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Chapter 6 Procedures PowerPoint Presentation
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Chapter 6 Procedures

Chapter 6 Procedures

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Chapter 6 Procedures

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  1. Chapter 6 Procedures

  2. 6.1 The 80x86 Stack

  3. Hardware Stack • Allocated with directive, for example.STACK 4096allocates 4096 uninitialized memory bytes • Visual Studio project setting can also be used to allocate stack space • Most access is indirect, through the stack point register ESP • Operating system initializes ESP to point to byte above stack • As program executes, it points to the last item pushed on the stack

  4. push instruction • Usual format: push source • source can in memory, register or immediate • doubleword or word pushed on the stack • Formats to use when the assembler cannot determine operand size • pushd source • pushw source

  5. push execution • ESP decremented by size of operand • Operand stored in stack where ESP points after being decremented • Flags not changed

  6. push Example

  7. pop instruction and execution • Usual format: pop destination • doubleword or word destination can in memory or register • Operand stored in stack where ESP points is copied to destination • ESP incremented by size of operand after the value is copied • Flags not changed

  8. pop Example

  9. Pushing/Popping Flags • pushfd pushes EFLAGS register contents onto stack • popfd pops doubleword from top of stack into EFLAGS

  10. Viewing the Stack with Debugger • Stop at breakpoint • View registers • ESP contains address of byte above stack • Subtract number of bytes to view from the address in ESP • Use this as the starting address at which to view memory • Example – if ESP contains 0042FFC4, using 0042FFC4 – 20 = 0042FFA4 lets you see the top 32 (2016) bytes of the stack

  11. 6.2 32-bit Procedures with Value Parameters

  12. Terminology • Procedure - a subprogram that is essentially a self-contained unit • Main program or another subprogram calls a procedure • A procedure may simply do a task or it may return a value • value-returning procedure is sometimes called a function

  13. Procedure Concepts • Transfer of control from calling program to procedure and back • Passing parameter values to procedure and results back from the procedure • Having procedure code that is independent of the calling program The cdecl protocol provides one standard implementation scheme

  14. Procedure Definition • In a code segment with body statements bracketed by PROC and ENDP directives giving procedure name .CODE procName PROC ; procedure body ... procName ENDP

  15. Transferring Control to a Procedure • In the “main” program, use callprocName • The next instruction executed will be the first one in the procedure

  16. Returning from a Procedure • In the procedure, use ret • The next instruction executed will be the one following the call in the “main” program

  17. How call Works • The address of the instruction following the call is pushed on the stack • The instruction pointer register EIP is loaded with the address of the first instruction in the procedure

  18. How ret Works • The doubleword on the top of the stack is popped into the instruction pointer register EIP • Assuming that this was the address of the instruction following the call, that instruction will be executed next • If the stack has been used for other values after the call, these must be removed before the ret instruction is executed

  19. Alternative ret Format • ret n • n is added to ESP after the return address is popped • This is most often used to logically remove procedure parameters that have been pushed onto the stack • Not used in cdecl protocol

  20. Parameter Terminology • A procedure definition often includes parameters (also called formal parameters) • These are associated with arguments (also called actual parameters) when the procedure is called • For a procedure's in (pass-by-value) parameters, values of the arguments are copied to the parameters when the procedure is called • These values are referenced in the procedure using their local names (the identifiers used to define the parameters)

  21. Implementing Value Parameters • Parameter values normally passed on the stack • Pushed in reverse order from argument list • Example • Design or high-level code: sum := add2(value1, value2) • 80x86 implementation:push ecx ; assuming value2 in ECXpush value1 ; assuming value1 in memorycall add2 ; call procedure to find sumadd esp, 8 ; remove parameters from stackmov sum, eax ; sum in memory • With the cdecl protocol the calling program must remove parameters from the stack • A single integer value normally returned in EAX

  22. Procedure Entry Code • Since the stack pointer ESP may change, a procedure starts with entry code to set the base pointer EBP to an address in the stack • This location is fixed until exit code restores EBP right before returning • In the procedure body, parameters are located relative to EBP • Entry code also saves contents of registers that are used locally within the procedure body • Exit code restores these registers

  23. Example Procedure add2 PROC ; add two words passed on the stack ; return the sum in the EAX register push ebp ; save EBP mov ebp,esp ; establish stack frame mov eax,[ebp+8] ; copy first parameter value add eax,[ebp+12] ; add second parameter value pop ebp ; restore EBP ret ; return add2 ENDP

  24. Stack frame upon entry to procedure add2 and after EBP established in entry code

  25. Accessing Parameters in a Procedure • Use based addressing • Since the value is actually on the stack, [EBP+n] references the value • Example:mov eax,[ebp+8]copies the last parameter pushed to EAX

