Register and Register Transfer Language

Bus and Memory Transfer

Arithmetic, Logic and Shift µ-operation

Arithmetic Logic Shift Unit

Register Transfer and Micro-Operation

Register

Group of flip-flops capable of storing one bit of information.

They may have combinational gates to perform certain data processing.

Are at the top of memory hierarchy and provide fastest way of data access.

Register Transfer Language

Digital System is interconnection of modules.

Modules are constructed using digital components and interconnected using common data and control paths.

They are best defined by the operation they perform on the stored data(µ-operation).

Example of µ-operations are shift, clear , load etc.

The internal hardware of the computer is best defined by specifying:

Set of register it contains and their functions

µ-operations that can be performed on the stored binary data

Control that initiates the sequence of µ-operation.

It is possible to specify the µ-operations in words but that will be lengthy.

Special symbolic notation is used to describe the µ-operation among the register called Register Transfer Language(RTL).

Register Transfer

Computer registers are denoted by capital letters(sometimes followed by numerals) to denote the function of the register.

Examples:

MAR: Memory Address Register

PC: Program Counter

R1: Processor Register

Information transfer from one register to another is designated as: R2 R1.

Normally we want the transfer to occur under a predetermined condition as

If(P=1) then (R2 R1. )

P is the control signal. Generally control function in included in the statement as follows:

P: R2 R1

Every statement in RTL need hardware for above RTL hardware will be as:

Bus and Memory Transfers

Digital computer have many registers and paths must be provided to transfer information.

No. of wires will be excessive if we connect each register with the other one.

A more efficient scheme for transferring information in multiple register system is by the use of the common bus system.

Control signals are used to select the register.

One way of constructing a common bus system is by the use of multiplexer.

Figure below show the 4 register common bus system.

It consist of Multiplexer and selection lines

In general, a bus system will multiplex k registers of n bits each to produce an n-line common bus.

Common bus for 8 registers of 16 bits requires, 16 multiplexer having 8 data input lines.

The transfer of information from bus to one of many register can be accomplished by connecting bus lines to the inputs of all register and activating the load signal of required one.

If we need to transfer the content of register C to register R1, it can be represented as:

It is convenient to show the direct transfer as:

Three State Bus Buffer

A bus system can also be constructed using three state gate.

Three state gate has 3 outputs one low and other high as like other gates.

The third output is the high impedance state which behave like the open circuit and does not have any logic significance.

The most commonly used gate in bus design is the is 3 state buffer gate.

Memory Transfer

Transfer of information from memory word to outside environment is read operation.

Transfer of new information to be stored into the memory is called write operation.

Memory word is symbolized by M and address by enclosing in the square bracket.

Read and write operation can be stated as:

Read: DR M[AR]

Write: M[AR] R1

Micro operations

µ-operation is an elementary operation performed with the data stored in register. They can be classified as:

Register transfer micro-operations

Arithmetic micro-operations

Logic micro-operations

Shift micro-operations

Arithmetic operation divide and multiply are not listed in table as they are not included in the basic set of micro-operations.

Multiplication is implemented by addition and shift operations.

Division is implemented by subtraction and shift operation.

Logic Micro-operations

These operation consider each bit of register separately and treat them as binary variables.

Example exclusive-OR between R1 and R2 can be symbolized as:

Special symbol are used for logical operations.

ˆ denote AND operation and ˇ denote OR operation.

When + symbol occur in micro-operation it denote arithmetic addition but in control signals it denote the OR.

Shift Micro-Operations

These are used for serial data transfer.

The information transferred through the serial input determines the type of shift.

There are three types of shift:

Logical

Circular

Arithmetic

Logical Shift

It is the one that transfer 0 through the serial input.

Symbol shl and shr are used to represent left logical shift and right logical shift respectively.

Circular Shift

It circulates the bits of the register around the two ends without loss of information.

It is represented as cil or cir for left and right circular shift.

Arithmetic Shift

It shift the signed binary number to left or right.

Arithmetic shift left multiply the signed binary number by 2.

Arithmetic shift right divide the signed binary number by 2.

It must leave the sign bit unchanged.

It is represented by ashl and ashr.

Arithmetic Logic Shift Unit

Computer systems employ number of storage register connected to common operational unit called ALU(Arithmetic Logic Unit) to perform micro-operation.

ALU is combinational circuit so transfer from source to ALU and ALU to destination is done only in one clock cycle.

Shift micro-operation is performed in separate unit or can be made part of overall ALU.