Roadmap Memory & data Section 5: Procedures & Stacks - PowerPoint PPT Presentation

University of Washington University of Washington Roadmap Memory & data Section 5: Procedures & Stacks Integers & floats Machine code & C C: Java: x86 assembly � Stacks in memory and stack operations Car c = new Car(); car *c = malloc(sizeof(car)); Procedures & stacks c->miles = 100; c.setMiles(100); Arrays & structs � The stack used to keep track of procedure calls c.setGals(17); c->gals = 17; Memory & caches float mpg = get_mpg(c); float mpg = � Return addresses and return values Processes free(c); c.getMPG(); Virtual memory � Stack-based languages Memory allocation Assembly get_mpg: Java vs. C pushq %rbp � The Linux stack frame language: movq %rsp, %rbp ... � Passing arguments on the stack popq %rbp ret � Allocating local variables on the stack OS: Machine � Register-saving conventions 0111010000011000 100011010000010000000010 code: � Procedures and stacks on x64 architecture 1000100111000010 110000011111101000011111 Computer system: Procedures and Stacks Procedure Calls University of Washington University of Washington Memory Layout Memory Layout 2 N -1 local variables; Managed “automatically” Stack Stack procedure context writable; not executable (by compiler) Dynamic Data Dynamic Data variables allocated with writable; not executable new or malloc Managed by programmer (Heap) (Heap) static variables Static Data Static Data writable; not executable Initialized when process starts (including global variables (C)) Literals Literals literals (e.g., “example”) Read-only; not executable Initialized when process starts Instructions Instructions Initialized when process starts Read-only; executable 0 Procedures and Stacks Procedures and Stacks

University of Washington University of Washington IA32 Call Stack IA32 Call Stack: Push Stack “Bottom” Stack “Bottom” � Region of memory managed � pushl Src with a stack “discipline” Increasing � Grows toward lower addresses Addresses Increasing � Customarily shown “upside-down” Addresses � Register %esp contains lowest stack address Stack Grows = address of “top” element Down Stack Grows Stack Pointer: %esp Down Stack Pointer: %esp Stack “Top” Stack “Top” Procedures and Stacks Procedures and Stacks University of Washington University of Washington IA32 Call Stack: Push IA32 Call Stack: Pop Stack “Bottom” Stack “Bottom” � pushl Src � popl Dest � Fetch value from Src Increasing Increasing � Decrement %esp by 4 (why 4?) Addresses Addresses � Store value at address given by %esp Stack Grows Stack Grows Down Down Stack Pointer: %esp -4 Stack Pointer: %esp Stack “Top” Stack “Top” Procedures and Stacks Procedures and Stacks

University of Washington University of Washington IA32 Call Stack: Pop Section 5: Procedures & Stacks Stack “Bottom” � Stacks in memory and stack operations � popl Dest � The stack used to keep track of procedure calls � Load value from address %esp Increasing � Write value to Dest � Return addresses and return values Addresses � Increment %esp by 4 � Stack-based languages � Item still remains, we are just not � The Linux stack frame referencing it any more � Passing arguments on the stack � Not really moving out of memory � Allocating local variables on the stack Stack Grows Stack Pointer: %esp � Register-saving conventions Down +4 � Procedures and stacks on x64 architecture Stack “Top” Procedures and Stacks Procedure Calls University of Washington University of Washington Procedure Call Overview Procedure Call Overview Caller Caller … … Callee <set up args> <save regs> Callee call <set up args> <create local vars> <clean up args> call <save regs> … <find return val> <clean up args> <create local vars> <set up return val> … <restore regs> … <destroy local vars> <find return val> <set up return val> return … <destroy local vars> <restore regs> return � Callee must know where to find args � Callee must know where to find “return address” � The convention of where to leave/find things is called the � Caller must know where to find return val procedure call linkage � Details vary between systems � Caller and Callee run on same CPU → use the same registers � We will see the convention for IA32/Linux in detail � Caller might need to save registers that Callee might use � What could happen if our program didn’t follow these conventions? � Callee might need to save registers that Caller has used Procedure Calls Procedure Calls

