A expansion rom is selected by writing a code to the rom select I/O address.
The rom select I/O address is not fully decoded, and at a minimum bit 13 of the I/O address must be set to "0" for roms to be selected. To avoid conflict with other devices, the remaining bits in the I/O address should be set to "1". The recommended I/O address is &DFxx.
The rom select I/O address is write only.
An expansion rom is identified with a code which is in the range 0-255. BASIC is defined with a code of 0 and AMSDOS with a code of 7. BASIC will be selected if an attempt is made to select a rom which is not connected.
This process will select a rom, but will not allow access to the rom data.
To access the rom data:
To select rom 7. (AMSDOS)
ld bc,&7f00 ;Gate Array ld a,%10000100 ;enable upper rom, disable lower rom, mode 0 out (c),a ;send it ld bc,&DF00 ;expansion rom select port ld a,7 ;expansion rom wanted out (c),a ;select it ret
ld bc,&7f00 ;Gate Array ld a,%10001100 ;upper and lower rom disabled, mode 0 out (c),a ;send it ret