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CS 3843 Computer Organization Prof. Qi Tian Fall 2013 1.

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1 CS 3843 Computer Organization Prof. Qi Tian Fall

2 Chapter 3 Machine-Level Representations of Programs 11/11/2013 (Monday) – Section Procedure – Quiz 4 2

3 Chapter 3 Machine-Level Representations of Programs 11/08/2013 (Friday) – Tracking a recursive procedure Section – Solution is posted under Resources. 11/06/2013 (Wednesday) – Tracking a procedure Section – Tracking a recursive procedure Section – Reminder: Quiz on Friday Nov. 8 11/04/2013 (Monday) – Section Procedure – 2 nd Midterm Exam on Friday Nov. 15 3

4 Chapter 3 Machine-Level Representations of Programs 11/01/2013 (Friday) – Loop slides – Questions on Assignment 4 10/30/2013 (Wednesday) – Practice Problems on Conditional Flags – Reminder: Quiz on Friday Nov. 1st 10/28/2013 (Monday) – Jump Instructions slides – Assignment 4 is due Nov. 4. 4

5 Chapter 3 Machine-Level Representations of Programs The week of 10/21-10/25 – Replacement Lectures by Prof. Turgay Korkmaz and TA – Slides

6 Chapter 3 Machine-Level Representations of Programs 10/18/2013 (Friday) – Shift Operations – Examples 4-8 – Note: Conference Travel Oct Replacement Lectures by Prof. Turgay Korkmaz and TA 10/16/2013 (Wednesday) – Examples 2-3 – Arithmetic and Logical Operations – Practice Problems 4 and 5 – Slides /14/2013 (Monday) – Movement Instructions – Practice Problems 2 and 3, Example 1 – Slides

7 Chapter 3 Machine-Level Representations of Programs 10/11/2013 (Friday) – Operand forms – Practice Problem 1 10/09/2013 (Wednesday) – An introduction to Assembly Code Read Sections Slides

8 Example of Assembly Codes Example 1 – sum.c int add(int x, int y) { int z; z=x+y; return z; } What is its assembly code? gcc –O1 –S sum.c Department Machines: elk01(~08).cs.utsa.edu 8

9 sum.s.file "sum.c".text.globl add.type add: pushl %ebp movl %esp, %ebp movl 12(%ebp), %eax addl 8(%ebp), %eax popl %ebp ret.size add,.-add.ident "GCC: (Ubuntu ubuntu4) Ignore the lines that start with. The pushl and popl save and restore %ebp In movl, the first argument is the source, and the second is the destination addl adds the source and destination and stores the results in the destination %eax is used to hold the return value. x and y are at 8(%ebp) and 12(%ebp) Stack set-up and completion 9

10 Assembly Code Highly machine specific Why study it? – Being able to read and understand it is an important skill for serious programmers. – Shifted over the years from one of being able to write programs directly in assembly to one of being able to read and understand the code generated by compilers. 10

11 An Introduction to Assembly Language IA32 – Intel Architecture 32-bits – The dominant machine language of most computers, and x86- 64, its extension to run on 64-bit machine. – All the examples in this chapter are related mainly to 32-bits IA32 – it is our focus. – Computers execute machine code. Sequences of bytes encoding low-level operations. – Assembly Code: a textual representation for the machine code giving the individual instructions in the program. – Complier, e.g., gcc C complier invokes an assembler and a linker to generate the executable machine code from the assembly code. We take a close look at machine code and its human- readable representation as assembly code. 11

12 ATT versus Intel Assembly-code formats In our representation, we use ATT-format – The default format for GCC, OBJDUMP, and the other tools Other programming tools, including those from Microsoft as well as the documentations from Intel, use Intel-format. – gcc –O1 –S –masm=intel sum.c 12

13 In the simplest assembly language model, a computer consists of - A main memory – An array of bytes – Consecutive numbered start at 0. These numbers are called memory addresses. A program counter or PC – Hold a memory address. – Called %eip in IA32. A register file containing a small number of named locations. – Each location (register) can hold a fixed amount of information corresponding to the word size of the machines Typical word size is 4 bytes (32-bits machine) %eax, %edx, %ecx, %ebx, %esi, %edi, %esp, %ebp (8 registers) Conditional code registers – Contain information about the last arithmetic or logical operation. – For example, ZF (zero flag) is set if the last operation resulted in 0. – For example, SF (sign flg) is set if the last operation yielded a negative value. A set of floating-point registers for holding floating-point data 13

14 Section 3.1 History of Intel Processor Line 1972: 8008 (3.5K) - first Intel microprocessor with 8-bit words. The instruction set was designed by Datapoint Corporation which was a leading maker of programmable CRT terminals. Datapoint was based in San Antonio, so you might say that the Intel architecture started just a few miles from here. 1974: 8080 (4.5K) - first successful Intel microprocessor, had some 16-bit instructions. 1978: 8086 (29K) - One of the first 16-bit microprocessors. 20-bit addresses with segmented address space. 1979: 8088 (29K) - An 8086 with an 8-bit external bus - basis of the original IBM PC 1980: 8087 (45K) - A floating point coprocessor for the 8086 and 8088, formed the bases for IEEE floating point standard. 1982: (134K) - basis of the IBM PC-AT and MS Windows 1985: (275K) (also called i386 – expanded the architecture to 32 bits) - added flat address space, could run Linux. 1989: (1.2M) - integrated the floating point processor 1993: Pentium (3.1M) - improved performance 1995: PentiumPro (5.5M) - new processor design 1997: Pentium 2 (7M) - more of the same 1999: Pentium 3 (8.2M) - new floating point instructions 2000: Pentium 4 (42M) - double precision floating point and many new instructions. 2004: Pentium 4E (125M) - added hyperthreading 2006: Core2 Duo (291M) - multiple cores, not hyperthreading 2008: Core i7 Quad (781M) - multiple cores and hyperthreading 2010: Itanium Tukwila (2B) - instruction-level parallelism 2011: Xeon Westmere (2.6B) - 10 cores 14

