Ithaca College Machine-Level Programming VII: Procedures Comp 21000: Introduction to Computer Systems & Assembly Lang Spring 2017

Mechanisms in Procedures • y = Q(x); print(y) } Passing control To beginning of procedure code Back to return point Passing data Procedure arguments Return value Memory management Allocate during procedure execution Deallocate upon return Mechanisms all implemented with machine instructions x86-64 implementation of a procedure uses only those mechanisms required int Q(int i) { int t = 3*i; int v[10]; • return v[t]; }

Today Procedures Stack Structure Calling Conventions Passing control Ithaca College Today Procedures Stack Structure Calling Conventions Passing control Passing data Managing local data Illustration of Recursion

x86-64 Stack Stack “Bottom” Ithaca College x86-64 Stack Stack Pointer: %rsp Stack Grows Down Increasing Addresses Stack “Top” Stack “Bottom” Region of memory managed with stack discipline Grows toward lower addresses Register %rsp contains lowest stack address address of “top” element

x86-64 Stack: Push Stack “Bottom” pushq Src Stack Pointer: %rsp Ithaca College x86-64 Stack: Push Stack “Bottom” pushq Src Fetch operand at Src Decrement %rsp by 8 Write operand at address given by %rsp Increasing Addresses Stack Grows Down Stack Pointer: %rsp Stack “Top” -8

x86-64 Stack: Pop Stack “Bottom” popq Dest Stack Pointer: %rsp Ithaca College x86-64 Stack: Pop Stack “Bottom” popq Dest Read value at address given by %rsp Increment %rsp by 8 Store value at Dest (must be register) Increasing Addresses Stack Grows Down +8 Stack Pointer: %rsp Stack “Top”

pushl and popl instructions Effect Description pushq S R[%rsp] R[%rsp]–8; M[R[%rsp]]S Push on runtime stack popq D DM[R[%rsp]]; R[%rsp]R[%rsp]+8 Pop from runtime stack Notes: 1. both instructions take a single operand 2. Stack is a place in memory allocated to a process 3. Stack grows from smaller addresses to larger addresses 4. %rsp holds the address of the current top of stack 5. When pushq a value, first increment %rsp by 8, then write the value at the new top of stack address. Effect of a pushq %rbp instruction: subq $8, %rsp movq %rbp, (%rsp) Effect of a popq %rax instruction: movq (%rsp), %rax addq $8, %rsp

Initially pushq %rax popq %rdx 0x123 0x108 %rax %rdx %rsp 0x123 0x100 %rax %rdx %rsp 0x123 0x108 %rax %rdx %rsp • Stack “top” Stack “bottom” Stack “bottom” Stack “bottom” • • Increasing address Stack Grows Down 0x108 0x108 0x108 0x123 0x123 0x100 Stack “top” Stack “top” 1. By convention we draw the stack with the top towards the bottom 2. stack grows toward lower addresses

Today Procedures Stack Structure Calling Conventions Passing control Ithaca College Today Procedures Stack Structure Calling Conventions Passing control Passing data Managing local data Illustration of Recursion

Code Examples void multstore (long x, long y, long *dest) { long t = mult2(x, y); *dest = t; } 0000000000400540 <multstore>: 400540: push %rbx # Save %rbx 400541: mov %rdx,%rbx # Save dest 400544: callq 400550 <mult2> # mult2(x,y) 400549: mov %rax,(%rbx) # Save at dest 40054c: pop %rbx # Restore %rbx 40054d: retq # Return long mult2 (long a, long b) { long s = a * b; return s; } 0000000000400550 <mult2>: 400550: mov %rdi,%rax # a 400553: imul %rsi,%rax # a * b 400557: retq # Return

Procedure Control Flow Ithaca College Procedure Control Flow Use stack to support procedure call and return Procedure call: call label Push return address on stack Jump to label Return address: Address of the next instruction right after call Example from disassembly 804854e: e8 3d 06 00 00 call 8048b90 <main> 8048553: 50 pushl %eax Return address = 0x8048553 Procedure return: ret Pop address from stack Jump to address

Control Flow Example #1 • 0x130 0000000000400540 <multstore>: • 400544: callq 400550 <mult2> 400549: mov %rax,(%rbx) 0x128 0x120 %rsp 0x120 %rip 0x400544 0000000000400550 <mult2>: 400550: mov %rdi,%rax • 400557: retq

Control Flow Example #2 • %rsp 0x120 0x128 0x130 0x118 %rip 0000000000400540 <multstore>: • 400544: callq 400550 <mult2> 400549: mov %rax,(%rbx) 0x400549 0x118 0x400550 0000000000400550 <mult2>: 400550: mov %rdi,%rax • 400557: retq

Control Flow Example #3 • %rsp 0x120 0x128 0x130 0x118 %rip 0000000000400540 <multstore>: • 400544: callq 400550 <mult2> 400549: mov %rax,(%rbx) 0x400549 0x118 0x400557 0000000000400550 <mult2>: 400550: mov %rdi,%rax • 400557: retq

Control Flow Example #4 • 0x130 0000000000400540 <multstore>: • 400544: callq 400550 <mult2> 400549: mov %rax,(%rbx) 0x128 0x120 %rsp 0x120 %rip 0x400549 0000000000400550 <mult2>: 400550: mov %rdi,%rax • 400557: retq