Types of Instructions and Addressing Modes in Assembly

Three-Address Instruction

An instruction with two source operands and one destination (e.g., ADD R1, R2, R3).

Two-Address Instruction

One operand acts as both a source and destination (e.g., ADD AX, BX).

One-Address Instruction

Uses accumulator and one memory operand (e.g., ADD B).

Zero-Address Instruction

Uses a stack; operands are implied (e.g., postfix A B +).

Addressing Modes

Techniques for locating data/operands used in instructions.

Direct Addressing

Address holds value.

Indirect Addressing

Address holds location of value.

Register Indirect Addressing

Register holds address of value.

Displacement Addressing

Used for arrays, structures, stack frames (offset + base).

Stack Addressing

Accesses operands via stack (LIFO), using stack pointer (SP).

Instruction Length

Affects performance, memory usage, and code speed.

Assembly Language

Human-readable machine code with mnemonics.

Advantage of Assembly

Precise control and high efficiency.

Disadvantage of Assembly

Harder to learn and debug; not portable.

Parts of an Assembly Statement

Label, mnemonic, operands, comment.

Pseudo-instructions

Assembler-only directives (e.g., .DATA, .ORG, .END).

Logic for expression (A AND B) OR (C AND NOT D)

Use 2 AND gates, 1 NOT, and 1 OR gate.