System Timers
Three programmable timer/counters are used by the system as
follows: Channel 0
is
a general-purpose timer providing a
constant time base for implementing a time-of-:day clock, Channel
1 times and requests refresh cycles from the Direct Memory
Access (DMA) channel, and Channel 2 supports the tone
generation for the speaker. Each channel has a minimum timing
resolution
of
1.05-,us.
System Interrupts
Of
the eight prioritized levels
of
interrupt, six are bussed to the
system expansion slots for use by feature cards. Two levels are
used
on
the system board.
Level 0, the highest priority,
is
attached to Channel 0 of the
timer/counter and provides a periodic interrupt for the
time-of-day clock.
Level 1
is
attached to the keyboard adapter circuits and receives
an interrupt for each scan code sent by the keyboard. The
non-maskable interrupt (NMI) of the 8088
is
used to report
memory parity errors.
System
Board
1-11
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