assembly language

Fundamentals of Computer Systems (8hrs) · Computer Fundamentals & Emerging Technologies

1. Definition

Assembly language is a low-level language that represents machine instructions in a human-readable, symbolic form using mnemonics (short names such as MOV, ADD, JMP) instead of binary opcodes. It is translated into machine language by a program called an assembler. There is (almost) a one-to-one correspondence between an assembly instruction and a machine instruction, which is why it is called a symbolic machine language.

  Machine language :  0011 1110 0000 0101     (3E 05 H)
  Assembly language:  MVI A, 05H
  Meaning          :  put the value 5 into the accumulator

2. Elements of an Assembly Program

Element	Purpose	Example
Mnemonic (opcode)	The operation	`MOV`, `ADD`, `INR`, `JZ`
Operands	Data, register or address the opcode works on	`A`, `B`, `2050H`
Label	A name for an address, used for branches and data	`LOOP:`, `SUM:`
Directive / pseudo-op	Instruction to the assembler, generates no code	`ORG`, `EQU`, `DB`, `DW`, `END`
Comment	Documentation for the reader	`; sum in A`

3. Addressing Modes

Mode	Meaning	Example
Immediate	Data is part of the instruction	`MVI A, 05H`
Register	Operand is a register	`MOV A, B`
Direct	Address of the operand is given	`LDA 2050H`
Indirect	A register pair holds the address	`MOV A, M` (M = memory at H-L)
Indexed / based	Address = base + index + displacement	`MOV AX, [BX+SI+4]`
Relative	Target is given as an offset from PC	`JNZ LOOP`
Implied / stack	Operand is implicit	`CMA`, `PUSH B`, `RET`

4. Sample Program (8085-style — add five numbers)

        ORG 2000H        ; start of program
        LXI H, 2050H     ; H-L = address of first number
        MVI C, 05H       ; C = counter = 5
        MVI A, 00H       ; A = sum = 0
LOOP:   ADD M            ; A = A + memory[H-L]
        INX H            ; point to next number
        DCR C            ; counter = counter - 1
        JNZ LOOP         ; repeat while counter is not zero
        STA 2060H        ; store the sum
        HLT              ; stop

5. How the Assembler Works

An assembler translates mnemonics into machine code in (usually) two passes:

Pass 1 builds a symbol table — each label is assigned the address of the instruction/data following it; this resolves forward references (a jump to a label that appears later).
Pass 2 substitutes opcode codes for mnemonics and label addresses for symbols, and emits the object code, listing, and error diagnostics.

A macro assembler also expands user-defined macros, and a cross assembler runs on one machine but generates code for another (typical in embedded development).

6. Assembly vs High-Level Language

Aspect	Assembly language	High-level language (C, Java)
Level	Low (near hardware)	High (near problem domain)
Translation	Assembler, 1 mnemonic ↔ 1 instruction	Compiler, 1 statement ↔ many instructions
Portability	None — CPU specific	Good to excellent
Program length	Long, verbose	Short, readable
Speed of code	Fastest, minimal overhead	Slightly slower, compiler-dependent
Hardware access	Full (registers, ports, interrupts)	Restricted (usually through libraries)
Development time	Very long, error-prone	Short, easier to debug
Debugging	Difficult (registers, flags)	Easier (variables, functions)

7. Advantages, Limitations and Uses

Advantages: gives complete control of CPU registers, flags, ports and interrupts; produces the smallest and fastest code; essential where timing and memory are critical; makes the hardware behaviour visible (good for learning).
Limitations: machine dependent, so a program must be rewritten for each CPU; long development and debugging time; unfriendly syntax; easy to introduce subtle bugs.
Applications: embedded firmware and microcontrollers, bootloaders, device drivers, real-time interrupt handling, cryptographic and DSP inner loops, low-level system code, and reverse engineering / malware analysis.

8. Exam Points

Define assembly language and list its elements (mnemonic, label, directive, operand) — 5 marks.
Explain any five addressing modes with examples — 5 marks, very frequent.
Difference between assembler and compiler; assembly vs high-level language — 5 marks.
Write a short assembly program (sum of N numbers / largest of N numbers) — 5-7 marks.
Explain the two-pass assembler with a symbol table — 5 marks.
Why is assembly language called a low-level language? — 2-3 marks.

assembly language