Register Organization in Computer Architecture

Register Organization in Computer Architecture – A register is a group of flip-flops with each flip-flop capable of storing one bit of information. An n-bit register has a group of n flip-flops and is capable of storing any binary information of n bits. Various types of registers are available commercially.

The simplest one consists of only flip-flops with no external gates. These different types of registers are used with computers and affect the nature of computers. Some computers have special-purpose register sets, while others have general-purpose registers.

Register Organization in Computer Architecture

On the basis of the nature of registers, these can be organize/classified into two categories:

1. General-purpose registers
2. Special-purpose registers

General Purpose Registers

General-purpose registers are mainly used to minimise the memory references by the programmer. The general-purpose register is a small amount of storage available in the processor itself, whose contents can be accessed more quickly than storage available elsewhere.

Typically, this storage is not considered a part of the normal memory storage. These registers are also available to programmers to use in their applications. A processor contains several types of general-purpose registers that can be classified according to their content or instructions that operate on them.

User-accessible Register

The most common registers are user-accessible registers. As the name suggests, these are available to the user for programming.

Data Register

These registers are used to hold numeric values such as integer and floating point values. These registers can not be used to store the address of an operand.

Address Registers

These registers hold addresses that immediately access memory. Some processors contain registers that may only be used to hold an address or only to hold a numeric value (index value). These registers are denoted as address registers. A wide variety of addressing modes are specified by these registers. Examples of address registers are the segment pointer, index register, stack pointer, etc.

Special Purpose Registers

Special-purpose registers are control and status registers which are used to control the operation of the processor. These registers are not visible to the user and hence are not considered part of the register set of the machine. These can further be classified as follows:

Control Registers

Control registers are used to control the processor operations. For example program counter (PC) is responsible for instruction sequencing. The program counter holds the address of the next instruction to be executed. Another example of a control register is the Instruction Register (IR). This register holds the instruction which is currently being executed.

Status Registers

Most computers have a set of bits called processor status bits, often known as the status register or Program Status Word (PSW). The status register has its own special purposes. It holds a result indicator for arithmetic and logical operations in different bits. These bits are called flags. Common flag bits are sign, zero, carry and parity flags. Although each status bit has a different and independent function, the instruction set provides instructions for saving and restoring the entire set of processor status bits.