INT 4A - Acorn BBC Master 512 - "OSWORD" - MISC FUNCTIONS USING CONTROL BLOCK
        AL = function code
            FAh transfer data between 80186 and 65C12 I/O processor
        DS:BX -> control block (see below)
Return: FLAGS destroyed
        control block updated
Note:   there are more functions than are listed here, but details are not
          available
SeeAlso: INT 4B"Acorn",INT 4C"Acorn"

Format of control block for function FAh:
Offset  Size    Description
 00h    BYTE    number of parameters sent to I/O processor (0Dh,0Eh)
 01h    BYTE    number of parameters read from I/O processor (01h)
 02h    DWORD   I/O processor address
 06h    DWORD   80186 segment:offset address
 0Ah    WORD    number of bytes to transfer
 0Ch    BYTE    operation type
                00h write to 65C12 at 24 us/byte
                01h read from 65C12 at 24 us/byte
                02h write to 65C12 at 26 us/word
                03h read from 65C12 at 26 us/word
                04h write to 65C12 at 10 us/byte using 256-byte blocks
                05h read from 65C12 at 10 us/byte using 256-byte blocks
 0Dh    BYTE    65C12 memory access control (only used if offset 00h = 0Eh)
                bit 7: unused
                bit 6: always use main screen memory if I/O addr 3000h-7FFFh
                        (overrides bit 5)
                bit 5: use shadow screen memory if screen address specified
                bit 4: use current ROM rather than ROM selected by bits 3-0
                        (only if I/O address between 8000h and BFFFh)
                bits 3-0: paged ROM number