Registers

Flip-flop is a 1 bit memory cell which can be used for storing the digital data. To increase the storage capacity in terms of number of bits, we have to use a group of flip-flop. Such a group of flip-flop is known as a Register. The n-bit register will consist of n number of flip-flop and it is capable of storing an n-bit word.
A processor register (CPU register) is one of a small set of data holding places that are part of the computer processor.
A register may hold an instruction, a storage address, or any kind of data (such as a bit sequence or individual characters). Some instructions specify registers as part of the instruction. 
For example, an instruction may specify that the contents of two defined registers be added together and then placed in a specified register.
A register must be large enough to hold an instruction - for example, in a 64-bit computer, a register must be 64 bits in length. In some computer designs, there are smaller registers - for example, half-registers - for shorter instructions. Depending on the processor design and language rules, registers may be numbered or have arbitrary names.
A processor typically contains multiple index registers, also known as address registers or registers of modification. The effective address of any entity in a computer includes the base, index, and relative addresses, all of which are stored in the index register. A shift register is another type. Bits enter the shift register at one end and emerge from the other end. Flip flops, also known as bistable gates, store and process the data.
These Registers are used for performing the various Operations. While we are working on the System then these Registers are used by the CPU for Performing the Operations. When We Gives Some Input to the System then the Input will be Stored into the Registers and When the System will gives us the Results after Processing then the Result will also be from the Registers. So that they are used by the CPU for Processing the Data which is given by the User. Registers Perform:-
1)    Fetch: The Fetch Operation is used for taking the instructions those are given by the user and the Instructions those are stored into the Main Memory will be fetch by using Registers.
2)    Decode: The Decode Operation is used for interpreting the Instructions means the Instructions are decoded means the CPU will find out which Operation is to be performed on the Instructions. 
3)    Execute: The Execute Operation is performed by the CPU. And Results those are produced by the CPU are then Stored into the Memory and after that they are displayed on the user Screen. 

Registers are the most important components of CPU. Each register performs a specific function. A brief description of most important CPU's registers and their functions are given below:
1. Memory Address Register (MAR):
This register holds the address of memory where CPU wants to read or write data. When CPU wants to store some data in the memory or reads the data from the memory, it places the address of the required memory location in the MAR.
2. Memory Buffer Register (MBR):
This register holds the contents of data or instruction read from, or written in memory. The contents of instruction placed in this register are transferred to the Instruction Register, while the contents of data are transferred to the accumulator or I/O register.
In other words you can say that this register is used to store data/instruction coming from the memory or going to the memory.
3. I/O Address Register (I/O AR):
I/O Address register is used to specify the address of a particular I/O device.
4. I/O Buffer Register :
I/O Buffer Register is used for exchanging data between the I/O module and the processor.
5. Program Counter (PC)
Program Counter register is also known as Instruction Pointer Register. This register is used to store the address of the next instruction to be fetched for execution. When the instruction is fetched, the value of IP is incremented. Thus this register always points or holds the address of next instruction to be fetched.
6. Instruction Register (IR):
Once an instruction is fetched from main memory, it is stored in the Instruction Register. The control unit takes instruction from this register, decodes and executes it by sending signals to the appropriate component of computer to carry out the task.
7. Accumulator Register:
The accumulator register is located inside the ALU, It is used during arithmetic & logical operations of ALU. The control unit stores data values fetched from main memory in the accumulator for arithmetic or logical operation. This register holds the initial data to be operated upon, the intermediate results, and the final result of operation. The final result is transferred to main memory through MBR.
8. Stack Control Register:
A stack represents a set of memory blocks; the data is stored in and retrieved from these blocks in an order, i.e. First In and Last Out (FILO). The Stack Control Register is used to manage the stacks in memory. The size of this register is 2 or 4 bytes.
9. Flag Register:
The Flag register is used to indicate occurrence of a certain condition during an operation of the CPU. It is a special purpose register with size one byte or two bytes. Each bit of the flag register constitutes a flag (or alarm), such that the bit value indicates if a specified condition was encountered while executing an instruction.
For example, if zero value is put into an arithmetic register (accumulator) as a result of an arithmetic operation or a comparison, then the zero flag will be raised by the CPU. Thus, the subsequent instruction can check this flag and when a zero flag is "ON" it can take, an appropriate route in the algorithm.
The binary data in a register can be moved within the register from one flip-flop to another. The registers that allow such data transfers are called as shift registers. There are four mode of operations of a shift register.
  • Serial Input Serial Output
  • Serial Input Parallel Output
  • Parallel Input Serial Output
  • Parallel Input Parallel Output

Serial Input Serial Output

Let all the flip-flop be initially in the reset condition i.e. Q3 = Q2 = Q1 = Q0 = 0. If an entry of a four bit binary number 1 1 1 1 is made into the register, this number should be applied to Din bit with the LSB bit applied first. The D input of FF-3 i.e. D3 is connected to serial data input Din. Output of FF-3 i.e. Q3 is connected to the input of the next flip-flop i.e. D2 and so on.


Block Diagram



Operation

Before application of clock signal, let Q3 Q2 Q1 Q0 = 0000 and apply LSB bit of the number to be entered to Din. So Din = D3 = 1. Apply the clock. On the first falling edge of clock, the FF-3 is set, and stored word in the register is Q3Q2 Q1 Q0 = 1000.
diagram 

Apply the next bit to Din. So Din = 1. As soon as the next negative edge of the clock hits, FF-2 will set and the stored word change to Q3 Q2 Q1 Q0 = 1100.
diagram 

Apply the next bit to be stored i.e. 1 to Din. Apply the clock pulse. As soon as the third negative clock edge hits, FF-1 will be set and output will be modified to Q3 Q2 Q1 Q0 = 1110.

diagram

Similarly with Din = 1 and with the fourth negative clock edge arriving, the stored word in the register is Q3 Q2 Q1 Q0 = 1111.

diagram

truth table


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