Theory of Operation
INTERRUPTS
The ZT 8809A supports both maskable and non-maskable interrupts.
This section discusses system level issues related to these interrupts.
Refer to Chapter 12 for more information on the operation and
programming of the maskable interrupt controller.
Interrupt Request Assignments
The 8259A Programmable Interrupt Controller (PIC) on board the
ZT 8809A has eight interrupt input requests,
each with two
possibilities for an interrupt source.
Figure 3-1 shows these
possibilities, which are assigned via jumper selections W2 through
W11. This figure indicates the factory default assignments with a
dagger symbol (†).
Three of these interrupt assignments are critical for STD DOS: the
counter/timer 0 for the Systick Timer and the two serial port interrupt
requests at levels 3 and 4. In addition, if the ZT 8844 EGA board is
placed in the system, then the keyboard interrupt at level 1 is also
critical.
Do not reassign these interrupt selections.
Notice that
although level 7 may receive the line printer interrupt request, STD
DOS does not communicate with the line printer via interrupts.
Therefore, level 7 may be reassigned unless an application program
requires it.
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