06-05-2022, 06:21 PM
(06-05-2022, 12:00 AM)mjelic Wrote: But does that mean if either of the extra Roms are enabled (bit 2 is set low), does that mean that video ram gets crunchecd by the extra rom, unless you also change Bit 4, to move the video ram to B000h?
Bit 2 enables ROM 2 if low, ROM 3 if High (ie, you can't have ROM 2 and ROM 3 in memory at the same time.
ROM 2 or 3, depending on which you select as above, appear in memory at C000h to FFFFh.
As far as I am aware, Video takes precedence, so if video is enabled (bit 3 LOW) for mapping to F000h to FFFFh (bit 4 low)
access to ROM 2 or 3 will only be in the range of C000h to EFFFh.
Likewise, if video is enabled (bit 3 LOW) for mapping to 8000h to 8FFFh (Bit 4 High) the boot rom will only be in the range 9000h to BFFFh
and ROM 2 / ROM 3 will be fully addressable at C000h to FFFFh.
By the way, if you disable the ROMs (bit 2 HIGH) DRAM bank 0, block 1 (the second 32k) appears from 8000h to either EFFFh or FFFFh depending on
whether you have video enabled for F000h to FFFFh (bit 4 LOW) or from 9000h to FFFFh if you have video enabled for 8000h to 8FFFh (bit 4 HIGH).
Of course, if you disable video access altogether, with ROMS disabled, you get full access to DRAM bank 0 block 1 at 8000h to FFFFh.