Microsoft KB Archive/26865

INF: Opening a Character Device in Raw Mode ID Number: Q26865

2.x 3.x 4.00 4.00a 4.01 4.01a 5.00 MS-DOS

Summary:

A character device driver cannot initialize itself in raw (binary) mode; the logic behind the MS-DOS device driver was not designed for this option.

The only way a character device driver can initialize is in cooked mode. Thus, the only way for a device driver to be used in raw mode is for the application that is using the device to set the device to raw mode after it has been initialized.

To do this, the application that wants to use the device in raw mode should use MS-DOS interrupt 21h, function 44h IOCTL interface, subfunction 1 (Set Device Parameters). Under this call, DX is the “device information”; bit 5 should be set to indicate that you want the device to operate in raw mode.

The following is an example of using this MS-DOS service:


 * Code fragment that sets the stdout file handle to raw mode mov dh, 0 ; Set upper byte of DX = 0 or dl, 20h ; Set raw mode bit in DL mov bx, 1 ; File handle (stdout) mov ax, 4401h ; IOCTL Set Device Parameters service int 21h ; MS-DOS interrupt 21h

The utilities you use to send data to your device must have the functionality to set the device to binary, or raw mode. For example, the MS-DOS COPY command’s /b switch does this. However, the MS-DOS TYPE command, when redirected to your device, does not do this. Some people have written small TSR programs that they distribute with their device drivers that intercept the file handle open service of interrupt 21h (service 3Dh); if the target is their device, it opens the file and sets the file to binary mode. The device would remain in raw mode while the application kept this file handle open.

Additional reference words: 2.x 3.x 4.00 4.00a 4.01 4.01a 5.00