Hardware Reference
In-Depth Information
Photo 1.3
Arcom's Pegasus embedded PC controller (photo courtesy of
Arcom)
The elements within the microcomputer system shown in Figure 1.1 (CPU,
ROM, RAM, and I/O) are connected together by three distinct bus systems:
1
The
address bus
along which address information is passed.
2
The
data bus
along which data is passed.
3
The
control bus
along which control signals are passed.
Data representation
The information present on the bus lines is digital and is represented by the two
binary logic states: logic 1 (or
high
) and logic 0 (or
low
). All addresses and data
values must therefore be coded in binary format with the
most significant bit
(MSB) present on the uppermost address or data line and the
least significant bit
(LSB) on the lowermost address or data line (labelled A0 and D0, respectively).
The bus lines (whether they be address, data, or control) are common to all
four elements of the system. Data is passed via the data bus line in parallel
groups of either 8, 16, 32, or 64 bits. An 8-bit group of data is commonly
known as a
byte
whereas a 16-bit group is usually referred to as a
word
.
As an example, assume that the state of the eight data bus lines in a system
at a particular instant of time is as shown in Figure 1.2.
The binary value (MSB first, LSB last) is 10100111 and its decimal value
(found by adding together the decimal equivalents wherever a '1' is present in
the corresponding bit position) is 167.
It is often more convenient to express values in hexadecimal (base 16) format
(see Appendix D). The value of the byte (found by grouping the binary digits into
two 4-bit
nibbles
and then converting each to its corresponding hexadecimal
character, is A7 (variously shown as A7h, A7H, HA7, or A7
16
in order to
indicate that the base is 16).
The data bus invariably comprises 8, 16, or 32 separate lines labelled D0 to
D7 (or D0 to D16, etc.), whilst the address bus may have as few as 20 lines in
