Topic       : Pure Assembler Documentation
Author      : John Kormylo
Version     : PASM.HYP 1.0
Subject     : Documentation/Pure Assembler
Nodes       : 740
Index Size  : 20262
HCP-Version : 3
Compiled on : Atari
@charset    : atarist
@lang       : en
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View Ref-File[ Interrupt Mask ]

Bits 8-10 of the Status Register are used to turn interrupts on or
off by priority.  The priority of the interrupt must be higher than
the value of the mask (0-7).  However, a level 7 interrupt can never
be turned off.

On the Atari, level 2 interrupts are generated at each horizontal
blank (to allow changing color pallets on the fly).  These are nomally
turned off (Mask = 3).

Level 4 interrupts are generated at each vertical blank (which occurs
50, 60 or 70 times a second, depending on the monitor).  These are
used for a variety of functions, including the Vblank queue of jump
vectors.

Level 5 interrupts are generated by the Z8530 SCC (TT only).

Level 6 interrupts are generated by the MK68901 MFP, which handles all
all other I/O interrupt requests.  The MFP supports 16 levels of
priorities internally.  To turn off I/O interrupts, use Mask = 7.

In addition, the (optional) VMEbus implements its own interrupts for
all levels.