Q60230: How MS-DOS Communicates with Serial Ports

Article: Q60230
Product(s): Microsoft Disk Operating System
Version(s): MS-DOS:3.3,3.3a,4.0,4.01,5.0
Operating System(s): 
Keyword(s): 
Last Modified: 03-DEC-1999

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The information in this article applies to:

- Microsoft MS-DOS operating system versions 3.3, 3.3a, 4.0, 4.01, 5.0 
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SUMMARY
=======

The information listed below describes how MS-DOS communicates with the serial
ports.

MORE INFORMATION
================

The traditional MS-DOS method of reading from and writing to the serial ports is
done through Interrupt 21H, Function 3 for AUX input. Interrupt 21H, Function 4
is used for AUX output. In MS-DOS Versions 2.00 and later, the handle-based Read
File or Write File, or Device Functions (Interrupt 21H, Functions 3fH and 40H)
can be used to read from or write to the auxiliary device. MS-DOS support for
the serial communications port is inadequate in several respects for
high-performance serial I/O applications. See "Article 5: Character Device Input
and Output" on Pages 161-162 of the "Microsoft MS-DOS Encyclopedia" for a
complete discussion of these limitations.

Whereas MS-DOS supports only two serial ports via COM1 and COM2, the BIOS also
provides support for RS-232C asynchronous communications via Interrupt 14H for
up to four serial ports. However, this service is not "interrupt-driven" and
also is limited. Telecommunication application programs commonly take complete
control of the serial ports and supply their own interrupt handler and internal
buffering for character read and write operations. See "Article 6:
Interrupt-Driven Communications" on Pages 167-246 of the "Microsoft MS-DOS
Encyclopedia" for complete details and examples.

The industry COMx standard for ports, I/O addresses, and interrupt request lines
is as follows:

  I/O PORT    Address    IRQ
  --------    -------    ---

  COM1        3F8 HEX    4
  COM2        2F8 HEX    3
  COM3        3E8 HEX    4
  COM4        2E8 HEX    3

Additional query words: 3.30 3.30a 4.00 4.01 5.00 noupd

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Keywords          :  
Technology        : kbMSDOSSearch kbMSDOS400 kbMSDOS330a kbMSDOS500 kbMSDOS330 kbMSDOS401
Version           : MS-DOS:3.3,3.3a,4.0,4.01,5.0

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