University of Washington University of Washington Procedure Control Flow Procedure Control Flow � Use stack to support procedure call and return � Use stack to support procedure call and return � Procedure call: call label � Procedure call: call label � Push return address on stack � Push return address on stack � Jump to label � Jump to label � Return address: � Address of instruction 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 return address from stack � Jump to address Procedure Calls Procedure Calls University of Washington University of Washington Procedure Call Example Procedure Call Example 804854e: e8 3d 06 00 00 call 8048b90 <main> 804854e: e8 3d 06 00 00 call 8048b90 <main> 8048553: 50 pushl %eax 8048553: 50 pushl %eax call 8048b90 call 8048b90 0x110 0x110 0x110 0x10c 0x10c 0x10c 0x108 123 0x108 123 0x108 123 0x104 %esp 0x108 %esp 0x108 %esp 0x108 %eip 0x804854e %eip 0x804854e %eip 0x804854e %eip: program counter %eip: program counter Procedure Calls Procedure Calls

University of Washington University of Washington Procedure Call Example Procedure Call Example 804854e: e8 3d 06 00 00 call 8048b90 <main> 804854e: e8 3d 06 00 00 call 8048b90 <main> 8048553: 50 pushl %eax 8048553: 50 pushl %eax call 8048b90 call 8048b90 0x110 0x110 0x110 0x110 0x10c 0x10c 0x10c 0x10c 0x108 123 0x108 123 0x108 123 0x108 123 0x104 0x104 0x8048553 %esp 0x108 %esp 0x108 %esp 0x108 %esp 0x108 0x104 %eip 0x804854e %eip 0x804854e 0x8048553 %eip 0x804854e %eip 0x804854e 0x8048553 %eip: program counter %eip: program counter Procedure Calls Procedure Calls University of Washington University of Washington Procedure Call Example Procedure Return Example 804854e: e8 3d 06 00 00 call 8048b90 <main> 8048591: c3 ret 8048553: 50 pushl %eax ret call 8048b90 0x110 0x110 0x110 0x10c 0x10c 0x10c 0x108 123 0x108 123 0x108 123 0x104 0x8048553 0x104 0x8048553 %esp 0x104 %esp 0x108 %esp 0x108 0x104 %eip 0x8048591 %eip 0x804854e %eip 0x8048553 + 0x000063d 0x8048b90 %eip: program counter %eip: program counter Procedure Calls Procedure Calls

University of Washington University of Washington Procedure Return Example Procedure Return Example 8048591: c3 ret 8048591: c3 ret ret ret 0x110 0x110 0x110 0x110 0x10c 0x10c 0x10c 0x10c 0x108 123 0x108 123 0x108 123 0x108 123 0x104 0x8048553 0x8048553 0x104 0x8048553 0x8048553 %esp 0x104 %esp 0x104 %esp 0x104 %esp 0x104 %eip 0x8048591 %eip 0x8048591 %eip 0x8048591 %eip 0x8048591 0x8048553 %eip: program counter %eip: program counter Procedure Calls Procedure Calls University of Washington University of Washington Procedure Return Example Return Values 8048591: c3 ret � By convention, values returned by procedures are placed in the %eax register ret � Choice of %eax is arbitrary, could have easily been a different register � Caller must make sure to save that register before calling a 0x110 0x110 callee that returns a value � Part of register-saving convention we’ll see later 0x10c 0x10c 0x108 123 0x108 123 � Callee placed return value (any type that can fit in 4 bytes – 0x104 0x8048553 0x8048553 integer, float, pointer, etc.) into the %eax register � For return values greater than 4 bytes, best to return a pointer to them %esp 0x104 %esp 0x108 0x104 � Upon return, caller finds the return value in the %eax register %eip 0x8048591 %eip 0x8048591 0x8048553 %eip: program counter Procedure Calls Procedure Calls