Introduction to Registers

Registers Accessible by Users: Contents in these can be altered by the instructions in the flow of program execution e.g Data and address Register.

Internal Registers: Used exclusively by CPU for internal operations and these are not accessible by instructions.

2. User Accessible Registers

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  Data Registers (DR): It holds variables used by the processor in the execution of instructions and in a transition state between the processor and peripheral devices. Its size is 16 bits and stores an integer, floating-point values, Characters and array of elements small in size.
  
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  Accumulator Registers (AC): It holds the results of any operation until it is consumed in the next operation by CPU. Its size is 16 bits.
  
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  Address Registers (AR): It holds the address of the main memory that holds data or instructions. It may hold the exact absolute address or offset value in numeric format and both the options exist in processors.
  
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  Program Counter (PC): Address of the main memory location which contains an instruction that has to be executed next after completing the current instruction, is stored in this register. It helps to maintain a flow in the execution of a program instruction after instruction in the right sequence. It can also be called an instruction pointer.
  
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  General Purpose Register (GPR): It stores addresses as well as data and it is known as integrated registers of both address and data. It has got the capability to store floating-point numbers as well.
  
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  Floating-point Registers (FPR): It stores Floating point numbers.
  
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  Vector Registers: It holds data of SIMD (Single instruction and multiple data) instructions.
  
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  Constant Registers: It holds constant values like pi, zero and one.
  
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  Status Registers: It holds true or false values that decide the execution of a statement.