Thread: Porting to protected mode

  1. #1

    Unhappy Porting to protected mode


    I am working on porting work. I have program(DOS) which does I/O with 8251. We are NOT using any BIOS calls. 8251 is working in interrupt mode. I have to port this program to protected mode program, running in privilge level. I would be wondering if anybody could provide me some info on getting knowledge base about
    1. 8251 pin out diagram and datasheet
    2. How 8251 should be programmed in protected mode?


  2. #2
    Super Moderator VirtualAce's Avatar
    Join Date
    Aug 2001
    I'm assuming you want some documentation on how to program in protected mode for the 8251 UART that was part of older pre-Pentium chipsets. You will probably want to check out Intel's website ( and browse through their data sheets. I'm not sure if any 8251 data sheets are there or not.

    If you cannot find any info at Intel, Dr. Dobbs frequently has information relating to hardware and data sheets. I forget the URL but look up Dr. Dobbs on your favorite search engine and you should be able to find it with no problem. Also, you can go to which is a site that has a wealth of information on a lot, a whole lot, of programming topics

    Given Intel's tendency to incorporate older chip designs into their newer ones any docs that you find on programming the 16550 UART will probably work to some extent on an 8251. To port this code I would recommend using the DJGPP protected mode-compiler. The docs that come with DJGPP will tell you which interrupts you can use, and which ones you should not use based on how they manipulate and deal with the registers. This compiler is available from

  3. #3
    Banned master5001's Avatar
    Join Date
    Aug 2001
    Visalia, CA, USA
    I hate to break it to you but just because you use interrupts doesn't mean that it will just port over without at least one headache. I don't see why one would "need" to make that sort of port.

  4. #4
    Registered User
    Join Date
    Aug 2001
    Memory problem in DOS. Application ran out of memory...

  5. #5
    Disagreeably Disagreeable
    Join Date
    Aug 2001
    Hehe...don't ya just love Bill Gates...

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