Zenith Z-100 PC series Service Manual page 77

Chapter 1
Introd uction
This service module describes the function of the CPU/memory card
used in the Z-158 model computer. Specifically, this service module
describes three cards: HE-181-5445 with 128K of RAM, HE-181-5857
with 256K of RAM, and HE-181-5601 with 320K of RAM. The only
difference between these cards is the number and type of RAM devices.
The RAM is contained in three banks of 64- and/or 256-kilobit devices,
and is expandable from 128K to 640K. For information on how to
configure the CPU/memory card for different RAM sizes, refer to
Chapter 2.
The CPU circuitry consists of an 8088-2 microprocessor, which makes all
of the operational decisions, controls all of the functions of the
computer, and communcates with all peripherals. The actions of this
microprocessor are governed by machine language instructions which
are contained in either the firmware (ROM), or software.
The 8088-2 can operate at clock frequencies of either 4.77 MHz or 8.0
MHz. The frequency is switch-selectable by means of a pushbutton
mounted above the keyboard connector, external to the computer. The
frequencies are derived by dividing the two crystal oscillator frequen­
cies (14.31818 MHz and 24.0 MHz) by 3. Also, the CPU/memory card was
designed so that an optional 8087 coprocessor may be added to speed
ari thmetic operations.
The 8088-2 microprocessor communicates with the rest of the system
through three separate busses. The address bus determines which device
is being talked to by the 8088-2. The data bus carries the actual message
to or from the addressed device. The control bus determines what type of
operation (such as read, write, or input/output) is to be performed and is
also used to control the timing of the operation. For more information on
the operation of the CPU/memory card, refer to Chapter 3.
Basically, the CPU/memory card combines the functions of CPU and
RAM circuitry onto a single card. This is possible because a gate array
device replaces several different components with a single IC. This gate
array performs all signal generation for the computer, including all
clock and control signals. It also performs all DMA functions, parity
generation, and interrupt acknowledge functions for the 8088-2. Refer
to Chapter 4 for more information on the gate array.