CD Notes

The Assembler

Learn about assembly language, assembler functions, machine code generation, and assembly instruction set details

The assembler is a system software that translates assembly language code into machine code (binary). Unlike compilers which translate high-level languages, assemblers perform almost a one-to-one translation from symbolic assembly to machine instructions.

What is an Assembler?

An assembler is a program that converts assembly language into executable machine code. Assembly language uses mnemonic instructions (like MOV, ADD, JMP) that are more human-readable than raw binary machine code. The assembler translates these mnemonics into their binary equivalents.

Assembly Code ↓ ┌──────────────┐ │ Assembler │ (One-to-one translation) └──────────────┘ ↓ Machine Code

Assembly Language Basics

Assembly language is specific to processor architectures. Here's a simple x86 example:

assembly example

; Simple x86-64 assembly
section .data
    message db "Hello", 0

section .text
global _start

_start:
    MOV EAX, 1          ; System call: sys_exit
    MOV EDI, 0          ; Exit code: 0
    SYSCALL             ; Call kernel

Common Assembly Instructions

Instruction	Purpose	Example
MOV	Move/Copy data	MOV EAX, 10
ADD	Addition	ADD EAX, EBX
SUB	Subtraction	SUB EAX, 5
MUL	Multiplication	MUL EBX
DIV	Division	DIV ECX
JMP	Unconditional jump	JMP label
JE	Jump if equal	JE target
CALL	Function call	CALL function
RET	Return from function	RET
PUSH	Push to stack	PUSH EAX
POP	Pop from stack	POP EAX

Assembler Functions and Workflow

Two-Pass Assembler

Most assemblers use two passes:

Example

Pass 1: Symbol Resolution ──────────────────────── Source Assembly Code ↓ Read all labels and symbols Build symbol table with addresses ↓ Symbol Table Created Pass 2: Code Generation ────────────────────── Source Assembly Code ↓ Read instructions Replace symbols with addresses from symbol table Generate machine code ↓ Object Code (Machine Code)

Assembler Process Diagram

┌─────────────────────────────────────────┐ │ Assembly Source File (.asm) │ └────────────┬────────────────────────────┘ │ ┌──────▼──────┐ │ PASS 1 │ │ Scanning │ │ Symbol │ │ Resolution │ └──────┬──────┘ │ ┌──────▼────────────────┐ │ Symbol Table Built │ │ label: 0x1000 │ │ var: 0x2000 │ └──────┬────────────────┘ │ ┌──────▼──────┐ │ PASS 2 │ │ Code │ │ Generation │ └──────┬──────┘ │ ┌──────▼──────┐ │ Machine Code │ │ (Object Code)│ └──────┬──────┘ │ ┌──────▼──────┐ │ Linker/ │ │ Loader │ └──────┬──────┘ │ ┌──────▼──────┐ │ Executable │ │ Program │ └─────────────┘

Assembly Language Concepts

Labels and Symbols

Labels mark memory locations for jumps and data references:

Example

section .data count: db 5 ; Label 'count' points to data section .text loop_start: ; Label for jump target MOV EAX, [count] LOOP loop_start ; Jump back to loop_start

Addressing Modes

Different ways to specify operands:

Example

; Immediate addressing: Literal value MOV EAX, 10 ; Register addressing: Data in register MOV EAX, EBX ; Direct addressing: Memory location MOV EAX, [0x1000] ; Register indirect: Register contains address MOV EAX, [EBX] ; Indexed addressing: Base + offset MOV EAX, [EBX + 4] ; Scaled indexed: Base + scaled index MOV EAX, [EBX + ECX*4]

Symbol Table in Assemblers

The symbol table maintains mapping of labels to memory addresses:

Symbol	Type	Address	Size
main	label	0x401000	-
count	data	0x402000	1 byte
buffer	data	0x402001	256 bytes
loop_start	label	0x401010	-

Example: Simple Assembly Program

High-Level C Code:

c example

int add(int a, int b) {
    return a + b;
}

int main() {
    int x = 5;
    int y = 3;
    int z = add(x, y);
    return z;
}

Assembly Code (x86-64):

assembly example

section .text
global add
global main

add:
    ; a is in EDI, b is in ESI (x86-64 calling convention)
    MOV EAX, EDI    ; EAX = a
    ADD EAX, ESI    ; EAX = a + b
    RET

main:
    PUSH RBP
    MOV RBP, RSP
    
    MOV EDI, 5      ; First arg (x)
    MOV ESI, 3      ; Second arg (y)
    CALL add        ; Call add, result in EAX
    
    MOV RSP, RBP
    POP RBP
    RET

Generated Machine Code (hex):

Example

add: 89 c7 mov edi, eax 01 f0 add eax, esi c3 ret main: 55 push rbp 48 89 e5 mov rsp, rbp bf 05 00 00 00 mov edi, 5 be 03 00 00 00 mov esi, 3 e8 f3 ff ff ff call add 48 89 ec mov rsp, rbp 5d pop rbp c3 ret

Assembler Output: Object Code

After assembly, the output is object code containing:

Object File Format ┌──────────────────┐ │ Header │ │ (Magic number, │ │ version, etc) │ ├──────────────────┤ │ Symbol Table │ │ (Labels, │ │ exported symbols) ├──────────────────┤ │ Code Section │ │ (Machine code) │ ├──────────────────┤ │ Data Section │ │ (Initialized │ │ data) │ ├──────────────────┤ │ Relocation Info │ │ (For linker) │ ├──────────────────┤ │ Debugging Info │ │ (Optional) │ └──────────────────┘

Assembler vs Compiler

Aspect	Assembler	Compiler
Input	Assembly	High-level language
Output	Machine code	Assembly/Machine code
Complexity	Low	High
Optimization	Minimal	Extensive
Translation ratio	~1:1	Many high-level statements to few machine instructions
Portability	Architecture-specific	More portable

Interview Q&A

Q: What is the difference between an assembler and a compiler? A: A compiler translates high-level languages to machine code or assembly, handling complex transformations. An assembler does nearly one-to-one translation from symbolic assembly to machine code.

