libasm

The goal of this project is to get familiar with assembly language.

Basic Concepts

Registers

Assembly programming primarily involves working with registers. Registers can be thought of as small, predefined storage locations in the CPU that hold data.

The general-purpose registers in x86-64 are: rax, rcx, rdx, rbx, rsp, rbp, rsi, rdi, r8 to r15, and rip.

It’s important to handle registers with care because they may be used by the system at any time (for both reading and writing). Here are some key examples:

• rax: Stores the return value of a function.
• rcx: Commonly used as a counter in loops.
• rbp: Holds the base pointer, typically for stack frames.
• rdi, rsi, rdx, rcx, r8, r9: Used for passing function arguments, in the order: function(rdi, rsi, rdx, rcx, r8, r9).

Execution Flow

Assembly instructions are executed sequentially, from top to bottom.

Instructions can be grouped into labels, which act as markers in the code. A label is defined with a name followed by a colon (:).

Note

Although labels may resemble functions in higher-level languages like C, they are not functions.

Syntax

The syntax used in this repository follows Intel syntax, the most common assembly syntax. This syntax requires the destination operand to be on the left and the source operand to be on the right.

Instructions

In assembly, most lines consist of an instruction followed by its operand(s). Operands may represent data, addresses, or registers.

Behavior

Each assembly instruction may implicitly alter program behavior. For instance:

• The cmp instruction compares two operands and sets flags in the flags register based on the result. These flags can then influence subsequent instructions, such as conditional jumps.

Addresses and Values

Similar to C, assembly allows working with addresses.

To dereference an address, use square brackets []. For example:

cmp rax, [0x1234]

This compares the value at memory address 0x1234 with the value in the rax register.

Note

In practice, you should specify the size of the data being referenced (e.g., BYTE, WORD, DWORD, QWORD). For simplicity, this example omits the size specifier.

The rsp register

The rsp register stands for Stack Pointer. It is a special-purpose register that points to the top of the stack in memory. The stack is a region of memory used for storing data such as function arguments, return addresses, and local variables during program execution.

Key points about the rsp register:

• Automatically managed: The rsp register is adjusted automatically when certain instructions like push or pop are used:
  • push decreases the value of rsp and stores data at the new stack location.
  • pop retrieves data from the top of the stack and increases rsp.
• Stack growth: In x86-64 architecture, the stack grows downward (towards lower memory addresses). This means that when data is pushed onto the stack, the value of rsp decreases.
• Manual adjustment: In some cases, assembly programmers adjust rsp directly (e.g., for aligning the stack or reserving space for local variables).

example

push rax    ; Decrease rsp, store rax on the stack
pop rbx     ; Retrieve value from stack into rbx, increase rsp

Important

Ensure rsp is 16-byte aligned before function calls to meet Application Binary Interface (ABI) requirements.

Exporting Symbols

To use assembly functions in a C program, you need to export them using the global directive. For example:

global ft_strlen

This makes ft_strlen symbol available to the linker.

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Testing All Functions

The test rule in the Makefile is used to compile and run tests for all implemented functions. It ensures that all functionality behaves as expected.

make test

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Sources

• Medium
• Cheat Sheet
• Another Cheat Sheet