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The Personal Computer (PC)
Interrupt Vector Table (IVT)

Web Presentation and Text Copyright©2006 by Daniel B. Sedory
NOT to be reproduced in any form without Permission of the Author !

What Is the IVT and Where is it Located?

The IVT (Interrupt Vector Table) generally contains far pointers to each of the PC's 256 Interrupts (each Segment:Offset pair; for a total of 4 bytes x 256 pointers = 1,024 bytes, or two 512-byte sectors).

This table is located at the very beginning of the PC's Memory! (That is: locations 0000:0000 through 0000:03FF) However, the PC commonly uses less than half that space (under 512 bytes) for actual vectors.


  Interrupt Vector Table,
  Interrupt Descriptor Table.


Interrupt  1E   Vector to Diskette Controller Parameters (located at: 0000:0078)

                DWORD Address points to data table that is used by BIOS.

        Default location is at 0F000:0EFC7 with an 11-byte Table format:

        00h     4-bit Step Rate, 4-bit Head Unload Time
        01h     7-bit Head Load Time, 1-bit DMA Flag
        02h     54.9254 ms counts - delay till motor off (37-38 typical)
        03h     Sector Size:
                              00h  =   128 bytes
                              01h  =   256 bytes
                              02h  =   512 bytes
                              03h  =  1024 bytes

        04h     Last Sector on track (8 or 9 typical)
        05h     Gap between sectors on read/write (42 typical)
        06h     Data Length for DMA transfers (0FFh typical)
        07h     Gap Length between sectors for FORMAT (80 typical)
        08h     Sector Fill Byte for FORMAT command (0F6h typical)
        09h     Head Settle Time (in milliseconds) (15 to 25 typical)

                DOS 1.0    0
                DOS 2.10  15
                DOS 3.1    1

        10h     Motor Start Time (in 1/8 second intervals) (2 to 4 typical)

                DOS 2.10   2

Notes: 1) This vector is pointed to the ROM BIOS diskette table on system

       2) IBM recommends that this table needs to be modified, that it should
          be copied into RAM and only the required changes be made.




Updates: July 31, 2006 (2006.07.31).
Last Update: December 8, 2021 (2021.12.08).

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