Q: Why do assemblers need two passes? A: First pass builds the symbol table by scanning all labels and their addresses. Second pass generates code using this symbol table to resolve forward references.

Q: What is an object file? A: An object file is the output of an assembler containing machine code and metadata (symbol table, relocation information) that a linker uses to create executable programs.

Q: What are relocation entries? A: Relocation entries in object files tell the linker how to adjust addresses when linking multiple object files together.

Q: What is calling convention? A: Calling convention defines how arguments are passed to functions and where return values are placed. Different architectures have different conventions (x86-64: EDI/ESI for first two args, EAX for return).

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for The Assembler.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Compiler Design topic.

Search Terms

compiler-design, compiler design, compiler, design, language, processing, system, assembler

Continue learning this concept

Language Processing SystemCompiler OverviewComprehensive guide to compiler architecture, phases, and how source code is transformed into executable code Language Processing SystemComplete Compilation ProcessDetailed walkthrough of the entire compilation pipeline from source code to program execution Language Processing SystemThe LinkerComprehensive guide to linking, symbol resolution, relocation, and executable creation Language Processing SystemThe LoaderDetailed guide to program loading, memory allocation, relocation, and execution setup Language Processing SystemThe PreprocessorLearn about preprocessors, macro expansion, file inclusion, and conditional compilation in detail IntroductionLanguage ProcessorsComplete guide to language processors including compilers, interpreters, assemblers, linkers, loaders, and preprocessors in the compilation system.

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CD Notes

The Assembler

Learn about assembly language, assembler functions, machine code generation, and assembly instruction set details

What is an Assembler?

Assembly Code ↓ ┌──────────────┐ │ Assembler │ (One-to-one translation) └──────────────┘ ↓ Machine Code

Assembly Language Basics

Assembly language is specific to processor architectures. Here's a simple x86 example:

assembly example

; Simple x86-64 assembly
section .data
    message db "Hello", 0

section .text
global _start

_start:
    MOV EAX, 1          ; System call: sys_exit
    MOV EDI, 0          ; Exit code: 0
    SYSCALL             ; Call kernel

Common Assembly Instructions

Instruction	Purpose	Example
MOV	Move/Copy data	MOV EAX, 10
ADD	Addition	ADD EAX, EBX
SUB	Subtraction	SUB EAX, 5
MUL	Multiplication	MUL EBX
DIV	Division	DIV ECX
JMP	Unconditional jump	JMP label
JE	Jump if equal	JE target
CALL	Function call	CALL function
RET	Return from function	RET
PUSH	Push to stack	PUSH EAX
POP	Pop from stack	POP EAX

Assembler Functions and Workflow

Two-Pass Assembler

Most assemblers use two passes:

Example

Assembler Process Diagram

Assembly Language Concepts

Labels and Symbols

Labels mark memory locations for jumps and data references:

Example

section .data count: db 5 ; Label 'count' points to data section .text loop_start: ; Label for jump target MOV EAX, [count] LOOP loop_start ; Jump back to loop_start

Addressing Modes

Different ways to specify operands:

Example

Symbol Table in Assemblers

The symbol table maintains mapping of labels to memory addresses:

Symbol	Type	Address	Size
main	label	0x401000	-
count	data	0x402000	1 byte
buffer	data	0x402001	256 bytes
loop_start	label	0x401010	-

Example: Simple Assembly Program

High-Level C Code:

c example

int add(int a, int b) {
    return a + b;
}

int main() {
    int x = 5;
    int y = 3;
    int z = add(x, y);
    return z;
}

Assembly Code (x86-64):

assembly example

section .text
global add
global main

add:
    ; a is in EDI, b is in ESI (x86-64 calling convention)
    MOV EAX, EDI    ; EAX = a
    ADD EAX, ESI    ; EAX = a + b
    RET

main:
    PUSH RBP
    MOV RBP, RSP
    
    MOV EDI, 5      ; First arg (x)
    MOV ESI, 3      ; Second arg (y)
    CALL add        ; Call add, result in EAX
    
    MOV RSP, RBP
    POP RBP
    RET

Generated Machine Code (hex):

Example

Assembler Output: Object Code

After assembly, the output is object code containing:

Assembler vs Compiler

Aspect	Assembler	Compiler
Input	Assembly	High-level language
Output	Machine code	Assembly/Machine code
Complexity	Low	High
Optimization	Minimal	Extensive
Translation ratio	~1:1	Many high-level statements to few machine instructions
Portability	Architecture-specific	More portable

Interview Q&A

Q: What are relocation entries? A: Relocation entries in object files tell the linker how to adjust addresses when linking multiple object files together.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for The Assembler.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Compiler Design topic.

Search Terms

compiler-design, compiler design, compiler, design, language, processing, system, assembler

Continue learning this concept

PreviousWhat is Compiler Design?NextCompiler Overview