Hardware Reference
In-Depth Information
Central processing unit (CPU)
Control
unit
Arithmetic
logical unit
(ALU)
I/O devices
Registers
Main
memory
Disk
Printer
Bus
Figure 2-1.
The organization of a simple computer with one CPU and two I/O
devices.
The CPU also contains a small, high-speed memory used to store temporary
results and certain control information. This memory is made up of a number of
registers, each having has a certain size and function. Usually, all the registers
have the same size. Each register can hold one number, up to some maximum de-
termined by its size. Registers can be read and written at high speed since they are
internal to the CPU.
The most important register is the
Program Counter
(
PC
), which points to
the next instruction to be fetched for execution. ( The name ''program counter'' is
somewhat misleading because it has nothing to do with
counting
anything, but the
term is universally used.) Also important is the
Instruction Register
(
IR
), which
holds the instruction currently being executed. Most computers have numerous
other registers as well, some of them general purpose as well as some for specific
purposes. Yet other registers are used by the operating system to control the com-
puter.
The internal organization of part of a simple von Neumann CPU is shown in
Fig. 2-2 in more detail. This part is called the
data path
and consists of the regis-
ters (typically 1 to 32), the
ALU
(
Arithmetic Logic Unit
), and several buses con-
necting the pieces. The registers feed into two ALU input registers, labeled
A
and
B
in the figure. These registers hold the ALU input while the ALU is performing
