OS Memory Management (x86)

Umm.. Actually ☝️

The x86 architecture and MIPS share a lot of common ideas. The % registers we talk about below are just different flavours of the $ registers from MIPS.

Components

• Like MIPS, you have program registers:
  • %eax - The main register for holding results of arithmetic
  • %ebx - The main register for holding data.
  • %esp - Extended Stack Pointer - Points to the top of the stack. This updates every new operation that’s done.
  • %ebp - Extended Base Pointer - Whenever you jump to a new function, this points to the base of that function.

Memory Address Space in x86:

Let’s walk through the provisioning of memory in x86… Below is a diagram of it, as well as of C code we’re about to run.

Quickly! What’s a stack frame!

Every time you run a function, you’re actually jumping to a different location in memory. But there’s stuff you need to remember. For example:

• The parameters you’re bringing along with you.
• Where to return to once you’re done. (Return Address)
• Stack Base Pointer - Look at %ebp above.
• etc.

Note: Remember this is Virtual Memory, so the addresses are actually all split up and correspond to different physical addresses. Fucking love OS.

Step By Step:

1. Firstly, you add the function arguments to memory, in reverse order.
2. You push the return address (the place to return to once the function is done).
3. Stack Base Pointer %ebp - look above.
4. Exception handlers
5. Canary goes here. Cos if your local variables are set to overwrite the base pointer, they’d trip up here first
6. Local Variables

Careful!

• You should be incredibly careful when dealing directly with memory. Improper handling can lead to Data Overflow