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FN2784.6
September 8, 2015
Operation Command Words (OCWs)
After the Initialization Command Words (lCWs) are
programmed into the 82C59A, the device is ready to accept
interrupt requests at its input lines. However, during the
82C59A operation, a selection of algorithms can command
the 82C59A to operate in various modes through the
Operation Command Words (OCWs).
Operation Command Word 1 (OCW1)
OCW1 sets and clears the mask bits in the Interrupt Mask
Register (lMR) M7 - M0 represent the eight mask bits. M = 1
indicates the channel is masked (inhibited), M = 0 indicates
the channel is enabled.
Operation Command Word 2 (OCW2)
R, SL, EOI - These three bits control the Rotate and End of
Interrupt modes and combinations of the two. A chart of
these combinations can be found on the Operation
Command Word Format.
L2, L1, L0 - These bits determine the interrupt level acted
upon when the SL bit is active.
Operation Command Word 3 (OCW3)
ESMM - Enable Special Mask Mode. When this bit is set to 1
it enables the SMM bit to set or reset the Special Mask
Mode. When ESMM = 0, the SMM bit becomes a “don’t
care”.
SMM - Special Mask Mode. If ESMM = 1 and SMM = 1, the
82C59A will enter Special Mask Mode. If ESMM = 1 and
SMM = 0, the 82C59A will revert to normal mask mode.
When ESMM = 0, SMM has no effect.
Fully Nested Mode
This mode is entered after initialization unless another mode
is programmed. The interrupt requests are ordered in priority
from 0 through 7 (0 highest). When an interrupt is
acknowledged the highest priority request is determined and
its vector placed on the bus. Additionally, a bit of the Interrupt
Service register (IS0 - 7) is set. This bit remains set until the
microprocessor issues an End of Interrupt (EOI) command
immediately before returning from the service routine, or if
the AEOI (Automatic End of Interrupt) bit is set, until the
trailing edge of the last INTA
. While the IS bit is set, all
further interrupts of the same or lower priority are inhibited,
while higher levels will generate an interrupt (which will be
acknowledged only if the microprocessor internal interrupt
enable flip-flop has been re-enabled through software).
After the initialization sequence, IR0 has the highest priority
and IR7 the lowest. Priorities can be changed, as will be
explained in the rotating priority mode or via the set priority
command.
OPERATION COMMAND WORDS (OCWs)
A0D7D6 D5D4D3D2D1D0
OCW1
1 M7M6 M5M4M3M2M1M0
OCW2
0 R SL EOI 0 0 L2 L1 L0
OCW3
0 0 ESMM SMM 0 1 P RR RIS
82C59A82C59A
