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[ VMEbus ]

... is an industry standard bus architecture for interfacing computer
cards.  It uses asynchronous non-multiplexed transfers with 16 data
lines and 24 address lines.  Some cards use only 16 address lines.
Double height cards (6U) support 32 address and data lines.

Cards are divided into slaves and masters.  Only masters can drive the
address or bus request lines, however slaves can request an interrupt
in order to initiate a data transfer (the status byte will be used to
specify an exception vector, or the autovector for the interrupt level
specified by lines IRQ1 - IRQ7 can be used).

VMEbus cards are accessed simply by reading or writing to the
corresponding memory locations.  The VMEbus address corresponds to the
lower 24 or 16 bits of the full address.

  $A00000 -   $DEFFFF (24 bit address cards) Mega STE
  $DF0000 -   $DFFFFF (16 bit address cards) Mega STE
$FE000000 - $FEFEFFFF (24 bit address cards) TT030
$FEFF0000 - $FEFFFFFF (16 bit address cards) TT030

The TT030 and Mega STE support one single height slave card slot, and
do not support block data transfers.  Exception Vectors 128 to 191 are
reserved for VMEbus card interrupts.  Interrupts can be masked using
the System Control Unit registers.

Examples for the Atari are Crazy Dots and the CyRel Sunrise M16-1280
graphics cards.

See also Atari Memory Map and Exceptions.