Concept of Registers

Small, permanent storage locations within the CPU used for a particular purpose
Manipulated directly by the Control Unit
Wired for specific function
Size in bits or bytes (not MB like memory)
Can hold data, an address or an instruction
How many registers does the LMC have?

Use of Registers;
Scratchpad for currently executing program
Holds data needed quickly or frequently
Stores information about status of CPU and currently executing program
Address of next program instruction
Signals from external devices

General Purpose Registers;
User-visible registers
Hold intermediate results or data values, e.g., loop counters
Equivalent to LMC’s calculator
Typically several dozen in current CPUs

Special-Purpose Registers
Program Count Register (PC)
Also called instruction pointer
Instruction Register (IR)
Stores instruction fetched from memory
Memory Address Register (MAR)
Memory Data Register (MDR)
Status Registers
Status of CPU and currently executing program
Flags (one bit Boolean variable) to track condition like arithmetic carry and overflow, power failure, internal computer error

Register Operations
Stores values from other locations (registers and memory)
Addition and subtraction
Shift or rotate data
Test contents for conditions such as zero or positive

(source: handbook 3rd edition Wilson Wong, Linda Senne, Bentley College)