IBM 5150 PC Technical Reference (6025005, August, 1981) (PDF)

SYSTEM BOARD
The System Board fits horizontally in the base of the System Unit
and has dimensions of approximately 8-1/2 inches by 11 inches. The
System Board isa multilayer single land-per-channel design, with
ground and power 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 the speaker. Five 62-pin card edge sockets are also
mounted on the System Board. The system I/O channel is bussed
across these five I/O slots.
There are 16 (13 used) Dual In-line Package (DIP) switches mounted
on the card which can be read under program control. These switches
are used to indicate to the system software what options are installed.
They are used to indicate amounts of installed storage, both on the
System Board and in the System Expansion slots, type of display
adapter installed, and desired operation modes upon power-up; ie,
coloror black and white and 80- or 40 character lines. Switches are also
used to identify when the operating system is to be loaded from
diskette, and how many diskette drives are attached.
The major elements of the System Board are divided into five major
functional areas. They are, the processor subsystem and its support
elements, the Read-Only Memory (ROM) subsystem, the Read/Write
(RlW) Memory subsystem, integrated I/O adapters, and the I/O
channel. All functions are described in detail in this section, except for
the I/O channel, which has its own section. Figure 2.0 "System Board
Data Flow" page 2-6, illustrates these functional areas.
The heart of the System Board is the Intel 8088 microprocessor.
This processor is an 8-bit bus version of the 16-bit 8086 processor by
Intel. Itis software compatible with the 8086 and, thus, supports 16-bit
operations including multiply and divide. The processor supports 20
bits of addressing (1 megabyte of storage). The processor is
implemented in maximum mode so a co-processor can be added as a
feature. The processor is operated at 4.77 Mhz. This frequency is
derived from a 14.31818 Mhz crystal which is divided by three for. the
processor clock and by four to obtain the 3.58 Mhz color burst signal
required for color televisions.
At the 4.77 Mhz clock rate, the 8088 bus cycles are four clocks of
210 ns or 840 ns. I/O cycles take five 210 ns clocks or 1.05 microsec
(m sec).
2-3