15 Stack – Some region of memory – A data structure where values can be added or deleted, but only according to a “last-in, first-out” discipline – push: add data – pop: remove data 15

16 Consider the following int sum(int x, int y) { return x + y; } Before the function is entered, a stack is set up with the stack pointer contained in a designated register (%esp). The stack grows toward low memory. The stack pointer points to the last item pushed on the stack. The values of x and y are pushed on the stack. The return address is also pushed on the stack. Assume %esp is the stack pointer and all items are 4 bytes. The return address is at 0(%esp) and the return value stored in %eax. x is at 4(%esp). y is at 8(%esp). 16

17 Machine code cc –c sum.s objdump –d sum.o which produces sum.o: file format elf32-i386 Disassembly of section.text: : 0: 55 push %ebp 1: 89 e5 mov %esp,%ebp 3: 8b 45 0c mov 0xc(%ebp),%eax 6: add 0x8(%ebp),%eax 9: 5d pop %ebp a: c3 ret To inspect the contents of machine-code files, a class of programs known as disasemblers can be invaluable. objdump (for “object dump”) generates a format similar to assembly code from the machine code. Each instruction takes up 1 to 15 bytes Common instructions such as push, pop, or ret, are short 17

18 Machine Code To use this program, we need a main to call it: e1.c int add(int x, int y); int main() { int x = 12; int y = 31; int z; z=add(x, y); printf("x is %d, y is %d, and z is %d\n", x, y, z); return 0; } We do: cc –O1 –S e1.c to create: e1.s which is… 18

19 e1.s – Machine code main: leal 4(%esp), %ecx andl $-16, %esp pushl -4(%ecx) pushl %ebp movl %esp, %ebp pushl %ecx subl $20, %esp movl $31, 4(%esp) movl $12, (%esp) call add movl %eax, 12(%esp) movl $31, 8(%esp) movl $12, 4(%esp) movl $.LC0, (%esp) call printf movl $0, %eax addl $20, %esp popl %ecx popl %ebp leal -4(%ecx), %esp ret 19

20 Moore’s Law In 1965, Gordon Moore, the founder of Intel predicted that the number of transistors that could fit on a chip would double every year for 10 years. Actually has about doubled every 18 months to 2 years for 45 years! 20

21 Section 3.2 Program Encoding gcc –O1 –o sum sum.c – The -O1 is a compiler directive telling it to limit the optimizations used. – The compiler generates assembly code: sum.s – The assembler converts the assembly code into object code: sum.o – The linker combines the object code with the libraries to produce an executable: sum – The sum.s file is not saved by default. You can look at the assembly code generated using: gcc -O1 -S sum.c – This produces a file sum.s in ATT format. gcc –O1 –o sum sum.c – This produces a file sum.s in Intel format. gcc –O1 –S –masm=intel sum.c 21

22 IA32 32-bit registers Eight 32-bits registers %eax: accumulator %ecx: counter %edx: data %ebx: base %esi: source %edi: destination %esp: stack pointer %ebp: frame pointer 22

23 Section 3.4 Access Information IA32 Registers −8 8-bit registers −8 16-bit registers −8 32-bit registers The first 6 32-bits registers can be considered general purpose registers, but historically they had specific uses. You can modify the 8-bit registers without modifying the rest of the bits of the corresponding 32-bit register. 23

24 Why these strange names? goes back to the 8080, an 8-bit machine with registers: A, B, C, D, etc. – The 8086 had 16-bit registers: ax, bx, cx, dx, where ax was made up of 2 8-bit registers, al and ah. – Similarly with bx, cx, and dx. – The 32-bit version (80386) extended these to 32 bits, making eax, ebx, etc. – The low 16 bits of eax are just ax, and ax is made up of ah and al. The 64-bit architecture has bit registers called r0 - r

25 Section 3.3 Data Formats for IA 32 b Byte: 8 bits (of course) – used for char w Word: 16 bits (for compatability with 16-bit architecture) – used for short l Double Word: 32 bits – used for int, long, and pointers s Single Precision: 32 bits – used for float l Double Precision: 64 bits – used for double t Extended Precision: 80 or 96 bits – used for long double No direct support for long long (64-bit ints). Operations must be done in pieces. 25

26 Section Operand Specifiers There are 11 basic forms for operands. – 1 for immediate (constant) values – 1 for registers – The rest are for memory. Three operand types: – Immediate, is for constant values Written with a $ followed by an integer, e.g., $-577 or $0x17 – Register, denote the contents of one of the registers Its value R[E a ] – Memory, M b [Addr] to denote the b-byte value stored in memory starting at address Addr 26

