Compiler \to Linker \to Loader

Compiler: Turns high-level source code into machine code.

Assembler: Encodes assembly code into machine code.

Note: Javac

• Java is special in that it compiles into bytecode for the JVM rather than a particular CPU’s machine code.
  • “Write once, run everywhere”

Linker: Combine object files into an executable.

• e.g., Doing -lm in gcc to link the math library for a C program.
• Makes separate compilation/assembly possible.

Loader: Brings executable into memory and starts execution.

• Initializes registers, stack, arguments to first functions
• Jumps to entry point

Example: Manually calling the compiler, assembler, and linker

# Compiler
gcc -S sum.c
# Assembler
as -o sum.o sum.s
# Linker
ld -o sum sum.o
# Execute (Uses Loader)
./sum

• Etymology: The linker is unintuitively named ld because it stands for “link editor”

Assembly and Machine Language

\text{Assembly Language} \to \text{(Assembler)} \to \text{Machine Language}

Assembly Language: Symbolic representation of machine language.

• More readable, use symbols for bits
• Uses labels for address

Machine Language: Set of instruction codes used to represent operations and date.

• Binary form.

When to Use Assembly:

1. Speed and size is critically important
2. To use specialized instructions not available in high-level languages
   • e.g., INT, single instruction loop, SIMD, etc.
3. Platform only supports assembly.

Assembly Drawbacks:

• ISA dependent (architecture dependent)
  • Not portable
• Longer than high-level language
• No high-level constructs
• Hard to read.
  • This is why good comments are important.

More on Assembly:

• A programming language
• Provides only a few, simple types of data and control flow
• Typically no types.
  • Programmers define the type
• All control flow implemented via go to.
• Difficult and error-prone

More on Assemblers

\text{Assembler's Job: } \text{Source} \to \text{Object}

Assembler: Translate a file of assembly language statements (source) into a file of binary machine instructions and binary data (object).

Two Major Parts

1. Identifies memory locations with symbolic references (labels), both code and data
2. Translate assembly statement into opcodes, register specifies, and labels not valid instructions.

Object File Contains:

• Machine instructions
• Data
• Other information
  • e.g., Symbol table, references, etc.
• Can’t be executed until it’s processed by the linker