Computer Architecture

What is Registers ?

  • A register is a very small amount of very fast memory that   is built into the CPU (central processing unit).
  • Contents can be accessed at extremely high speeds.
  • Registers are used to store data temporarily during the execution of a program.
  • Different processors have different register sizes.
  • Registers are normally measured by the number of bits they can hold, for example, an 8-bit register means it can store 8 bits of data or a 32-bit register means it can store 32 bit of data.


The processor register AC consists of 16-bits. It is used to hold the results or partial results of arithmetic and logical operations. An accumulator is a register in which intermediate arithmetic and logic results are stored. The 16-bit inputs to the Adder / logic circuit come from the outputs of AC. They are used to implement register micro operations such as complement and shift the  contents of AC.


The register DR consists of 16-bits and it is used to hold memory operands (data). This register contains the data to be written into memory or receives the data read from memory.


Temporary registers have 16-bits and it provides temporary storage of variables or results.


The instruction register consists of 16-bits. The purpose of the instruction register is to hold a copy of the instruction which

the processor is to execute. In our basic computer, the instruction register (IR) holds instruction code which is read from memory.


This register specifies the address in memory for next read or write operations. The address register consists of 12-bits.


Program counter has 12-bits and it holds the address of the next instruction to be read from memory after the current execution is executed. The instructions are read sequentially because the program counter automatically increments after fetching the current instruction.


Input register has 8-bits. The INPR register receives a character from an input device and delivers it to the AC.


Output register has 8-bits. The output register receives information from AC and transfers it to the output device. 

Common bus system


A wire or a collection of wires that carry some multi-bit information is known as bus. Main purpose of bus is to transfer information form one system to another.


   The basic computer has eight registers (AC, PC, DR, AC, IR, TR, INPR, OUTR), a memory unit and a control unit. Paths must be provided to transfer information from one register to another and between memory and registers. The number of wires will be excessive if connections are made between the output of each register and input of other registers. A more efficient scheme is to use a common bus. Thus a common bus provides a path between memory unit and registers.

Five registers have three control inputs: L(load), INR (increment) and CLR (clear). Two registers have only a LD input

Load (LD):

 The lines from the common bus are connected to the inputs of each register and the data inputs of the memory. The particular register whose LD input is enabled receives the data from the bus.

 Increment (INR)) and Clear (CLR):

The contents of the particular register are incremented when its INR signal is enabled and cleared when its CLR signal is enabled. 

Memory Unit:

The memory receives the 16-bit information from the bus when its write input is enabled and the memory places its 16-bit information onto the bus when its read input is activated and S2S1S0 = 111.

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