27 Operand Forms Operands can denote immediate (constant) values, register values, or values from memory. The scaling factor s must be either 1, 2, 4, or 8 The general form is shown at the bottom of the table. 27

28 Practice Problem 1 Assume the following values are stored at the indicated memory addresses and registers Address Values Register Values x100 0xFF %eax 0x100 0x104 0xAB %ecx 0x1 0x108 0x13 %edx 0x3 0x10C 0x11 Fill the following table: Operand Value %eax ________ 9(%eax, %edx) __________ 0x104 ________ 260(%ecx, %edx) __________ $0x108 _________ 0xFC(, %ecx, 4) __________ (%eax) _________ (%eax, %edx, 4) __________ 4(%eax) _________ 28

29 Practice Problem 1 - Solution Assume the following values are stored at the indicated memory addresses and registers Address Values Register Values x100 0xFF %eax 0x100 0x104 0xAB %ecx 0x1 0x108 0x13 %edx 0x3 0x10C 0x11 Fill the following table: Operand Value %eax _0x100___ 9(%eax, %edx) _0x11_____ 0x104 _0xAB____ 260(%ecx, %edx) _0x13_____ $0x108 _0x108____ 0xFC(, %ecx, 4) _0xFF_____ (%eax) __0xFF____ (%eax, %edx, 4) _0x11_____ 4(%eax) __0xAB___ 29

30 Data Movement Instructions MOV classes – movb, movw, movl – Operate on the data size of 1, 2, and 4 bytes, respectively – movs, movz classes movsbw, movsbl, movswl – Sign-extended movzbw, movzbl, movzwl – Zero-extended 30

31 Data Movement Instructions InstructionEffectDescription MOV S, D D  S Move movbMove bytes movwMove words movlMove double words MOVS S, D D  SignExtend(S) Move with sign extension movsbwMove sign-extended byte to word movsblMove sign-extended byte to double word movswlMove sign-extended word to double word MOVZ S,D D  ZeroExtend(S) Move with zero extension movzbwMove zero-extended byte to word movzblMove zero-extended byte to double word movzwlMove zero-extended word to double word pushl S R[%esp]  R[%esp]-4 Push double word M[R[%esp]]  S popl D D  M[R[%esp]]; Pop double word R[%esp]  R[%esp]+4

32 Practice Problem 2 Assume initially that %dh = 0xCD, %eax = 0x movb %dh, %al %eax =? 2.movsbl %dh, %eax %eax = ? 3.movzbl %dh, %eax %eax = ? 32

33 Practice Problem 2- Solution Assume initially that %dh = 0xCD, %eax = 0x movb %dh, %al %eax = 0x987654CD 2.movsbl %dh, %eax %eax = 0xFFFFFFCD 3.movzbl %dh, %eax %eax = 0x000000CD 33

34 Practice Problem 3 What’s wrong with each line? 1.movb $0xF, (%bl) 2.movl %ax, (%esp) 3.movw (%eax), 4(%esp) 4.movb %ah, %sh 5.movl %eax, $0x123 6.movl %eax, %dx 7.movb %si, 8(%ebp) 34

35 Practice Problem 3 - Solution What’s wrong with each line? 1.movb $0xF, (%bl) Ans: cannot use %bl as address register 2. movl %ax, (%esp) Ans: mismatch between suffix with register ID 3. movw (%eax), 4(%esp) Ans: cannot have both source and destination be memory address 4. movb %ah, %sh Ans: no register named %sh 5. movl %eax, $0x123 Ans: Cannot have immediate as destination 6. movl %eax, %dx Ans: Destination operand incorrect size 7. movb %si, 8(%ebp) Ans: Mismatch between instruction suffix with register ID. 35

36 Example 1 Example 1: int simple(int x) { return x+17; } Complies to: simple: pushl %ebp movl %esp, %ebp movl 8(%ebp), %eax // x into %eax addl $17, %eax // x+17 into %eax popl %ebp ret 36

37 Example 2 Example 2: int array(int* s, int i) { return s[i]; } Complies to: array: pushl %ebp movl %esp, %ebp movl 12(%ebp), %eax movl 8(%ebp), %edx movl (%edx,%eax,4), %eax popl %ebp ret Question: if we changed this to an array of short, could we just change the 4 to 2? 37

38 Example 2 Example 2: int array(int* s, int i) { return s[i]; } Complies to: array: pushl %ebp movl %esp, %ebp movl 12(%ebp), %eax // i into %eax movl 8(%ebp), %edx // s into %edx movl (%edx,%eax,4), %eax // M[S+4*i] -> %eax popl %ebp ret Question: if we changed this to an array of short, could we just change the 4 to 2? 38

39 Example 3 short array(short* s, int i) { return s[i]; } Complies to: array: pushl %ebp movl %esp, %ebp movl 12(%ebp), %eax movl 8(%ebp), %edx movzwl (%edx,%eax,2), %eax popl %ebp ret Questions: 1) what does the movzwl do? 2) What value would be returned in %eax if the array contained -1? 39

40 Example 3 short array(short* s, int i) { return s[i]; } Complies to: array: pushl %ebp movl %esp, %ebp movl 12(%ebp), %eax // i into %eax movl 8(%ebp), %edx // s into %edx movzwl (%edx,%eax,2), %eax // M[s+i*2] -> %eax popl %ebp ret Questions: 1) what does the movzwl do? 2) What value would be returned in %eax if the array contained -1? 40

