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Figure 5.16 Microinstructions for memory read and write
Figure 5.17 Microinstructions for fetching an instruction
in the ALU field, which means that out-bus1 will be connected to in-bus. The field
source 2 will be set to “10000”, which means none of the registers will be selected.
5.6. SUMMARY
The CPU is the part of a computer that interprets and carries out the instructions con-
tained in the programs we write. The CPU's main components are the register file,
ALU, and the control unit. The register file contains general-purpose and special reg-
isters. General-purpose registers may be used to hold operands and intermediate results.
The special registers may be used for memory access, sequencing, status information,
or to hold the fetched instruction during decoding and execution. Arithmetic and logi-
cal operations are performed in the ALU. Internal to the CPU, data may move from one
register to another or between registers and ALU. Data may also move between the
CPU and external components such as memory and I
O. The control unit is the com-
ponent that controls the state of the instruction cycle. As long as there are instructions to
execute, the next instruction is fetched from main memory. The instruction is executed
based on the operation specified in the op-code field of the instruction. The control unit
generates signals that control the flow of data within the CPU and between the CPU and
external units such as memory and I
/
O. The control unit can be implemented using
hardwired or microprogramming techniques.
/
EXERCISES
1. How many instruction bits are required to specify the following:
(a) Two operand registers and one result register in a machine that has 64
general-purpose registers?
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