OSWORD &B0
==========
XY+0 : Input control block length
XY+1 : Output control block length
XY+2 : Command
XY+3 : Subcommand
XY+4 : Return byte - Set to &01 on entry. Supported calls will return &00.
       Unsupported calls will return &01 or leave left set to &01.
XY+5 : Extra return byte
XY+6... : Returned data

Command &00
-----------
Does nothing, returns XY+4=&01

Command &01 - GoMMC status
--------------------------
Subcommand &01 - return GoMMC ROM number in XY+6
Subcommand &02 - return GoMMC card size in XY+6..XY+9

Command &02
-----------
Returns data in XY+5

Subcommand &09
Subcommand &0A

Command &03 - Read/Write data
-----------------------------
XY+6..9   : Address
XY+10..13 : Disk byte address
XY+14..17 : Number of bytes
Returns data in XY+5

Subcommand &00 Read within current disk
Subcommand &01 Write within current disk
Subcommand &40 Read absolute sector
Subcommand &41 Write absolute sector

Command &04 - Read/Write within current disk
--------------------------------------------
XY+6..9   : Address
XY+10..13 : Disk byte address
XY+14..17 : Number of bytes
Returns data in XY+5

Subcommand &00,&01 - As for Command &03