Referring back to Fig.8-17, bits L2-L0 of OCW2 are for
designating an IR level, bit 5 is for giving EOI commands
and bits 6 and 7 are for controlling the IR levels.
Recall that when the AEOI bit in ICW4 is 1, the ISR bit,
which is set by the interrupt request, is reset automatically
at the end of the second /INTA pulse, but if AEOI=0, then
the ISR bit must be explicitly cleared by an EOI command,
which consists of sending an OCW2 with bit 5 equal to 1.
When an EOI command is given the meaning of the four
possible combinations of bit 7, the R (rotate) bit, and bit
6, the SL (set level) bit, are:
R SL
0 0 Nonspecific, normal priority mode
0 1 Specifically clears the ISR bit indicated by L2-L0
1 0 Rotate priority so that a device sfter being serviced
has the lowest priority
1 1 Rotate priority until position specified by L2-L0
is lowest
Under the normal priority mode, if ISRn is set, the
priority resolver will not recognize any requests on
IR7 through IR(n+1), but will recognize unmasked
requests on IR(n-1) through IR0.
As an example of the normal priority mode, suppose that
initially AEOI=0 and all ISR and IMR bits are clear. Also
suppose that, as shown in Fig.8-18, requests occur
simultaneously on IR2 and IR4, then a request arrives at
IR1 and last a request arrives at IR3 and that these are
the only requests.
Any ISR bit can be explicitly cleared by sending an
OCW2 with the R, SL and EOI bits set to 011 and putting
the number of the bit to bi\e cleared in L2-L0. if
01100011 is sent to OCW2, ther ISR3 will be cleared.
In addition to the normal priority mode discussed above,
OCW2 can rotate the priority by assigning bottom priority
to any one of the IR levels. In this case the other
priorities will follow as if the normal ordering had been
rotated. For instance, if the lowest priority is given
to IR4, then the order of priorities will be:
IR5, IR6, IR7, IR0, IR1, IR2, IR3, IR4 (i.e. IR5 is
rotated into the top-priority position). A rotation by one
can be obtained by letting the combination for the R and SL
bits be 10. If the R and SL bit combination is 11, then
the IR level with the lowest priority is the one
specified by L2-L0. If IR5 currently has top priority
and 10100000 is sent to OCW2, then the new priority
ordering would be IR6, IR7, IR0, IR1, IR2, IR3, IR4, IR5.