41 3.5 Arithmetic and Logical Operations InstructionEffectDescription leal S, D D  &S Load effective address INC D D  D+1 Increment DEC D D  D-1 Decrement NEG D D  -D Negate NOT D D  ~D Complement ADD S, D D  S+D Add SUB S, D D  D-S Subtract IMUL S, D D  D*S Multiply XOR S, D D  D^S Exclusive-or OR S, D D  D | S Or AND S, D D  D & S And SAL k, D D  D<< k Left Shift SHL k, D D  D<< k Left Shift (same as SAL) SAR k, D D  D>> A k Arithmetic right shift SHR k, DD  D>> L kLogical right shift Figure 3.7 Integer arithmetic operations. The load effective address (leal) instruction is commonly used to perform simple arithmetic. The remaining ones are more standard unary or binary operations. We use the notation >>A and >>L to denote arithmetic and logical right shift, respectively. 41

42 Section Unary and Binary Operations Unary operations: inc, dec, neg, not Binary operations: – Operate on source and destination, storing results in destination – add, sub, imul – xor, or, and Bitwise operations 42

43 Practice Problem 4 Suppose register %eax holds value x and %ecx holds value y. Fill in the table below with formulas indicating the value that will be stored in register %edx for each of the given assembly code instructions: Instruction Result ______________________________________________________________________ leal 6(%eax), %edx _____________ leal (%eax, %ecx), %edx _____________ leal 7(%eax, %eax, 8), %edx _____________ leal 0xA(, %ecx, 4), %edx _____________ leal 9(%eax, %ecx, 2), %edx _____________ 43

44 Practice Problem 4 - Solution Suppose register %eax holds value x and %ecx holds value y. Fill in the table below with formulas indicating the value that will be stored in register %edx for each of the given assembly code instructions: Instruction Result ______________________________________________________________________ leal 6(%eax), %edx ____6+x______ leal (%eax, %ecx), %edx ___x+y______ leal 7(%eax, %eax, 8), %edx ___7+x+8y___ leal 0xA(, %ecx, 4), %edx ___10+4y____ leal 9(%eax, %ecx, 2), %edx ___9+x+2y___ 44

45 Practice Problem 5 Assume the following values are stored at the indicated memory addresses and registers Address Values Register Values x100 0xFF %eax 0x100 0x104 0xAB %ecx 0x1 0x108 0x13 %edx 0x3 0x10C 0x11 Fill the following table: Instruction Destination Value ________________________________________________________ addl %ecx, (%eax) ________ ________ subl %edx, 4(%eax) ________ ________ imul $16, (%eax, %edx, 4) ________ ________ incl 8(%eax) ________ ________ decl %ecx ________ ________ subl %edx, %eax ________ ________ 45

46 Practice Problem 5 Solution Assume the following values are stored at the indicated memory addresses and registers Address Values Register Values x100 0xFF %eax 0x100 0x104 0xAB %ecx 0x1 0x108 0x13 %edx 0x3 0x10C 0x11 Fill the following table: Instruction Destination Value ________________________________________________________ addl %ecx, (%eax) _0x100__ __0x100_ subl %edx, 4(%eax) _0x104__ __0xA8__ imul $16, (%eax, %edx, 4) _0x10C_ __0x110__ incl 8(%eax) _0x108__ __0x14_ decl %ecx _%ecx__ __0x0___ subl %edx, %eax _%eax___ _0xFD__ 46

47 Section 3.5.3: Shift Operations D=[x n-1,x n-2, …, x 0 ] Left Shift – SAL, SHL are same – D<> A k = [x n-1, x n-1, …, x n-1,x n-2, …, x k ] – SHR: logical right shift D>> L k = [0, 0, …, 0,x n-1,x n-2, …, x k ] Shift Amounts – k is encoded as a single byte, since only shift amounts between 0 and 31 are possible (only the low-order 5 bits of the shift amounts are considered) – Shift amount is given either as an immediate or in the single byte register element %cl 47

48 Practice Problem 6 Suppose we want to generate assembly code for the following C function: int shift_left2_rightn(int x, int n) { x << = 2; x >> = n; } The code that follows is a portion of the assembly code that performs the actual shifts and leaves the final value in register %eax. Two key instructions have been omitted. Parameters x and n are stored at memory locations with offsets 8 and 12, respectively to the address in register %ebp. 1.movl 8(%ebp), %eax // get x 2._____________________ // x << =2 3.movl 12(%ebp), %ecx // get n 4._____________________ // x >> = n 48

49 Practice Problem 6 - Solution Suppose we want to generate assembly code for the following C function: int shift_left2_rightn(int x, int n) { x << = 2; x >> = n; } The code that follows is a portion of the assembly code that performs the actual shifts and leaves the final value in register %eax. Two key instructions have been omitted. Parameters x and n are stored at memory locations with offsets 8 and 12, respectively to the address in register %ebp. 1.movl 8(%ebp), %eax // get x 2._sall $2, %eax_____ // x << =2 3.movl 12(%ebp), %ecx // get n 4.__sarl %cl, %eax____ // x >> = n 49

