If supported by the driver, this subfunction can be used to request hardware-dependent operations (such as setting baud rate for a serial port) that are not supported by other MS-DOS function calls.Transfers control data from an application to 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 output to the physical device.
Call with:
AH = 44H
AL = 03H
BX = handle
CX = number of bytes to write
DS:DX = segment:offset of data
Returns:
If function successful
Carry flag = clear
AX = bytes transferred
If function unsuccessful
Carry flag = set
AX = error code
Notes:
If supported by the driver, this subfunction can be used to request hardware-dependent operations (such as setting baud rate for a serial port) that are not supported by other MS-DOS function calls.
Character-device drivers are not required to support IOCTL Subfunction 03H. 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 03H 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:
Write a control string from the buffer buff to the standard list device driver. The length of the string is assumed to be in the variable ctllen.
stdprn equ 4 ; standard list handle
buflen equ 64 ; length of buffer
ctllen dw ? ; length of control data
buff db buflen dup (?) ; contains control data
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mov ax,4403h ; function & subfunction
mov bx,stdprn ; standard list handle
mov dx,seg buff ; buffer address
mov ds,dx
mov dx,offset buff
mov cx,ctllen ; length of control data
int 21h ; transfer to MS-DOS
jc error ; jump if write failed
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