Description
The system board fits horizontally in
the
base of the system unit
and
is
approximately 215.5 mm
(8-1/2
in.) x 304.8 mm (12 in.).
It
is
a multilayer, single-land-per-channel design with ground and
internal planes provided. Dc power and a signal from the power
supply enter the board through two six-pin connectors. Other
connectors
on
the board are for attaching the keyboard, audio
cassette and speaker. Five 62-pin card edge-sockets are also
mounted
on
the board. The
I/O
channel
is
bussed across these
five
I/O
slots.
Two dual-in-line package (DIP) switches (two eight-switch
packs) are mounted
on
the board and can
be
read under program
control. The DIP switches provide the system software with
information about the installed options, how much storage the
system
board
has, what type
of
display adapter
is
installed, what
operation modes are desired when power
is
switched
on
(color
or
black-and-white, 80
or
40-character lines), and the number
of
diskette drives attached.
The system board consists
of
five functional areas: the
microprocessor subsystem and its support elements, the read-only
memory (ROM) subsystem, the
read/write
(R/W)
memory
subsystem, integrated
I/O
adapters,
and
the
I/O
channel. The
read/write
memory is also referred to as random access memory
(RAM). All are described in this section.
Microprocessor
The heart of the system board
is
the Intel 8088 Microprocessor.
This is
an
8-bit external-bus version
of
Intel's 16-bit 8086
Microprocessor, and is software-compatible with the 8086. Thus,
~
the 8088 supports 16-bit operations, including multiply and
divide, and supports
20
bits of addressing
(1
megabyte
of
storage).
It
also operates in maximum mode, so a
comicroprocessor can
be
added as a feature. The microprocessor
operates at 4.77-MHz. This frequency, which is derived from a
System
Board
1-3
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