; Note: this code assumes the adapter is set to XGA instance 6, video ROM @ seg. C000h

; -----------------------------------------------------------------------------
; make Nth ROM section visible to host
;------------------------------------------------------------------------------

                 mov     dx, 216Ah       ; DX - 21xAh: XGA Index Register I/O address
                 out     dx, al
                 mov     al, 71h         ; XGA reg. idx 71h - ROM paging
                 out     dx, al
                 inc     dl              ; DX - 21xBh: Data Register 0
                 xor     al, al          ; 0 - map ROM offset 0 to host
                 out     dx, al

; wait for the mapped ROM area to switch FROM offset 6000h (optional)

                 mov     ax, C000h
                 push    ax
                 pop     ds              ; DS = C000h
check_again:
                 mov     al, ds:0        ; C000:0h
                 cmp     al, 55h         ; 55h - value of first byte of mapped ROM (55AAh magic)
                 jz      short check_again
 
; -----------------------------------------------------------------------------
; make XGA-2 ROM section visible to host
;------------------------------------------------------------------------------

                 mov     dx, 216Ah       ; DX - 21xAh: XGA Index Register I/O address
                 out     dx, al
                 mov     al, 71h         ; XGA reg. idx 71h - ROM paging
                 out     dx, al
                 inc     dl              ; DX - 21xBh: Data Register 0
                 mov     al, 3fh         ; 3fh - map ROM offset 6000h to host
                 out     dx, al

; wait for the mapped ROM area to switch TO offset 6000h (optional)

                 mov     ax, C000h
                 push    ax
                 pop     ds              ; DS = C000h
check_again:
                 mov     al, ds:0        ; C000:0h
                 cmp     al, 55h         ; 55h - value of first byte of mapped ROM (55AAh magic)
                 jnz     short check_again