The Stack

Objectives

Notes

• This is Jorgensen chapter 9
• The stack is necessary for function calls.
  • It allows us to build a place to store function local data
• Remember
  • A stack is a data structure
  • It holds data in a last-in-first-out manner
  • Operations include
    • push: add something to the top of the stack
    • pop: remove something from the top of the stack
• In general, the stack is a portion of memory shared with the heap.
  • Usually the stack is in the high portion of this memory and grows down.
  • The heap grows up.
• The stack growing down is sort of counter intuitive to our general notion of a stack
  • Just remember, pushing something will decrease the address of the top of the stack.
  • And popping something will do the opposite.
• If you think about it, when you are busy you probably employ a stack
  • You are working on a task.
  • You get an email, so you push the task on the stack and start working on the email
  • You have someone stop in, so you push the email and start consulting with the visitor
  • You get an important phone call, so you push the visitor (?) and start working on the phone call.
  • You finish the phone call, so you return to the visitor (by popping them off the stack)
  • You finish the visitor, so you return to the email (by popping them off the stack)
  • You finish the email, so you return to your base task. (ya right)
• We use the stack for
  • Temporary storage of registers inside a function
  • passing parameters
  • Storing information necessary for function call maintenance
• Intel supports two instructions for manipulating the stack
  • push operand64 places the operand on the stack and changes the value of the stack pointer by 8
  • pop operand64 removes the operand on the stack and changes the value of the stack pointer by 8
  • The operand my not be an immediate.
• When using a the stack, we generate a data structure
  • The activation record or the stack frame
  • This holds the local temporary data for the current function.
  • The beginning of this (high memory) is pointed to by the bp (rbp, ebp, bp)
  • The end of this (lower memory) is pointed to by the sp, (rsp, esp, sp)
• A calling convention
  • Is a set of agreed on policies so that programs can work together.
  • Describes how registers and memory should be used for function calls
  • This is where the temporary and preserved registers are defined.
• Our main concern here is that the stack pointer must be 16-byte alligned.
  • Or the stack pointer must contain a multiple of 16
  • Or it must end in a multiple of 0x10 (0x1000, 0x1010, 0x1020...)
  • Or the last digit must be a 0 in hex
• Remember addresses are byes in any case.
• So subtracting 1 from the stack pointer creates a 1 byte "padding" in the stack.
• You can push a byte or a quad word.
  • So you will either need to add 8 or 15 to the stack to get it alligned.