Registers

A  register is a device for storing data. It is a small amount of very fast computer
memory used to speed the execution of computer programs by providing quick access to
commonly used values. These registers are the top of the memory hierarchy, and are the
fastest way for the system to manipulate data. It is common to measure registers by the
number of bits it can hold, for example, an "8-bit register" or "32-bit register". Registers
are now usually implemented as an array of SRAMs, but they have also been
implemented using individual flip flops, high speed core memory, thin film memory, and
other ways in various machines.
There are several other classes of registers:
Data registers are used to store integer numbers. 
Address registers hold memory addresses and are used to access memory. 
General Purpose registers can store both data and addresses. 
Floating Point registers are used to store floating point numbers. 
Constant registers hold read-only values (e.g zero or one). 
Vector registers hold data for Single Instruction Multiple Data (SIMD) instructions. 
Special Purpose registers which store internal CPU data like the stack pointer or
processor status words. 
The ALU & FPU store intermediate and final results from their calculations in these
registers. Then the processed data goes back to the data cache and then to main memory
from these registers.