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Today’s topics

Today’s topics. Procedures Passing values to/from procedures Saving registers Documenting procedures The system stack Review for Midterm Exam. Passing values/addresses to/from procedures. Methods: Pass parameters in registers Use shared memory (global variables)

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Today’s topics

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  1. Today’s topics • Procedures • Passing values to/from procedures • Saving registers • Documenting procedures • The system stack • Review for Midterm Exam

  2. Passing values/addresses to/from procedures • Methods: • Pass parameters in registers • Use shared memory (global variables) • Pass parameters on the system stack

  3. 1. Pass parameters in registers • Set up registers before call and/or before return • Generally … it’s a bad idea to pass parameters in registers • Procedure might change/restore register contents • However • some Irvine library procedures require values in registers (e.g., “Receives” and “Preconditions” for ReadString) • some Irvine library procedures return values in registers (e.g., “Returns” for ReadInt)

  4. 2. Use shared memory(global variables) • Set up memory contents before call and/or before return • Generally … it’s a bad idea to use global variables • Procedure might change memory contents needed by other procedures (unwanted side-effects) • For now … we use globals. • Later we will pass parameters on the system stack.

  5. In all cases, when a procedure is called: • be aware of preconditions • What conditions must be true before the procedure can perform its task? • be aware of what registers are changed (document!) • save and restore registers if necessary

  6. Saving registers • If a procedure changes any registers, the calling procedure might lose important data • Registers may be saved before call, and restored after return • Registers may be saved by the procedure, and restored before the return

  7. Saving/restoring registers • Methods: • Move register contents to named memory locations, then restore after procedure returns. • Use pushad and popad • Option 1: calling procedure pushes before call, pops after return • Option 2: procedure pushes at beginning, and pops before the return • Use the USES directive • Save selected registers on the system stack • (Much) more later about this

  8. Method 1: Save register contents in memory • Example (in main): mov aReg, eax ;save registers mov bReg, ebx mov eax, count ;set parameters mov ebx, OFFSET val call someProc mov eax, aReg ;restore registers mov ebx, bReg …

  9. Methods 2: Save all registers on the system stack* • PUSHAD pushes the 32-bit general-purpose registers onto the stack • order: EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI • POPAD pops the same registers off the stack in reverse order • Note: it’s best to use 32-bit (DWORD) operands • * More about this soon …

  10. Method 2: Save all registers on the system stack • Example (Option 1: in calling procedure): pushad ;save registers call someProc popad ;restore registers …

  11. Method 2: Save all registers on the system stack • Example (Option 2: in the called procedure): calcSum PROC pushad ;save registers … ;procedure body … popad ;restore registers ret calcSum ENDP

  12. Method 2: Save all registers on the system stack • Warnings: • Be sure that values don't get lost. • Be sure that the system stack is properly aligned • More later …

  13. Method 3: USES Directive • Specifies which registers will be preserved • Saves specified registers on the system stack before any procedure statements are executed. • Restores registers before the procedure returns. • MASM adds (invisible) save/restore code for specified registers • at the beginning and end of the procedure

  14. Method 3: USES Directive • Example (in the procedure): calcSum PROC USES esi ecx … ;procedure body … ret calcSum ENDP • Notes: • no commas • Be careful !USES affects the system stack

  15. Documenting Procedures • Documentation for each procedure: • A description of all tasks accomplished by the procedure. • Receives: A list of input parameters; state usage and requirements. • Returns: A description of values returned by the procedure. • Preconditions: List of requirements that must be satisfied before the procedure is called. • Registers changed: List of registers that may have different values than they had when the procedure was called If a procedure is called without satisfying its preconditions, the procedure's creator makes no promise that it will work.

