Lecture 2: x86 Assembly and Call Stack

• Slides
• Playlist (length: 51:17)
• Optional: Some students have found these slides from Matthias Vallentin helpful: Normal x86 function call, a crash, a control-flow diversion, and code injection.

This entire lecture is optional and won’t be tested directly on exams, but you may find it useful for Project 1. You can use the sanity check questions to determine which section(s) you want to review.

Number Representation

How many bytes are represented by `0x36`? Why?

1 byte. Each hex digit represents 1 nibble = 4 bits, so two hex digits represent 2 nibbles = 8 bits = 1 byte.

C Memory

The compiler, assembler, linker, and loader are used to execute a C program. Which one is responsible for converting assembly code into machine code?

The assembler.

From lowest address to highest address, what are the four sections of C memory? What is each section used for? Which section grows up during program execution? Which section grows down during program execution?

From lowest address to highest address: the code section contains machine code instructions for the program. The static section contains constants declared outside functions. The heap grows up, and contains dynamically allocated memory (from calling malloc). The stack grows down, and contains local variables from function calls.

Little-endian Words

Stack Analogy

Stack Design

In x86, what are the registers ebp, esp, and eip responsible for? What do we mean when we say these registers contain pointers?

ebp determines the top of the current stack frame. esp determines the bottom of the current stack frame (and the bottom of the entire stack). eip determines what instruction (in the code section) the program is currently executing. All three registers contain pointers because the values stored in them are addresses in memory.

Function Call

Function Call in x86 Assembly

You can find the complete set of function call slides here.