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Programming the 8259 with no slaves
Now in this topic we assume that 8085 is the processor which is used in this microcomputer system. In this slave, no 8259 slaves are used. We should examine properly before 8259 PIC is used in the microcomputer system for performing the interrupt control application. 8259 is configured in such a fantastic way that is found that a variety of information are provided like for IR0 request IV, interrupts like level or edge-triggered, whether 8259s are used single or many, if ICW4 is in need or not and whether for the interrupt requests masking should be done or not. This information is only provided to 8259 if and only if the processor issues some of the following commands LIKE Initial Command Words and Operation Command Words which are stated below:
Operation command word1 (OCW1)
Operation command word2 (OCW2)
Operation command word3 (OCW3)
Initial command word1 (ICW1)
Initial command word2 (ICW2)
Initial command word3 (ICW3) and so on.
The point to be noted that only two initialization command words (ICW1 and ICW2) should be used since they are compulsory for performing the programming task. When slave 8259 is in the system ICW3 or the third initial command is provided. When the processor used is 8086 or some special modes of8259 is used ICW4 or the forth initial command is needed. Here in comparison, we can say that operation command words are not compulsory. Hence we may term operation command words as optional command words. But there is a rule for writing the command words that is they should be written only to the low port or to the high port. On the low port of 8259, the command words ICW1, ICW2, and ICW3 are written. And to the high port of 8259, the command words like ICW3, ICW2, OCW1, and OCW1 commands are written. In the system, the port address depends only on the chip used. Let us assume that the low port address is 50H selected when A0=0, similarly the address of the high port is 51H selected when A0=1.
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