Lexicon MPX-1 - Page 42

So what good is the automatic EOI mode with problems
like those just covered? Well, again, like the other EOls,
selection
is dependent upon the application. If inter·
rupts are controlled at a predetermined rate, so
as not to
cause the problems mentioned above, the automatic
EOI mode works perfect just the way it is. However, if in·
terl'unt!LhannAn !l:.naf~dt"'JloJhL4'--l!tP'\ inl'lAtArmiru.tA rAtA
157 Ise ISS IS. 153 152 IS1 ISO
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IEFORE
COMMAND
...... ",;11'1,; CVI l;ommana. Aller it'S reset tnough, the cor·
responding
IR level is assigned lowest priority. Other IR
priorities rotate to conform to the fUlly nested mode
based on the newly assigned low priority
Figures 13A and B show how the rotate on non·specific
EOI command effects the interrupt priorities. Let's
assume the
IR priorities were assigned with IRO the
highest and IA7 the lowest, as in 13A. IAe and
IR4 are
already in service but
ne~her is completed. Being the
higher
priority routine, IA4 is necessarily the routine
being executed. During the IA4 routine a rotate on non.
specific
EOI command is executed. When this happens,
bit 4 in the
ISR is reset. IR4 then becomes the lowest
priority and IA5 becomes the highest as in
13B.
43
levels will conform to the fully nested mode based on
the newly assigned low priority.
An example of how the set
priority command works is
shown
In Figures 14A and 14B. These figures show the
status
of the ISA and the relative priorities of the inter-
rupt levels before and after the set priority command.
Two interrupt routines are shown to be in service in
Figure 14A. Since IA2 is the highest priority, it is
necessarily the routine being executed. During the
IR2
routine. priorities are altered so that IRS is the highest.
This is done
simply by issuing the set priority command
to the 8259A. In this case, the command specifies IR4 as
being the lowest priority, The result of this set priority
command is shown in Figure 148. Ellen though tA7 now