Example - Layer 2 esxdos

M_GETSETDRV  equ $89
F_OPEN       equ $9a
F_CLOSE      equ $9b
F_READ       equ $9d
F_WRITE      equ $9e
F_SEEK       equ $9f
F_GET_DIR    equ $a8
F_SET_DIR    equ $a9

FA_READ      equ $01
FA_APPEND    equ $06
FA_OVERWRITE equ $0C

ESXDOS      macro   command
        db  $cf
        db  command
endm
        
        org     32768

start:
        ld      a, '*'          ; use current drive
        ld      b, FA_READ      ; set mode
        ld      ix, bmp         ; filename in ix
        
        ESXDOS  F_OPEN
        ret     c               ; return if failed

        ld      (handle), a     ; store handle

        ld      l, 0            ; seek from start of file
        ld      bc, 0
        ld      de, 1078        ; 54 byte header and 1024 palette
        ESXDOS  F_SEEK

        ld      a, 3            ; enable write bit and make visible

   writeLoop:
        push    af              ; store bitmask
        ld      bc, $123b       ; set port for writing
        out     (c), a

        ld      a, (handle)     ; restore handle

        ld      ix, $0          ; read 16k into addr 0
        ld      bc, $4000
        ESXDOS  F_READ
        
        pop     af              ; restore bit mask
        add     a, 64           ; next page
        jr      nc, writeLoop

        ESXDOS  F_CLOSE         ; close file

        ld      bc, $123b       ; reset write bit
        ld      a, 2            ; but keep visible
        out     (c), a

        ret                     ; have a guess?

handle: db      0

bmp:    db      "test.bmp"  

end     start