COMPUTER ORGANIZATION AND ARCHITECTURE DESIGNING FOR PERFORMANCE NINTH EDITION
The ALU is that part of the computer that actually performs arithmetic and logical
operations on data. All of the other elements of the computer system—control unit,
registers, memory, I/O—are there mainly to bring data into the ALU for it to pro-
cess and then to take the results back out. We have, in a sense, reached the core or
essence of a computer when we consider the ALU.
An ALU and, indeed, all electronic components in the computer are based on
the use of simple digital logic devices that can store binary digits and perform simple
Boolean logic operations.
Figure 10.1 indicates, in general terms, how the ALU is interconnected with
the rest of the processor. Operands for arithmetic and logic operations are pre-
sented to the ALU in registers, and the results of an operation are stored in regis-
ters. These registers are temporary storage locations within the processor that are
connected by signal paths to the ALU (e.g., see Figure 2.3). The ALU may also set
flags as the result of an operation. For example, an overflow flag is set to 1 if the
result of a computation exceeds the length of the register into which it is to be stored.
The flag values are also stored in registers within the processor. The processor
provides signals that control the operation of the ALU and the movement of the
data into and out of the ALU.