  16. Procedure Documentation (example) ;calculate: procedure to calculate ; the summation of integers from ; a to b. ;receives: none (a, b, sum are global) ;returns: sum = a+(a+1)+ ... +b ;preconditions: a <= b ;registers changed: eax,ebx,ecx

  17. Questions on procedures?

  18. System Stack(Runtime Stack) • The operating system maintains a stack • Implemented in memory • LIFO structure • Managed by the CPU, using two registers • SS: address of stack segment • ESP: stack pointer (always points to “top” of stack) • i.e., ESP contains the address of the top of the stack

  19. PUSH Operation • A push operation • decrements the stack pointer by 4 • copies a value into the location pointed to by the stack pointer. • Actual decrement depends on the size of the operand • Note: it’s best to use 32-bit (DWORD, 4-byte) operands

  20. Example PUSH Stack Segment in Memory Suppose that ecx contains 317 and esp contains 0200h. In this case, [esp] is 25 * The next instruction is push ecx Execute push ecx 317 esp: 01FCh [esp]:317 esp: 0200h [esp]: 25 Note: esp is decremented, then 317 is stored in the stack *Note: [esp] means “contents of memory at the address in esp”

  21. POP Operation • A pop operation • Copies value at ESP into a register or variable. • increments the stack pointer by 4 • Actual increment depends on the size of the operand • Note: it’s best to use 32-bit (DWORD , 4-byte) operands

  22. Example POP Stack Segment in Memory Suppose that esp contains 01FCh. In this case, [esp] is 317. The next instruction is pop eax Execute pop eax eax now contains 317 esp: 0200h [esp]:25 esp: 01FCh [esp]: 317 Note: 317 is copied to EAX, then then ESP is incremented. Memory contents unchanged.

  23. PUSH and POP Instructions(32-bit) • PUSH syntax: • PUSH r/m32 • PUSH immed • POP syntax: • POP r/m32

  24. Using PUSH and POP Save and restore registers when they contain important values. POP operands occur in the opposite of the order of PUSH operands. push ecx ; save registers push ebx mov ecx,100h mov ebx,0 ; etc. pop ebx ; restore registers pop ecx

  25. Example: Nested Loop Push the outer loop counter before entering the inner loop. Pop the outer loop counter when the inner loop terminates. mov ecx,100 ; set outer loop count L1: ; begin the outer loop push ecx ; save outer loop count mov ecx,20 ; set inner loop count L2: ; begin the inner loop ; ; loop L2 ; repeat the inner loop pop ecx ; restore outer loop count loop L1 ; repeat the outer loop

  26. When not to push • Be sure that PUSH does not hide a return address • Be sure that POP does not lose a return address and/or replace needed values

  27. Questions? • Midterm Exam Review

  28. Midterm Exam • Format: • 40% Calculations, definitions, short answer, multiple choice • 40% MASM code tracing • 20% Writing MASM code • Covers: • Lectures # 1 – 10 • MASM programming • Programs #1 & 2 • Homework #1 • Quizzes #1 & 2 • Calculator and one 8x11 notecard permitted (both sides) • No sharing

  29. Midterm Exam Topics • How computers work • CISC: microprograms • Memory, CPU, registers, ALU, buses, etc. • VonNeumann architecture, Instruction Execution Cycle, pipelining, etc. • Internal representation • Numbers, characters, instructions, addresses, etc. • Floating-point, hamming codes, etc. • External representation • Binary, decimal, hexadecimal • Peripheral devices • Magnetic disk drives • IA-32 architecture • Register names, etc. • Byte-ordering (little-endian)

  30. Midterm Exam Topics • Assembly Language • Related to high-level languages • Related to machine languages • Related to architecture • Motivations for using assembly language • Program development • Modularization • Incremental development/testing • Debugging

  31. Midterm Exam Topics • MASM • Program structure, commenting • Assembly, linking, etc. • Segments, directives, etc. • Labels, identifiers, constants • Data types, declarations • BYTE, WORD, DWORD, etc. • strings • Addressing modes • immediate, register, direct (i.e., memory), indirect (i.e., [register]) • Instructions: • mov, push, pop • add, sub, mul, div, inc, dec • cmp, jmp, j<condition>, loop • call, ret

  32. Midterm Exam Topics • MASM • Control structures • Procedures, procedure calls • return address, etc • Decisions • Repetition • Data validation • System stack • Procedure calls and returns • Fundamentals of push and pop

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