50 Example 4 void array_set(int* s, int i, int value) { s[i]= value; } Compiles to: array_set: pushl %ebp movl %esp, %ebp // add comments movl 16(%ebp), %ecx // movl 12(%ebp), %edx // movl 8(%ebp), %eax // movl %ecx, (%eax,%edx,4) // popl %ebp ret 50

51 Example 4 void array_set(int* s, int i, int value) { s[i]= value; } Compiles to: array_set: pushl %ebp movl %esp, %ebp movl 16(%ebp), %ecx // value into %ecx movl 12(%ebp), %edx // i into %edx movl 8(%ebp), %eax // s into %eax movl %ecx, (%eax,%edx,4) // value into memory at (s + 4*i) popl %ebp ret 51

52 Example 5 Example 5: Examples 4 using short void array_set(short* s, short i, short value) { s[i]= value; } Compiles to: array_set: pushl %ebp movl %esp, %ebp // add comments here movl 16(%ebp), %ecx // movl 12(%ebp), %edx // movl 8(%ebp), %eax // movw %cx, (%eax,%edx,2) // popl %ebp ret Note: the use of movw and cx instead of movl and ecx Note: 4 bytes are used to store value on the stack, even though only 2 are needed. 52

53 Example 5 Example 5: Examples 4 using short void array_set(short* s, short i, short value) { s[i]= value; } Compiles to: array_set: pushl %ebp movl %esp, %ebp movl 16(%ebp), %ecx // value into %ecx movl 12(%ebp), %edx // i into %edx movl 8(%ebp), %eax // s into %eax movw %cx, (%eax,%edx,2) // value into memory at (s+ 2*i) popl %ebp ret Note: the use of movw and cx instead of movl and ecx Note: 4 bytes are used to store value on the stack, even though only 2 are needed. 53

54 Example 6 Example 6: using long long long long array(long long* s, int i) { return s[i]; } Compiles to: array: pushl %ebp movl %esp, %ebp // add comments here movl 12(%ebp), %edx // movl 8(%ebp), %eax // leal (%eax,%edx,8), %edx // movl (%edx), %eax // movl 4(%edx), %edx // popl %ebp ret 54

55 Example 6 Example 6: using long long long long array(long long* s, int i) { return s[i]; } Compiles to: array: pushl %ebp movl %esp, %ebp movl 12(%ebp), %edx // mov i into %edx movl 8(%ebp), %eax // s into %eax leal (%eax,%edx,8), %edx // address of s[i] into %edx movl (%edx), %eax // low 32 bits of s[i] into %edx movl 4(%edx), %edx // high 32 bits of s[i] into %edx popl %ebp // 64-bit return value in %edx, %eax ret 55

56 Example 7: Using Pointer Parameter void exchange(int *xp, int *yp) { int temp; temp = *xp; *xp = *yp; *yp = temp; } Compiles to exchange: pushl %ebp movl %esp, %ebp pushl %ebx // add comments movl 8(%ebp), %edx // movl 12(%ebp), %ecx // movl (%edx), %ebx // movl (%ecx), %eax // movl %eax, (%edx) // movl %ebx, (%ecx) // popl %ebx // popl %ebp // ret Question: It takes 4 movl instructions to do the exchange. The C source code does this in 3 moves? Why? 56

57 Example 7: Using Pointer Parameter void exchange(int *xp, int *yp) { int temp; temp = *xp; *xp = *yp; *yp = temp; } Compiles to exchange: pushl %ebp movl %esp, %ebp pushl %ebx movl 8(%ebp), %edx // %edx = xp movl 12(%ebp), %ecx // %ecx = yp movl (%edx), %ebx // %ebx = *xp movl (%ecx), %eax // %eax = *yp movl %eax, (%edx) // *xp = *yp movl %ebx, (%ecx) // *yp = *xp popl %ebx popl %ebp ret Question: It takes 4 movl instructions to do the exchange. The C source code does this in 3 moves? Why? 57

58 Example 8: Arithmetic and Logical Operations int arith(int x, int y, int z) { int t1 = x + y; int t2 = z + t1; int t3 = x + 4; int t4 = y * 48; int t5 = t3 + t4; int rval = t2 * t5; return rval; } Compiles to arith: pushl %ebp movl %esp, %ebp // add comments here movl 8(%ebp), %ecx // movl 12(%ebp), %edx // leal (%edx,%edx,2), %eax // sall $4, %eax // leal 4(%ecx,%eax), %eax // addl %ecx, %edx // addl 16(%ebp), %edx // imull %edx, %eax // popl %ebp ret 58

59 Example 8: Arithmetic and Logical Operations int arith(int x, int y, int z) { int t1 = x + y; int t2 = z + t1; int t3 = x + 4; int t4 = y * 48; int t5 = t3 + t4; int rval = t2 * t5; return rval; } Compiles to arith: pushl %ebp movl %esp, %ebp movl 8(%ebp), %ecx // %ecx = x movl 12(%ebp), %edx // %edx = y leal (%edx,%edx,2), %eax // %eax = y+2*y = 3y sall $4, %eax // %eax = 16*3y=48y =t4 leal 4(%ecx,%eax), %eax // %eax = 4+x+48y =t3+t4=t5 addl %ecx, %edx // %edx = x + y=t1 addl 16(%ebp), %edx // %edx = z+t1 = t2 imull %edx, %eax // %eax = t2*t5 popl %ebp ret 59

