Peritek
Theory of Operation
4-7
4.5 VMEbus Interrupt Controller
The interrupt controller FPLA is a D08 RORA (Release On Register
Access) interrupter. It may be used with a D08 interrupt handler, which is
the most common interrupter type, and includes CPUs that use the
"VIC068" chip.
If the board is not requesting an interrupt and it receives an IAKI it will
drive IAKO. As IAKI is internally synchronized it may take up to two
34020 clocks (50 ns) to drive IAKO after receipt of IAKI. IAKO is
asynchronously reset (immediately negated) upon the negation of AS, as
required by the VME specification. When the 34020 sets its HINT
interrupt flag, and the DEVINTEN in the CSR is set, the board will drive
one of IRQ1 through IRQ7 lines, depending on the IRQ jumper option
selection. The VME interrupt handler will then drive (true) VIACK,
IACKO, and AS, and drive (true or false) A01-A03, LWORD, DS1, and
DS0, depending if it wants a D32, D16, or D08 Status ID. A01-A03 reflect
the interrupt priority the interrupt handler is acknowledging. When the
interrupt controller receives these signals it compares A01-A03 with the
vector priority select jumpers (VPSEL0-2). If there is not a match it will
drive IAKO as outlined above. If there is a match it will cause a read of the
8 bit Interrupt Vector Register (IVAR). The board will drive the contents
of the IVAR into bits 0-7 of the transceivers. Bits D08-D31 are not driven
by the board. They are pulled high by the VMEbus terminators. The
interrupt cycle then terminates as a normal read cycle. The interrupt
handler uses the vector number read from the board to point to an
exception routine address. As the interrupter is a RORA device, the
exception routine should negate (or toggle) the DEVINTEN bit in the
CSR. The exception routine then executes its function and ends with an
RTE (return from exception) instruction.
4.6 System Arbitration
One of the most important pieces of logic on the graphics board is the
system arbitrator. The function of the arbitrator is to allow the VMEbus to
access the non-34020 related functions on the graphics board and to
provide handshaking between the 34020 and the VMEbus for 34020
related functions.
The graphics board has four addressable registers (CSR group) on the
VMEbus side of the board and 8 devices on the 34020 side (34020,
writemask register, color map/cursor controller, 2 DUART serial I/O
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