register organization

The number of registers in a processor unit may vary from just one processor register to as many as 64 registers or more.

1. One of the CPU registers is called as an accumulator AC or 'A' register. It is the main operand register of the ALU.
2. The data register (DR) acts as a buffer between the CPU and main memory. It is used as an input operand register with the accumulator.
3. The instruction register (IR) holds the opcode of the current instruction.
4. The address register (AR) holds the address of the memory in which the operand resides.

The program counter (PC) holds the address of the next instruction to be fetched for execution.

Additional addressable registers can be provided for storing operands and address. This can be viewed as replacing the single accumulator by a set of registers. If the registers are used for many purpose, the resulting computer is said to have general register organization. In the case of processor registers, a registers is selected by the multiplexers that form the buses.

When a large number of registers are included in the CPU, it is most efficient to connect them through a common bus system. The registers communicate with each other not only for direct data transfers, but also while performing various micro-operations. Hence it is necessary to provide a common unit that can perform all the arithmetic, logic and shift micro-operation in the processor.

A Bus organization for seven CPU registers:

---

The output of each register is connected to true multiplexer (mux) to form the two buses A & B. The selection lines in each multiplexer select one register or the input data for the particular bus. The A and B buses forms the input to a common ALU. The operation selected in the ALU determines the arithmetic or logic micro-operation that is to be performed. The result of the micro-operation is available for output and also goes into the inputs of the registers. The register that receives the information from the output bus is selected by a decoder. The decoder activates one of the register load inputs, thus providing a transfer both between the data in the output bus and the inputs of the selected destination register.

The control unit that operates the CPU bus system directs the information flow through the registers and ALU by selecting the various components in the systems.

R1 ® R2 + R3

(1) MUX A selection (SEC A): to place the content of R2 into bus A

(2) MUX B selection (sec B): to place the content of R3 into bus B

(3) ALU operation selection (OPR): to provide the arithmetic addition (A + B)

(4) Decoder destination selection (SEC D): to transfer the content of the output bus into R1

These form the control selection variables are generated in the control unit and must be available at the beginning of a clock cycle. The data from the two source registers propagate through the gates in the multiplexer and the ALU, to the output bus, and into the into of the destination registers, all during the clock cycle intervals.