit 5 of General Purpose Port A (F2 hex)
   Bank 0 memory is now enabled (0-8K)
3. Copy the data from the ROM to somewhere over 8K
4. Set bit 5 of General Purpose Port A
   Bank 0 memory is now disabled; you have RAM again from 0-8K
5. Enable interrupts

With Bank 0 active, you have to be careful not to use interrupts or any BDOS 
calls, because they need to access low RAM. But that's not too limiting. When 
I've done it, I just block moved the entire ROM to high RAM, and then did all 
the examining and saving of the data after returning to CP/M.
Lee A. Hart    814 8th Ave N    Sartell MN 56377    leeahart at earthlink.net
Delivered by the SEBHC Mailing List

More information about the Sebhc mailing list