60 3.5.4 Discussions What does the following instruction do? xorl %eax, %eax 60

61 3.5.4 Discussions What does the following instruction do? xorl %eax, %eax Ans: Set %eax to zero 61

62 Section 3.5. Special Arithmetic Operations 5 special operations – imull, mull, cltd, idivl, divl InstructionEffectDescription imull S R[%edx]: R[%eax]  S × R[%eax] Signed full multiply mull S R[%edx]: R[%eax]  S × R[%eax] Unsigned full multiply cltd R[%edx]: R[%eax]  SignExtend(R[%eax]) Convert to quad word idivl S R[%edx]  R[%edx] : R[%eax] mod S; Signed divide (remainder) R[%eax]  R[%edx] : R[%eax]  S; Quotient divl S R[%edx]  R[%edx] : R[%eax] mod S; Unsigned divide (remainder) R[%eax]  R[%edx] : R[%eax]  S;Quotient 62

63 imull and mull Take one operand: imull S Multiply the operand by %eax The resulting 64 bits are put in %edx (high bits) and %eax (low bits) – Compared to imull S, D imull throws away the high order bits that do not fit in the destination imull is for signed and mull is for unsigned 63

64 Example Suppose we have signed numbers x and y stored at positions 8 and 12 relative to %ebp, and we want to store their full 64-bit product as 8 bytes on top of the stack. 1.movl 12(%ebp), %eax // y into %eax 2.imul 8(%ebp) // x * y in R[%edx]:R[%eax] 3.movl %eax, (%esp) // store low 32 bits 4.movl %edx, 4(%esp) // store high 32 bits Note: Assume little endian machine 64

65 idivl and divl Take one operand: idvil S Divide the 64-bit R[%edx]:R[%eax] by the operand The low 32 bits of the quotient are put in %eax The remainder is put into %edx 65

66 Example 9 Suppose we have signed numbers x and y stored at positions 8 and 12 relative to %ebp 1.movl 8(%ebp), %eax 2.cltd 3.idivl 12(%ebp) 4.movl %eax, 4(%esp) 5.movl %edx, (%esp) 66

67 Example 9 Suppose we have signed numbers x and y stored at positions 8 and 12 relative to %ebp 1.movl 8(%ebp), %eax // x into %eax 2.cltd // sign extended into %edx 3.idivl 12(%ebp) // divide x by y 4.movl %eax, 4(%esp) // x/y 5.movl %edx, (%esp) // x % y = x mod y 67

68 Section 3.6 Control Section Conditional Codes IA32 uses four single-bit flags called conditional codes which are set by certain instructions based on the result of the instruction. FlagNameUse CFCarry flag Carry out of the most significant bit. Used to detect overflow for unsigned operations. ZFZero flagThe most recent operation yielded zero. SFSign flagThe most recent operation yielded a negative value. OFOverflow flagThe most recent operation caused a two's-complement overflow - either positive or negative. (for signed overflow) 68

69 Conditional Codes OF flag: result of add or sub has wrong sign – addl sets the OF if both operands have the same sign, but the result has a different sign. – Subl A, B calculates B-A and sets OF if B>0, A<0, and B-A <0 or B 0, and B-A > 0 69

70 Conditional Codes CF flag: carry out of high bit – addl sets CF if unsigned result does not fit. e.g., 8-bit unsigned operation: = 2 – For shift operations CF is set to be the last bit shifted out. sal and shl set the carry bit to the former MSB (most significant bit) sar and shr set the carry bit to the former LSB (least significant bit) 70

71 Conditional Codes The following instructions set the conditional codes appropriately: inc, dec, neg, not, add, sub, mul, imul, div, idiv, xor, or, and, sal, shl, sar, shr The following instructions do not modify the condition codes: mov, leal, push, pop, call, ret, cltd 71

72 Comparison and Test Instructions InstructionBased onDescription CMP S 2, S 1 S 1 - S 2 Compare cmpbCompare byte cmpwCompare word cmplCompare double word TEST S 2, S 1 S 1 & S 2 Test testbTest byte testwTest word testlTest double word These do not store the resulting computation in the destination, only the conditional codes are set. 72

73 Example Suppose we used one of the ADD instructions to perform the equivalent of the C assignment t=a+b, where variables a, b, and t are integers. CF: (unsigned) t < (unsigned) a Unsigned Overflow ZF: (t == 0) Zero SF: (t < 0) Negative OF: (a < 0 == b < 0) && (t < 0 != a < 0) signed overflow 73

74 3.6.2 Accessing the Condition Codes You can set a byte to 0 or 1 on the condition flags with the set instructions. These take a single byte operand as the destination: either an 8-bit register or a single byte of memory. 74

75 The SET instructions InstructionSynonymEffectDescription sete DsetzD = ZFEqual or zero Setne DsetnzD=~ZFNot equal or not zero sets DD = SFnegative setns DD = ~SFnonnegative setg Dsetnle D  ~(SF^OF) & ~ZF Greater (signed >) setge Dsetnl D  ~(SF^OF) Greater or equal (signed >=) setl Dsetnge D  SF^OF Less (signed <) setle DSetng D  (SF^OF) | ZF Less or equal (signed <=) seta Dsetnbe D  ~CF & ~ZF Above (unsigned >) setae Dsetnb D  ~CF Above or equal (unsigned >=) setb Dsetnae D  CF Below (unsigned <) Setbe DsetnaD  CF|ZFBelow or equal (unsigned <=) The important part of this table is the effect field which shows how the 4 condition codes are related to various tests. The description field is based on a previous instruction of the form cmp S 2, S 1 negative refers to the value of S 1 -S 2 greater, less, above, or below refer to comparing S 1 to S 2 signed unsigned 75

