CD Notes

The Loader

Detailed guide to program loading, memory allocation, relocation, and execution setup

The loader is a system software component that reads an executable file from disk and loads it into memory for execution. It's the final step in the compilation/linking process that prepares a program for CPU execution.

What is a Loader?

A loader is a program (often part of the operating system) that:

Reads executable files from disk
Allocates memory sections
Transfers control to the program's entry point
Sets up runtime environment

Executable File on Disk │ ▼ ┌──────────────┐ │ Loader │ (OS component) │ (Reads .exe) │ └──────────────┘ │ ▼ Program in Memory │ ▼ Execution

Loader Functions

1. Allocation

Allocate memory sections for code, data, stack, and heap:

Memory Layout After Loading: High Address ┌─────────────────┐ │ Command-line │ │ Arguments │ ├─────────────────┤ │ Environment │ ├─────────────────┤ │ Stack │ ← Grows downward │ (local vars) │ ├─────────────────┤ │ Unused │ ├─────────────────┤ │ Heap │ ← Grows upward │ (dynamic mem) │ ├─────────────────┤ │ .bss │ (uninitialized) │ .data │ (initialized) │ .text │ (code) │ (Read-only) │ ├─────────────────┤ │ Header │ Low Address └─────────────────┘

2. Relocation

Adjusting final addresses if the program wasn't linked for a specific address:

If executable linked for 0x400000 but loaded at 0x500000

All addresses adjusted: 0x400000 → 0x500000

References updated

CODE @ 0x400100 → 0x500100

DATA @ 0x400500 → 0x500500

3. Linking (Dynamic Linking at Runtime)

Resolving references to dynamically linked libraries:

Example

Program calls: printf(...) Loader checks: Is printf already loaded? If no: Load libc.so into memory Resolve printf to its address in memory Set up jump table entry

4. Transfer of Control

Pass control to program's entry point:

IP/PC register	_start or main() address
SP register	top of stack
Other registers	initialized values

Loading Process

┌─────────────────────────────┐ │ Executable File (.exe/.elf)│ └────────────┬────────────────┘ │ ┌──────▼──────────┐ │ 1. Read Header │ Get section info └──────┬──────────┘ │ ┌──────▼────────────────┐ │ 2. Allocate Memory │ Create sections └──────┬────────────────┘ │ ┌──────▼──────────────────┐ │ 3. Load .text (.code) │ └──────┬──────────────────┘ │ ┌──────▼──────────────────┐ │ 4. Load .data │ │ Load .bss │ └──────┬──────────────────┘ │ ┌──────▼──────────────────────┐ │ 5. Perform Relocations │ │ (if necessary) │ └──────┬──────────────────────┘ │ ┌──────▼──────────────────────┐ │ 6. Resolve External Refs │ │ (dynamic linking) │ └──────┬──────────────────────┘ │ ┌──────▼──────────────────────┐ │ 7. Setup Stack & Registers │ │ argc, argv, envp │ └──────┬──────────────────────┘ │ ┌──────▼──────────────────────┐ │ 8. Transfer to Entry Point │ │ Jump to _start/main │ └──────────────────────────────┘ │ ▼ Program Execution

Executable File Format

ELF Format (Linux)

ELF Header

Magic number (0x7F 'E' 'L' 'F')

Architecture (x86-64, ARM, etc.)

Entry point

Program header offset

Section header offset

Program Headers

Type, offset, virtual address, size

.text segment

.data segment

.bss segment

Sections

.text (code)

.data (initialized data)

.bss (uninitialized data)

.symtab (symbol table)

.strtab (string table)

...

String tables, symbol tables, etc.

PE Format (Windows)

Example

DOS Header PE Header Optional Header Section Headers Sections: .text (code) .data (data) .rsrc (resources) .reloc (relocations) ...

