If supported by the driver, this subfunction can be used to obtain hardware-dependent status and availability information that is not supported by other MS-DOS function calls.Reads control data from a character-device driver. The length and contents of the data are specific to each device driver and do not follow any standard format. This function does not necessarily result in any input from the physical device.
Call with:
AH = 44H
AL = 02H
BX = handle
CX = number of bytes to read
DS:DX = segment:offset of buffer
Returns:
If function successful
Carry flag = clear
AX = bytes read
and buffer contains control data from driver
If function unsuccessful
Carry flag = set
AX = error code
Notes:
If supported by the driver, this subfunction can be used to obtain hardware-dependent status and availability information that is not supported by other MS-DOS function calls.
Character-device drivers are not required to support IOCTL Subfunction 02H. A program can test bit 14 of the device information word returned by IOCTL Subfunction 00H to determine whether the driver supports this subfunction. If Subfunction 02H is requested and the driver does not have the ability to process control data, control returns to the program with the carry flag set and error code 0001H (invalid function) in register AX.
Example:
Read a control string from the standard list driver into the buffer buff.
stdprn equ 4 ; standard list handle
buflen equ 64 ; length of buffer
ctllen dw ? ; length of control string
buff db buflen dup (0) ; receives control string
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mov ax,4402h ; function & subfunction
mov bx,stdprn ; standard list handle
mov cx,buflen ; buffer length
mov dx,seg buff ; buffer address
mov ds,dx
mov dx,offset buff
int 21h ; transfer to MS-DOS
jc error ; jump if read failed
mov ctllen,ax ; save control string length
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