Zenith Z-100 PC series Service Manual page 85

Page
3-3
Theory of Operation
Interrupt Controller -- The interrupt controller is used to prioritize the
system interrupts. This device accepts up to eight interrupt inputs from
other parts of the system, assigns each input a priority for processing,
and then directs the microprocessor to the next instruction in the
interrupt routine.
Gate Array -- The gate array on the CPU/memory card replaces many of
the buffers, latches, and other devices normally required in CPU
circuitry. Primarily, the gate array performs signal generation,
including all control signals and clock frequencies. This device also
receives all keyboard interrupts for the microprocessor and performs
DMA functions. The gate array is described in more detail in Chapter 4.
Parallel Port -- The CPU/memory card is equipped with a Centronics­
compatible, 25-pin, D-type connector to provide access to a parallel
printer or other parallel device. The parallel port can be disabled by
removing a jumper. Also, the port's address can be changed to a second
fixed address by moving a jumper.
DIP Switches -- The DIP switches on the CPU/memory card inform the
microprocesor of the system configuration. This allows the micro­
processor to utilize the installed hardware correctly and efficiently. For
more information on configuring the CPU/memory card, refer to
Chapter 2.
RAM -- The CPU/memory card combines CPU circuitry and system
RAM onto a single card. This is possible because of the gate array, which
combines several functions into a single device. The RAM is contained in
three banks of 64-kilobit and/or 256-kilobit devices and is expandable
from 128K to 640K.
ROM -- The 256-kilobit ROM located on the CPU/memory card contains
the instructions which control the fundamental operation of the
microprocessor. These instructions tell the microprocessor how to load
software, read the keyboard, and perform many other functions. The
instructions for self-checks at power-up, as well as for more complete
diagnostics, are also contained in the ROM.
Oscillators -- The microprocessor can operate at either of two clock
frequencies, which are selected by means of a pushbutton switch. These
frequencies are derived from two crystals, with frequencies of 14.31818
MHz and 24.0 MHz, respectively. The selected frequency is divided by 3
in the gate array to provide either 4.77 or 8.0 MHz 33% duty cycle output
for the microprocessor.