Register

Description

In computing and digital electronics, a register is a small, fast storage location within a CPU used to hold temporary data and instructions during processing. Registers are essential for a computer’s performance because they provide direct access to data by the processor, which is significantly faster than accessing data from RAM or cache.

Registers are hardware components built directly into the CPU and operate at the same speed as the processor. They are used for a wide range of operations such as arithmetic computations, memory addressing, and instruction execution.

Types of Registers

Type	Description
Accumulator (ACC)	Stores intermediate results of arithmetic and logic operations
Program Counter (PC)	Holds the address of the next instruction to be executed
Instruction Register (IR)	Stores the current instruction being executed
Memory Address Register (MAR)	Holds memory address from which data will be fetched or written
Memory Data Register (MDR)	Contains data to be written or read from memory
Stack Pointer (SP)	Points to the top of the current stack in memory
General Purpose Registers	Multipurpose storage for variables, counters, addresses, etc.
Status Register / Flags Register	Holds flags that indicate the outcome of various CPU operations

Register Characteristics

Fastest memory type: Operates at CPU clock speed
Volatile: Data is lost when power is turned off
Size-limited: Typically 8, 16, 32, or 64 bits depending on architecture
Directly controlled by CPU instructions

Role in Instruction Cycle

Registers play a central role in the fetch-decode-execute cycle:

Fetch: Program Counter (PC) sends address to MAR → Data fetched into IR
Decode: Control Unit interprets instruction from IR
Execute: ALU uses operands in registers like ACC to perform operations

Registers vs Memory

Feature	Registers	RAM / Main Memory
Location	Inside the CPU	External to CPU
Speed	Extremely fast	Slower than registers
Size	Very limited (a few KB or less)	Much larger (MB to GB)
Access time	1 CPU cycle	Many CPU cycles
Volatility	Volatile	Volatile
Use	Temporary data and instruction storage	General-purpose memory

Examples in Assembly (x86 Syntax)

Adding Two Numbers

MOV AX, 5        ; Move 5 into AX register
MOV BX, 3        ; Move 3 into BX register
ADD AX, BX       ; Add BX to AX (AX = 8)

Loop with Counter Register

MOV CX, 10       ; CX used as loop counter
LOOP_LABEL:
  ; Do something
  DEC CX         ; Decrement CX
  JNZ LOOP_LABEL ; Jump if CX is not zero

Registers in Different Architectures

x86: AX, BX, CX, DX, SI, DI, SP, BP
ARM: R0 to R15, with R13 (SP), R14 (LR), R15 (PC)
MIPS: $t0–$t9 (temporary), $s0–$s7 (saved), $zero (constant 0)

Each architecture has its own register set and naming convention, but their purpose remains broadly similar.

Role in Optimization and Performance

Register Allocation: Compilers optimize performance by storing frequently used variables in registers
Loop Unrolling: Register usage is increased to reduce loop overhead
Register Spilling: Occurs when there are more variables than registers—data is moved to memory
Instruction Pipelining: Registers allow faster throughput of instructions