Intel mcs-48 manual Operation Command Words OCWs, Operation Control Word 1 OCW1, Ocwa

8259A

OPERATION COMMAND WORDS (OCWs)

After the Initialization Command Words (ICWs) are pro- grammed Into the 8259A, the chip Is ready to accept Interrupt requests at Its Input lines. However, during the 8259A operation, a selection of algorithms can com- mand the 8259A to operate In various modes through the Operation Command Words (OCWs).

OPERATION CONTROL WORD 1 (OCW1)

OCW1 sets and clears the mask bits In the Interrupt Mask Register (IMR). M:'-Mo represent the eight mask bits. M = 1 Indicates the channel is masked (Inhibited), M = 0 indicates the channel is enabled.

OPERATION CONTROL WORD 2 (OCW2)

OPERATION CONTROL WORDS (OCWI)

OCW1

AOD7 De D5 D4 D3 D2 D1 DO

[i]I M7 Me M5 M4 M3 M2 M1 MO I

OCW2

0I R SEOI EOI 0 0 L2 Ll LOI

ocwa

0 I 0 SSMM SMM 0 P SRIS RIS I

R, SEOI, 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 Opera· tlon Command Word Format.

L2, L1, La - These bits determine the Interrupt level acted upon when the SEOI bit Is active.

OPERATION CONTROL 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'tcare".

SMM - Special Mask Mode. If ESMM = 1 and SMM =1 the 8259A will enter Special Mask Mode. If ESMM =1 and SM M = 0 the 8259A will revert to normal mask mode. When ESMM = 0, SMM has no effect.

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