  26. Saving/Restoring Registers • Push registers in entry code after EBP is established • Exit code pops registers in the opposite order • Calling program’s flag values from can be saved with pushfd/popfd

  27. Entry and Exit Code Summary • Entry code: push ebp ; establish stack frame mov ebp, esp push ... ; save registers ... push ... pushfd ; save flags • Exit code: popfd ; restore flags pop ... ; restore registers ... pop ... pop ebp ; restore EBP ret ; return

  28. Procedure Call Summary

  29. 6.3 Additional 32-bit Procedure Options

  30. Reference Parameters • The address of the argument instead of its value is passed to the procedure • Reference parameters are used: • To send a large argument (for example, an array or a structure) to a procedure • To send results back to the calling program as argument values

  31. Passing an Address • lea instruction can put address of an argument in a register, and then the contents can be pushed on the stack lea eax, minimum ; 3rd parameter push eax

  32. Returning a Value in a Parameter • Get address from stack • Use register indirect addressing mov ebx, [ebp+16] ; get addr of min ... mov [ebx], eax ; min := a[i]

  33. Allocating Local Variable Space • save EBP and establish stack frame • subtract number of bytes of local space from ESP • save registers used by procedure • Access both parameters and local variables in procedure body using based addressing • return value, if any, goes in EAX • restore saved registers • copy EBP to ESP • restore EBP • return New entry and exit code actions are bold

  34. Recursive Procedure • Calls itself, directly or indirectly • Many algorithms are very difficult to implement without recursion • A recursive call is coded just like any other procedure call

  35. Separate Assembly • Procedure code can be in a separate file from the calling program • File with call has an EXTERN directive to describe procedure that is defined in another file • ExampleEXTERN minMax:PROC

  36. 64-bit procedures

  37. Stack Usage • Normally push and pop quadwords • call pushes a 64-bit address on the stack • ret pops a 64-bit address into RIP • Calling procedure’s entry code must reserve space on the stack for arguments. • Typical code includes sub rsp,120

  38. 64-bit protocol • Different from cdecl • First four arguments passed in registers • Later arguments copied to stack • Fifth argument is copied to [RSP+32]

  39. Register Usage • RAX, RCX, RDX, and R8-R11 are called volatile • A called procedure is free to change them • RBX, RDI, RSI, RBP, RSP and R12-R15 are called nonvolatile • A called procedure has the responsibility of preserving them • In practice, it is safest to preserve any register that you don’t want destroyed by a called procedure

  40. Procedure to Add Five Integers ; void add5(int x1,int x2,int x3,int x4,int x5); ; returns sum of arguments add5 PROC mov eax, ecx ; x1 add eax, edx ; x2 add eax, r8d ; x3 add eax, r9d ; x4 add eax, DWORD PTR [rsp+40] ; x5 ret ; return add5 ENDP

  41. 6.5 Macro Definition and Expansion

  42. Macro • Expands to the statements it represents • Expansion then assembled • Resembles a procedure call, but is different • Each time a macro appears in code, it is expanded • There is only one copy of procedure code

  43. Macro Definition name MACRO list of parameters assembly language statements ENDM • Parameters in the MACRO directive are ordinary symbols, separated by commas • The assembly language statements may use the parameters as well as registers, immediate operands, or symbols defined outside the macro

  44. Macro Example 1 Given the definition add2 MACRO nbr1, nbr2 ; put sum of two doubleword parameters in EAX mov eax, nbr1 add eax, nbr2 ENDM the macro call add2 value, 30 ; value + 30 expands to ; put sum of two doubleword parameters in EAX mov eax, value add eax, 30

  45. Macro Example 2 Given the definition add2 MACRO nbr1, nbr2 ; put sum of two doubleword parameters in EAX mov eax, nbr1 add eax, nbr2 ENDM the macro call add2 eax, ebx ; sum of two values expands to ; put sum of two doubleword parameters in EAX mov eax, eax add eax, ebx

  46. Macro Example 3 Given the definition add2 MACRO nbr1, nbr2 ; put sum of two doubleword parameters in EAX mov eax, nbr1 add eax, nbr2 ENDM the macro call add2 value expands to ; put sum of two doubleword parameters in EAX mov eax, value add eax, (won’t assemble)

  47. Macros in IO.H • Each expands to a statement that calls a procedure, for example atod MACRO source ; convert ASCII string ; to integer in EAX lea eax,source ; source address to AX push eax ; source parameter on stack call atodproc ; call atodproc(source) add esp, 4 ; remove parameter ENDM