76 Unsigned Comparison In interpreting the effect and description, consider the instruction: cmpl S 2, S 1 which calculates S 1 -S 2 If S 1 and S 2 are unsigned, S 1 is above S 2 if the result of S 1 -S 2 is not zero and does not set the carry flag. D  ~CF & ~ZF The other three comparisons can be understood from this one using de Morgan’s laws. 76

77 Signed Comparison The signed comparison are a bit more complicated Consider greater than or equal test condition. o Under what conditions S1 >= S2 o Answer: S1 – S2 >=0 o This would indicate that we just want SF =0 o But recall, that sometimes the SF is incorrect. o This is indicated by the OF flag. o So if SF is correct (OF=0), we just test SF=0, or ~SF. o If SF is incorrect (OF=1), we want SF=1. o This is ~(SF^OF) o ^ is exclusive or Other signed comparison can be gotten from >= using De Morgan’s laws. 77

78 Example Consider the following code segment: cmpl $10, $20 jle.L1 Does this jump? 78

79 Example Consider the following code segment: cmpl $10, $20 jle.L1 Does this jump? Ans: No 79

80 Section Jump Instructions and Their Encoding Jump instruction change the flow of control so that the next instruction executed is not the next instruction. Traditional instruction cycle, also called fetch-and-execute cycle or fetch-decode-execute cycle. The program counter (PC) register contains the address of the next instruction to execute. – Fetch: read the instruction whose address is in the PC – Increment PC: increment PC so that it points to the next instruction. – Decode: determine what instruction this is. – Execute: do what the instruction indicates. – Store: store the result. 80

81 Section Jump Instructions and Their Encoding InstructionSynonymJump conditionDescription jmp Label1Direct jump Jmp *Operand1Indirect jump je LabeljzZFEqual /zero jne Labeljnz~ZFNot equal / not zero js LabelSFNegative jns Label~SFNonnegative jg Labeljnle~(SF^OF) & ~ZFGreater (signed >) jge Labeljnl~(SF^OF)Greater or equal (signed >=) jl LabeljngeSF^OFLess (signed <) jle Labeljng(SF^OF) | ZFLess or equal (signed <=) ja Labeljnbe~CF & ~ZFAbove (unsigned >) jae Labeljnb~CFAbove or equal (unsigned >=) jb LabeljnaeCFBelow (unsigned <) jbe LabeljnaCF |ZFBelow or equal (unsigned <=) Figure The jump instructions. These instructions jump to a labeled destination when the jump condition holds. Some instructions have “synonyms”, alternate names for the same machine instructions. unconditional conditional 81

82 Unconditional jump Instruction 1. mov1 $0, %eax // set %eax to 0 2. jmp.L1 // goto.L1 3. movl (%eax), %edx // will be skipped 4..L1: 5. popl %edx IA 32 unconditional jump instructions: Two types: direct and indirect jmp Label jmp *Operand jmp *%eax // use the value in register %eax as the jump target jmp *(%eax) // use the value in register %eax as the read address Unconditional jumps are rarely used, except with conditional jumps. 82

83 Conditional jump Example An example: jump.c int simple_jump(int x, int y, int z) { if (x == 0) return y-z; return z-y; } After cc –O1 –S jump.c, jump.s contains simple_jump: pushl %ebp movl %esp, %ebp cmpl $0, 8(%ebp) jne.L2 movl 12(%ebp), %eax subl 16(%ebp), %eax jmp.L3.L2: movl 16(%ebp), %eax subl 12(%ebp), %eax.L3: popl %ebp ret 83

84 Conditional jump Example An example: jump.c int simple_jump(int x, int y, int z) { if (x == 0) return y-z; return z-y; } After cc –O1 –S jump.c, jump.s contains simple_jump: pushl %ebp movl %esp, %ebp cmpl $0, 8(%ebp) // compare x to 0 jne.L2 // jmp if x ! = 0 movl 12(%ebp), %eax // y into %eax subl 16(%ebp), %eax // y-z into %eax jmp.L3 // done.L2: // this is the case x != 0 movl 16(%ebp), %eax // get z into %eax subl 12(%ebp), %eax // z-y into %eax.L3: // common return popl %ebp ret 84

85 Jump instruction encoding There are several ways that jump instructions are encoded, the simplest of which is with PC- relative destination. After cc –c –O1 jump.c and objdump –d jump.o, we get : 0: 55 push %ebp 1: 89 e5 mov %esp,%ebp 3: 83 7d cmpl $0x0,0x8(%ebp) 7: jne 11 9: 8b 45 0c mov 0xc(%ebp),%eax c: 2b sub 0x10(%ebp),%eax f: eb 06 jmp 17 11: 8b mov 0x10(%ebp),%eax 14: 2b 45 0c sub 0xc(%ebp),%eax 17: 5d pop %ebp 18: c3 ret Labels have been replaced by the address relative to the start of the program. During the executable phase of the jne instruction at 7, the PC has the value 9 (point to the next instruction). The encoding of jne shows a jump offset of 8, 9+8 = 17=0x11 During the execution of jmp instruction at f, the PC has value 11. The jump offset is 6, giving 0x11 +0x6 = 0x17 85

