Topic       : The ATARI Compendium
Author      : Scott Sanders / JAY Software
Version     : 1.25 (20/6/2003)
Subject     : Documentation
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Compiled on : Atari
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                              GEMDOS  Vectors


GEMDOS reserves eight system interrupt vectors (of which only three are
used) for various system housekeeping. The BIOS function Setexc() should
be used to redirect these vectors when necessary. The GEMDOS vectors are
as follows:

Name             Setexc() Usage
                 Vector
                 Number

VEC_TIMER        0x0100   Timer Tick Vector: This vector is jumped through
                          50 times per second to maintain the time-of-day
                          clock and accomplish other system housekeeping.
                          A process intercepting this vector does not have
                          to preserve any registers but should jump
                          through the old vector when completed. Heavy use
                          of this vector can severly affect system
                          performance. Return from this handler with RTS.

VEC_CRITICALERR  0x0101   Critical Error Handler: This vector is used by
                          the BIOS to service critical alerts (an Rwabs()
                          disk error or media change request). When
                          called, the WORD at 4(sp) is a GEMDOS error
                          number. On return, D0.L should contain 0x0001000
                          to retry the operation, 0 to ignore the error,
                          or 0xFFFFFFxx to return an error code (xx).
                          D3-D7 and A3-A6 must be preserved by the
                          handler. Return from this handler with RTS.

VEC_PROCTERM     0x0102   Process Terminate Vector: This vector is called
                          just prior to the termination of a process ended
                          with ctrl-c. Return from this handler with RTS.

-                0y103-   Currently unused.
                 0x107