Computer Architecture And Organization – Register Transfer And Micro Operations: Computer Architecture And Organization

Register is a very fast computer memory, used to store data/instruction in-execution.

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 binary information of n-bits.

A register consists of a group of flip-flops and gates. The flip-flops hold the binary information and gates control when and how new information is transferred into a register. Various types of registers are available commercially. The simplest register is one that consists of only flip-flops with no external gates.

These days registers are also implemented as a register file.

Loading the Registers
The transfer of new information into a register is referred to as loading the register. If all the bits of register are loaded simultaneously with a common clock pulse than the loading is said to be done in parallel.

Register Transfer Language
The symbolic notation used to describe the micro-operation transfers amongst registers is called Register transfer language.

The term register transfer means the availability of hardware logic circuits that can perform a stated micro-operation and transfer the result of the operation to the same or another register.

The word language is borrowed from programmers who apply this term to programming languages. This programming language is a procedure for writing symbols to specify a given computational process.

Register Transfer –
Information transferred from one register to another is designated in symbolic form by means of replacement operator.

It denotes the transfer of the data from register R1 into R2.

Normally we want the transfer to occur only in predetermined control condition. This can be shown by following if-then statement: if (P=1) then (R2 ← R1)

Here P is a control signal generated in the control section.

Control Function
A control function is a Boolean variable that is equal to 1 or 0. The control function is shown as:

P: R2 ← R1

The control condition is terminated with a colon. It shows that transfer operation can be executed only if P=1.

Micro-Operations
The operations executed on data stored in registers are called micro-operations. A micro-operation is an elementary operation performed on the information stored in one or more registers.

Example: Shift, count, clear and load.

Arithmetic Micro-Operations
Some of the basic micro-operations are addition, subtraction, increment and decrement.

Add Micro-Operation
It is defined by the following statement:

R3 → R1 + R2
The above statement instructs the data or contents of register R1 to be added to data or content of register R2 and the sum should be transferred to register R3.

Subtract Micro-Operation
Let us again take an example:

R3 → R1 + R2′ + 1
In subtract micro-operation, instead of using minus operator we take 1’s compliment and add 1 to the register which gets subtracted, i.e R1 – R2 is equivalent to R3 → R1 + R2′ + 1

Increment/Decrement Micro-Operation
Increment and decrement micro-operations are generally performed by adding and subtracting 1 to and from the register respectively.

R1 → R1 + 1
R1 → R1 – 1

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