General Register Organization

A set of flip-flops forms a register. A register is a unique high-speed storage area in the CPU. They include combinational circuits that implement data processing. The information is always defined in a register before processing. The registers speed up the implementation of programs.

Registers implement two important functions in the CPU operation are as follows −

• It can support a temporary storage location for data. This supports the directly implementing programs to have fast access to the data if required.
• It can save the status of the CPU and data about the directly implementing program.

Example − Address of the next program instruction, signals get from the external devices and error messages, and including different data is saved in the registers.

If a CPU includes some registers, therefore a common bus can link these registers. A general organization of seven CPU registers is displayed in the figure.

The CPU bus system is managed by the control unit. The control unit explicit the data flow through the ALU by choosing the function of the ALU and components of the system.

Consider R1 ← R2 + R3, the following are the functions implemented within the CPU −

MUX A Selector (SELA) − It can place R2 into bus A.

MUX B Selector (SELB) − It can place R3 into bus B.

ALU Operation Selector (OPR) − It can select the arithmetic addition (ADD).

Decoder Destination Selector (SELD) − It can transfers the result into R1.

The multiplexers of 3-state gates are performed with the buses. The state of 14 binary selection inputs determines the control word. The 14-bit control word defines a micro-operation.

The encoding of register selection fields is specified in the table.

Encoding of Register Selection Field

Binary Code	SELA	SELB	SELD
000	Input	Input	None
001	R1	R1	R1
010	R2	R2	R2
011	R3	R3	R3
100	R4	R4	R4
101	R5	R5	R5
110	R6	R6	R6
111	R7	R7	R7

There are several micro-operations are implemented by the ALU. Few of the operations implemented by the ALU are displayed in the table.

Encoding of ALU Operations

OPR Select	Operation	Symbol
00000	Transfer A	TSFA
00001	Increment A	INCA
00010	Add A + B	ADD
00101	Subtract A - B	SUB
00110	Decrement A	DECA
01000	ADD A and B	AND
01010	OR A and B	OR
01100	XOR A and B	XOR
01110	Complement A	COMA
10000	Shift right A	SHRA
11000	Shift left A	SHLA

There are some ALU micro-operations are shown in the table.

ALU Micro-Operations

Micro-operation	SELA	SELB	SELD	OPR	Control Word
R1 ← R2 – R3	R2	R3	R1	SUB	010	011	001	00101
R4 ← R4 ∨ R5	R4	R5	R4	OR	100	101	100	01010
R6 ← R6 + R1	-	R6	R1	INCA	110	000	110	00001
R7 ← R1	R1	-	R7	TSFA	001	000	111	00000
Output ← R2	R2	–	None	TSFA	010	000	000	00000
Output ← Input	Input	-	None	TSFA	000	000	000	00000
R4 ← shl R4	R4	-	R4	SHLA	100	000	100	11000
R5 ← 0	R5	R5	R5	XOR	101	101	101	01100