CPC Guide - Expansion ROM SelectionSelecting an Expansion ROM
To select an expansion rom, simply send the number of the expansion rom to port 
&DFxx. (An expansion rom may have a number from 0-255). Where a value of 0 will 
select BASIC, and a value of 7 will select AMSDOS.

However this process only selects the expansion rom. It does not allow the user 
to access any routines in the rom. To allow routines to be accessed the user 
must enable the upper rom using the Gate Array. (See programming the gate array 
to see how this is done).

A programming example is given to show how to select and disable an expansion 
rom.

Note port &DFxx is a write only port, reading will return a nonsense value.




Programming Examples
a) Selecting an expansion rom
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




b) Disabling an expansion rom
ld bc,&7f00     ;Gate Array
ld a,%10001100  ;upper and lower rom disabled, mode 0
out (c),a       ;send it
ret