Runtime Environment Setup

Before execution, the loader sets up:

Stack Frame (at program start): High Address ┌──────────────────┐ │ Environment vars│ │ (PATH, HOME,...)│ ├──────────────────┤ │ Command-line │ │ Arguments │ ├──────────────────┤ │ Return address │ │ (to OS) │ ├──────────────────┤ │ Local variables │ │ (if any) │ ├──────────────────┤ │ Program data │ │ ... │ Low Address └──────────────────┘ ↑ SP register

Program Execution Flow

┌─────────────────────────────────────┐ │ Operating System │ │ (File I/O, Memory Management) │ └────────────────┬────────────────────┘ │ ┌────────▼────────┐ │ Execute Program │ │ File (click it) │ └────────┬────────┘ │ ┌────────▼──────────────┐ │ OS loads Loader │ │ (or linker+loader) │ └────────┬──────────────┘ │ ┌────────▼──────────────┐ │ Loader reads .exe/.elf│ │ from disk │ └────────┬──────────────┘ │ ┌────────▼──────────────┐ │ Allocate memory │ │ layout sections │ └────────┬──────────────┘ │ ┌────────▼──────────────┐ │ Load code & data │ │ into memory │ └────────┬──────────────┘ │ ┌────────▼──────────────┐ │ Resolve dynamic refs │ │ Load dynamic libs │ └────────┬──────────────┘ │ ┌────────▼──────────────┐ │ Set up stack/regs │ │ argc, argv, envp │ └────────┬──────────────┘ │ ┌────────▼──────────────┐ │ Jump to entry point │ │ Program starts! │ └────────┬──────────────┘ │ ┌────────▼──────────────┐ │ Program Execution │ │ running... │ └──────────────────────┘

Interview Q&A

Q: What does a loader do? A: A loader reads executable files from disk, allocates memory, loads code and data sections, resolves dynamic references, and transfers control to the program's entry point.

Q: What's the difference between linker and loader? A: The linker combines object files into executable files at compile time. The loader reads executable files from disk and loads them into memory at runtime for execution.

Q: What is relocation in loading? A: Relocation in loading adjusts addresses if the program is loaded at a different memory location than it was linked for. All addresses are offset accordingly.

Q: What is dynamic linking? A: Dynamic linking resolves references to shared libraries at runtime rather than at link time. Libraries are loaded into memory when needed.

Q: What information does executable file header contain? A: The header contains magic number, architecture, entry point address, file format version, program and section header offsets, and other metadata needed to load the file.

Exam Focus

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

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, loader

Continue learning this concept

Language Processing SystemThe AssemblerLearn about assembly language, assembler functions, machine code generation, and assembly instruction set details 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 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.

PreviousThe Linker NextThe Preprocessor

CD Notes

The Loader

Detailed guide to program loading, memory allocation, relocation, and execution setup

What is a Loader?

A loader is a program (often part of the operating system) that:

Reads executable files from disk
Allocates memory sections
Transfers control to the program's entry point
Sets up runtime environment

Loader Functions

1. Allocation

Allocate memory sections for code, data, stack, and heap:

2. Relocation

Adjusting final addresses if the program wasn't linked for a specific address:

If executable linked for 0x400000 but loaded at 0x500000

All addresses adjusted: 0x400000 → 0x500000

References updated

CODE @ 0x400100 → 0x500100

DATA @ 0x400500 → 0x500500

3. Linking (Dynamic Linking at Runtime)

Resolving references to dynamically linked libraries:

Example

Program calls: printf(...) Loader checks: Is printf already loaded? If no: Load libc.so into memory Resolve printf to its address in memory Set up jump table entry

4. Transfer of Control

Pass control to program's entry point:

IP/PC register	_start or main() address
SP register	top of stack
Other registers	initialized values

Loading Process

Executable File Format

ELF Format (Linux)

ELF Header

Magic number (0x7F 'E' 'L' 'F')

Architecture (x86-64, ARM, etc.)

Entry point

Program header offset

Section header offset

Program Headers

Type, offset, virtual address, size

.text segment

.data segment

.bss segment

Sections

.text (code)

.data (initialized data)

.bss (uninitialized data)

.symtab (symbol table)

.strtab (string table)

...

String tables, symbol tables, etc.

PE Format (Windows)

Example

DOS Header PE Header Optional Header Section Headers Sections: .text (code) .data (data) .rsrc (resources) .reloc (relocations) ...

Runtime Environment Setup

Before execution, the loader sets up:

Program Execution Flow

Interview Q&A

Q: What is relocation in loading? A: Relocation in loading adjusts addresses if the program is loaded at a different memory location than it was linked for. All addresses are offset accordingly.

Q: What is dynamic linking? A: Dynamic linking resolves references to shared libraries at runtime rather than at link time. Libraries are loaded into memory when needed.

Exam Focus

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

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, loader

Continue learning this concept

PreviousThe Linker NextThe Preprocessor