such away that the operation can be resumed from the
point of the break at a later time with exactly the same
conditions prevailing. CPUs follow a specific sequence
of events when processing an interrupt. Interrupt
processing has priority over normal program execution.
CONTROL MEMORY Control memory
consists of addressable storage registers. It is used as a
Access to control memory data
requires less time than access to main memory. This
speeds up CPU operation by reducing the number of
memory references for data storage and retrieval.
CACHE MEMORY Cache memory is a small,
high-speed RAM buffer located between the CPU and
main memory and used to hold a copy of the instructions
or data currently being used by the CPU. It is used to
speed up the flow of instructions and data into the CPU
from main memory.
READ-ONLY MEMORY Every computer is
supplied with a set of software instructions to enable the
computer to perform its I/O operations. These
permanent instructions (routines) reside in a read-only
memory (ROM). ROM is often referred to as
firmware: software permanently contained in
hardware. The instructions are considered permanent
or nonvolatile, since they are not erased each time the
computer loses power or is turned off. The ROM is
tailored to system requirements and initiates the boot
procedurethe steps followed when you turn on
ARITHMETIC LOGIC UNIT The arithmetic
logic unit (ALU) implements arithmetic and/or logical
operations required by the instructions. The
instructions tell the CPU which type of mathematical or
logical calculation the ALU is to carry out. The
registers and operands provide the computer the sources
of the data needed to perform the calculations. Tuning
in the ALU is provided by the CPUs timing circuits.
ALU OPERATIONS ALUs can perform
arithmetic and logical operations. An ALU can be
designed to perform arithmetic operations in
fixed-point representation (integers) and floating-point
representation (fractional). The types of arithmetic
operations range from add and subtract operations to
sophisticated trigonometric operations. Some
computers have a separate numeric data coprocessor or
math pacs to perform arithmetic functions independent
of the ALU.
INTERNAL BUSES Buses transfer information
internally in computers.
A bus is a parallel data
communication path over which information is
transferred a byte or word at a time. The direction of
signal flow may be unidirectional or bidirectional.
BUS OPERATIONS The bus control function is
performed by a bus interface unit, or logic circuitry
similar to it. Control of a bus line and the proper
protocol of requesting a bus depend on the design of the
computer. Bus transfers are done on a priority basis.
Basically two factors must be taken into consideration
in bus communications: transfer priority and
source/destination of the data being transferred.
By studying this chapter, you should have learned
how the CPU works through its control section and its
arithmetic logic unit. You also should have learned how
buses are used to transfer instructions, data, and
information throughout a computer. These concepts are
important to understanding how to troubleshoot and
diagnose malfunctions and repair or replace CPU parts.