As it is known the two functional units of the CPU are the control unit (CU)
and the arithmetic-logical unit (ALU). The control unit manages and
coordinates the entire computer system. It obtains instructions from the
program stored in main memory, interprets the instructions, and issues
signals that cause other units of the system to execute them.
The control unit operates by reading one instruction at a time from memory
and taking the action called for by each instruction. In this way it
controls the flow between the main storage and the arithmetic-logical unit.
The control unit has the following components: a counter that selects the instructions, one at a time,
from memory; a register that temporarily holds the instructions read
from memory while it is being executed; a decoder that takes the coded instruction and breaks it down
into individual commands necessary to carry it out; a clock, which
produces marks at regular intervals. These timing marks are electronic and
very rapid.
The sequence of control unit operations is as follows. The next instruction
to be executed is read out from primary storage into the storage register.
The instruction is passed from the storage register to the instruction
register. Then the operation part of the instruction is decoded so that the
proper arithmetic or logical operation can be performed. The address of the
operand is sent from the instruction register to the address register. At
last the instruction counter register provides the address register with the
address of the next instruction to be executed.
The
arithmetic-logical unit (AL U) executes
the processing operations called for by the instructions brought from main
memory by the control unit. Binary arithmetic, the logical operations and
some special functions are performed by the arithmetical-logical unit.
Data enter the ALU and return to main storage through the storage register. Theaccumulator serving as a
register holds the results of processing operations. The results of
arithmetic operations are returned to the accumulator for transfer to main
storage through the storage register. The comparer performs logical comparisons of the contents of
the storage register and the accumulator. Typically, the comparer tests for
conditions such as «less than», «equal to», or «greater than».
So as you see the primary components of the arithmetic-logical unit are
banks of bistable devices, which are called registers. Their purpose is to
hold the numbers involved in the calculation and hold the results
temporarily until they can be tranferred to memory. At the core of the ALU
is a very high- speed binary adder, which is used to carry out at least the
four basic arithmetic functions (addition, subtraction, multiplication and
division). The logical unit consists of electronic circuitry which compares
information and makes decisions based upon the results of the comparison. |