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Selecting an Expansion ROM

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
  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


Article créé le : Samedi 05 Décembre 2009 à 10 h 53
Dernière mise à jour le : Mercredi 29 Février 2012 à 20 h 37

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