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

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

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