Int 21H [1.0] Function 04H Auxiliary output

[1] Outputs a character to the first serial port.

[2.0+] Outputs a character to the standard auxiliary device. The default is the first serial port (COM1).

Call with

AH = 04H

DL = 8-bit data for output

Returns:

Nothing

Notes:

If the output device is busy, this function waits until the device is ready to accept a character.

There is no way to poll the status of the auxiliary device using this function. On the IBM PC, more precise control can be obtained by calling ROM BIOS Int 14H or by driving the communications controller directly.

If a Ctrl-C is detected at the keyboard, an Int 23H is executed.

[2.0+] You can also send strings to the auxiliary device by performing a write (Int 21H Function 40H) using the predefined handle for the standard auxiliary device (0003H) or using a handle obtained by opening the logical device AUX.

Example:

Output a "*'' character to the auxiliary device.

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mov ah,4 ; function number

mov dl,'*' ; character to output

int 21h ; transfer to MS-DOS

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