Microprocessor Fundamentals

A microprocessor is a programmable VLSI/ULSI integrated circuit that implements the CPU functions: arithmetic and logic, control, registers and machine-instruction execution. External memory, I/O interfaces, clocking, reset and power circuitry are normally added to form a complete computer.

The preceding section develops CPU organization, registers and the instruction cycle. Here the emphasis is on the properties visible when selecting or programming a processor. A microprocessor accepts binary instructions and data, processes them under stored-program control and produces results and control signals.

Principal Characteristics

Characteristic	Meaning and consequence
Word length	Number of bits naturally processed by the ALU and registers, commonly 8, 16, 32 or 64 bits. A longer word can represent a wider integer or address but may require more hardware and energy.
Clock speed	Clock cycles per second, measured in hertz. A higher frequency shortens a cycle, but does not by itself guarantee higher performance because instruction count, cycles per instruction, cache misses and pipeline stalls also matter.
Instruction set	The operations, data types, registers, instruction formats and addressing modes exposed to a programmer or compiler.
Address-bus width	With $n$ independent address lines, at most $2^n$ addresses can be selected. The corresponding byte capacity is $2^n$ bytes only when each address denotes one byte.
Data-bus width	Number of bits transferred in one bus transaction. A word wider than the external data bus may require several transfers.

Important microprocessor characteristics.

Do not equate word length, address width and data-bus width.: They may be equal in a simple processor, but they describe different properties and can have different numerical values.

Addressing Modes

An addressing mode tells the CPU where an operand is found or how its effective address (EA) is calculated. Let $A$ be an address field in the instruction, $R$ a selected register and $M[x]$ the contents of memory at address $x$.

Mode	Operand or effective address	Main use	Example
Immediate	Operand is the constant encoded in the instruction	Constants; no extra operand-memory read	MOV A, #05H
Register	Operand is the contents of $R$	Fast arithmetic on values already in the CPU	ADD A, B
Direct	$\mathrm{EA}=A$	Fixed memory location or I/O register	LOAD R1, [2050H]
Indirect	$\mathrm{EA}=R$ or $\mathrm{EA}=M[A]$, according to the instruction set	Pointers and dynamically selected data	LOAD R1, [R2]
Indexed	$\mathrm{EA}=A+R_{index}$	Arrays, tables and strings	LOAD R1, [TABLE+R2]

Common addressing modes.

Immediate and register modes do not need an effective memory address for the operand. Direct addressing needs one operand-memory access after instruction fetch; memory-indirect addressing may need an additional access to obtain the pointer. Exact syntax and whether an index is scaled by element size are instruction-set dependent.

Note

Example — Effective-address calculation

An array begins at byte address 2000H. An indexed instruction has $R_{index}=\texttt{0014H}$. Its effective address is

$$\mathrm{EA}=\texttt{2000H}+\texttt{0014H}=\boxed{\texttt{2014H}}.$$

If the page or cache containing that byte is not present, address calculation is still correct; the memory system handles the resulting miss or fault.

RISC and CISC

Reduced Instruction Set Computer (RISC) designs emphasize a small, regular set of simple instructions, whereas Complex Instruction Set Computer (CISC) designs provide a larger set that may perform multi-step operations in one instruction.

Feature	RISC tendency	CISC tendency
Instruction set	Fewer, simpler and more regular instructions	More numerous, specialized and complex instructions
Instruction format	Often fixed length with few formats	Often variable length with many formats
Memory access	Usually load/store: arithmetic uses registers	Instructions may operate directly on memory
Execution	Many instructions complete in one or a few cycles and pipeline regularly	Complex instructions may require several internal steps
Control hardware	Simpler hardwired decoding is common	More complex decode; microprogrammed control is common historically
Code size	May need more simple instructions	A complex instruction may replace several simple ones
Examples	ARM and RISC-V	x86 family

Typical RISC–CISC comparison.

These are design tendencies, not absolute performance rules. A cache miss can make a RISC load take many cycles, and a modern CISC processor may decode a complex instruction into simple internal micro-operations. Performance must therefore be compared for a workload, not inferred from the acronym alone.

Microprocessor and Microcontroller

A microcontroller combines a CPU with program/data memory and peripherals such as timers, GPIO, serial interfaces and often ADCs on one chip. It is optimized for dedicated embedded control; a microprocessor is normally the CPU-centered choice for systems requiring large external memory and high general-purpose performance.

Feature	Microprocessor	Microcontroller
On-chip integration	Primarily CPU, cache and bus interfaces	CPU, memory and I/O peripherals on one chip
External components	More external RAM, storage and controllers required	Fewer components required for a complete controller
Typical application	PCs, workstations, servers and high-performance systems	Appliances, instruments, vehicles and embedded control
Cost and power	Generally higher system cost and power	Generally lower cost, power and board area
Expansion	Large memory and rich external expansion	Limited by on-chip resources, though external interfaces may be provided

Microprocessor versus microcontroller.

Note

Key Point — Selection rule

Choose according to the complete system requirement. A battery sensor node normally favors an integrated microcontroller; a computer running a large OS and many applications normally favors a microprocessor with external memory.

Quick Review

• Microprocessor: CPU functions on a programmable chip; external memory and I/O complete the system.

• Addressing mode: specifies an operand or computes its effective address; immediate, register, direct, indirect and indexed are the standard forms.

• RISC: regular simple instructions and load/store organization; ARM and RISC-V are examples.

• CISC: larger instruction repertoire and potentially multi-step instructions; x86 is the standard example.

• Microcontroller: integrates CPU, memory and peripheral I/O for embedded control.

Tip

Exam Focus — Comparison answers

Start with one-sentence definitions, compare at least four corresponding features in a table, give one application of each, and qualify architectural tendencies rather than claiming that every RISC instruction takes exactly one cycle.