What is CPU Register?

CPU Registers - Registers are used to quickly process the data which is currently used by the processor. A CPU contains a number of registers which are used to store the intermediate result, or temporary data produced during the execution of a program.

Registers are used to quickly accept, store and transfer the data & instruction that are being used immediately by the processor.
In a computer, a register is the fastest memory & that's why, the cost of registers are too expensive.
Registers comes at the top of Memory Hierarchy Design.

Types of Registers - A CPU contains various types of registers whose number varies from processor to processor, some processors contain less registers & some more. On the basis of their specific functions, registers are classified into three parts.
1.General Purpose Register
2.Accumulator
3.Special Purpose Register

1. General Purpose Register - These registers are used to store data and intermediate results produced during the execution of a program. If a user is working in assembly language
then, these registers are accessible to the user through instructions.

2. Accumulator - Accumulators are the most important general purpose registers having multiple functions. Accumulators are most efficient in data movements, arithmetic and logical operations. Its functions depends on the design of the processor, i.e. whether the processor is accumulator based
or, general purpose register based. All the data transfer between the CPU and device/ports are performed through the accumulator.

3.Special Purpose Register - These registers are used for performing some specific function. A CPU contains a number of special purpose registers. These are:
1.Flag Register(or Status Register)
2.Stack Pointer(SP)
3.Program Counter(PC)
4.Instruction Register(IR)
5.Index Register
6.Memory Address Buffer(MAR)
7.Memory Buffer Register(MBR) or Data Register(DR)

Note - It is not essential that all CPU contains all these special purpose registers, but a powerful CPU have almost all these registers.

Microtricks Learning team made a special video on CPU Register.
You guys can watch that video on YouTube.
Link -  https://youtu.be/rlGYfZrKPQQ