• Register
• A synonym for memory in computer science
• General purpose reigster
• A memory cell
• Each general purpose register has a unique name
• It is used to 
• store (and recall) intermediate result of complex computations

Arithmetic and Logic Unit (ALU)

• It is a complex electrical circuit that can perform Mathematical (+,-,x,/) and logical operation (<, <=, >, >=, and, or)
• The output (result) of the computation (obtained by the ALU) is often stored in a general purpose register. 

Instruction Register (IR)

• Contains the current instruction being executed by the CPU
• The CPU will perform the operation indicated by the instruction code contained in the instruction register

• The program counter is a register (memory cell)!
• This register contains the address (location in memory) of the next instruction after the CPU finishes executing the current instruction in the instruction register
• The value in the program counter will be increased after the CPU finishes executing one instruction

Processor Status Register (PSR)

• This register contains the various information about the CPU.
• Among the information contained in the PSR is
• The result of a comparison operation
• When the CPU compares 2 numbers a and a and b,  the outcome of the comparison is stored in the PSR
• The outcome of a compare operation will allow the CPU to determine the following fact between a and b.