Ok, so I understand the SID Master 99 is mapped into the 0x4000 - 0x5FFF space when no other DSR roms are mapped in.
Does someone know by what mechanism that is done?
Is it that it decodes cru bit 0 of all card spaces ( effectively every 128'th cru-bit ) and tracks if anything else is on? Assuming only one will be on at a time?
Then is the remaining address space shareable if additional device memory addresses are fully decoded?
Also, I've seen that cards could use crubase's below 0x1000 as long as they don't need to be found by a DSRLNK routine. Were any such devices ever produced? If there is no ROM it is seems to me it requires community convention to allocate crubases do not collide, or memory addresses within a shared crubase that do not collide.
I've got a YM2413 (OPLL) FM synthesizer chip I'd like to hook up to the 4A... I'm just trying to decide how I want to make it addressable. The minimum requirement is 2 memory mapped write-only bytes.