From what I know; the effective change happens immediately upon the change from the hucard side. Well, whatever the propagation delay is, but it should be faster than any bus cycle of the cpu.
Only the lower 1megabyte range is changed; the upper range remains as is. The 64k in the upper range mapped by the original base CD system, is always there when attached. This allows Duos and SuperCDROM addon units access to the 64k when older system hucards are inserted (Bios 2.x and lower).
A word of caution about CD ram in bank $80/87 range: don't use it for page #1. For some reason, it causes issues (wrong values being read). Any of the memory from $68 to $7f is fine to map into page 1 (address $2000). I found this out when doing nes2pce stuffs. Otherwise, it works fine mapped anywhere else (including page #7 which houses the interrupt vectors).
IIRC, this is how the later version GE cards work on Duo/SuperCD units - bus access to rom changes on the hucard -> the card changes the state of the hucard detect pin, otherwise when accessing banks $68-7f it disables the hucard allows access to that ram. This is for the later GE cards. Mednafen author would probably know more (I never had any GE cards to test myself).
Also to note: there's open bus address ranges in the upper 1megabyte range. The Arcade card, both pro and duo, map in interface registers in the $1a00-1aff range (hardware bank #$FF). That's pretty convenient cause that bank is always mapped there. The reason they chose $1a00-ff range is interesting; maybe some hardware dev stuff had additional mapped things in around this address range as well.
The arcade card also mapped stuff in lower banks $40-43. These are mirrored corresponding I/O ports per 8k bank. I suspected that on the Duo card, they used the hucard detect pin, but now that I think about it - that's probably open bus range as well ($40-$67).
Topic: HuCard CARD_DETECT line on Duo/Duo-R/SuperCDROM? (Read 1133 times)
