Selecting an Expansion ROM

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:

• Select the rom using the method above,
• Enable the upper rom region using the Gate Array (see the document on the Gate Array for more information)
• The rom data can be accessed in the Z80 memory range &C000-&FFFF.

Programming Examples

1. 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
   

2. 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