CS208, Wednesday 26 Jan 2022

Today

• Map memory by hand
• Run-through of a few x86_64 instructions and addressing modes
• What does a C loop look like in x86_64? An if-statement? A function call? Return values? Different types of parameters?

For your entertainment (?)

• What exactly should we be automating?

Exercise: 15+5 minutes

• Write a little program, say "memory-mapper.c" on mantis
• Use the & operator and printf's %p to figure out where stuff gets placed in memory by the compiler.
  • local variables
  • literal strings
  • memory allocated by malloc
  • function parameters
  • functions themselves
  • sequences of local variables in the same function
  • local variables in one function vs. local variables in a function that the first one calls
  • anything else you can think of
• Try to explore the boundaries of these regions of memory
• Draw a map--here's some sample code

Some x86_64 instructions and addressing modes

• mov (and movq, movl, movw, movb)
• addressing modes
• lea
• add (2- and 3-argument versions)
• push, pop
• jmp, js, jl, jle, je
• test, cmp, and the FLAGS register
  
  (from https://nekosecurity.com/x86-64-assembly/part-6-arithmetic-operation)

A little note on optimization

• A tiny bit of compiler history
• Conflicting goals (performance, debuggability)
• Pedagogical goals (-O0? -Og?)

Mapping C constructs to x86_64 (probably Friday)

• if-else
• loop
• function call
• ??