Function 03H (3): I/O-Control Read

The IOCTL read function allows the device driver to pass information directly to the application program. This function is called only if bit 14 is set in the device attribute word. MS-DOS performs no error check on IOCTL I/O calls.

The IOCTL read function is called with

RH + 1 BYTE Unit code (block devices)

RH + 2 BYTE Command code = 3

RH + 13 BYTE Media descriptor byte

RH + 14 DWORD Transfer address

RH + 18 WORD Byte/sector count

RH + 20 WORD Starting sector number (block

devices)

It returns

RH + 3 WORD Status

RH + 18 WORD Actual bytes or sectors transferred