86 Practice Problem 7 In the following excerpts from a disassembled binary, some of the information has been replaced by Xs. Answer the following questions about these instructions A.What is the target of the je instructions below? (You don’t need to know anything about the call instruction here.) f: je XXXXXXX : e8 1e call 80482b4 B. What is the target of the jb instruction below? : 72 e7 jb XXXXXXX : c a movb $0x1, 0x804a010 C. What is the address of the mov instruction? XXXXXXX: je XXXXXXX: b mov $0x0, %eax 86

87 Practice Problem 7 Solution In the following excerpts from a disassembled binary, some of the information has been replaced by Xs. Answer the following questions about these instructions A.What is the target of the je instructions below? (You don’t need to know anything about the call instruction here.) f: je XXXXXXX : e8 1e call 80482b4 Ans: PC = ; Offset = 05; Dest = PC + Offset = x05 = B. What is the target of the jb instruction below? : 72 e7 jb XXXXXXX : c a movb $0x1, 0x804a010 Ans: PC = Offset = e7= 1110,0111=N*=-N=-[0001,1001]=-25=-0x19 Dest = PC + Offset = x19 = C. What is the address of the mov instruction? XXXXXXX: je XXXXXXX: b mov $0x0, %eax Ans: Dest = PC + Offset => PC = Dest – Offset = – 0x12 = F Address of Jump Instruction = F – 0x2= D 87

88 Practice Problem 8 D.In the code that follows, the jump target is encoded in PC-relative form as a 4-byte, two’s-complement number. The bytes are listed from least significant to most, reflecting the little-endian byte ordering of IA32. What is the address of the jump target? 80482bf: e9 e0 ff ff ff jmp XXXXXXX 80482c4: 90 nop E. Explain the relation between the annotation on the right and the byte coding on the left aa: ff 25 fc 9f jmp *0x8049ffc 88

89 Practice Problem 8 Solution D.In the code that follows, the jump target is encoded in PC-relative form as a 4-byte, two’s- complement number. The bytes are listed from least significant to most, reflecting the little-endian byte ordering of IA32. What is the address of the jump target? 80482bf: e9 e0 ff ff ff jmp XXXXXXX 80482c4: 90 nop Ans: Offset = ffff,ffe0 = -32 = -0x20; PC = 80482c4 Dest = PC + Offset = 80482c4 – 0x20 = 80482A4 E. Explain the relation between the annotation on the right and the byte coding on the left aa: ff 25 fc 9f jmp *0x8049ffc Ans: An indirect jump is denoted by instruction code ff 25. The address from which the jump target is to read is encoded explicitly by the following 4 bytes. Since the machine is little endian, these are given in reverse order as fc 9f

90 Section Loops C provides several looping constructs – namely, do-while, while, and for. No corresponding instructions exist in machine codes. Instead, combinations of conditional test and jumps are used to implement the effect of loops. 90

91 Section Loops Do-while loops While loops For loops do body-statement while(test-expr); while (test-expr) body-statement for (init-expr; test-expr; update-expr) body-statement The effect of the loop is to repeatedly execute body- statement, evaluate test- expr, and continue the loop if the evaluation result is nonzero. The body-statement is executed at least once. It differs from do-while in that test-expr is evaluated and the loops is potentially terminating before the first execution of body-statement. Identical to the following code: init-expr; while (test-expr) { body-statement update-expr; } 91

92 Example 1: A do-while loop int fact_do(int n) { int result = 1; do { result *= n; n--; } while (n > 1); return result; } And the corresponding assembly code: fact_do: pushl %ebp movl %esp, %ebp movl 8(%ebp), %edx movl $1, %eax.L2: imull %edx, %eax subl $1, %edx cmpl $1, %edx jg.L2 popl %ebp ret 92

93 Example 1: A do-while loop int fact_do(int n) { int result = 1; do { result *= n; n--; } while (n > 1); return result; } And the corresponding assembly code: fact_do: pushl %ebp movl %esp, %ebp movl 8(%ebp), %edx // n into %edx movl $1, %eax // result is in %eax, initial value =1.L2: imull %edx, %eax // result = result * n subl $1, %edx // n--; cmpl $1, %edx // compare n to 1 jg.L2 // jump if n > 1 popl %ebp ret 93

94 Example 2: A while loop C code: int fact_while(int n) { int result = 1; while (n > 1) { result *= n; n--; } return result; } The corresponding assembly code : fact_while: pushl %ebp movl %esp, %ebp movl 8(%ebp), %edx movl $1, %eax cmpl $1, %edx jle.L3.L6: imull %edx, %eax subl $1, %edx cmpl $1, %edx jg.L6.L3: popl %ebp ret 94

95 Example 2: A while loop C code: int fact_while(int n) { int result = 1; while (n > 1) { result *= n; n--; } return result; } The corresponding assembly code : fact_while: pushl %ebp movl %esp, %ebp movl 8(%ebp), %edx // get n into %edx movl $1, %eax // result in %eax cmpl $1, %edx // see if n > 1 jle.L3 // no, we are done.L6: imull %edx, %eax // yes, result = result * n subl $1, %edx // n--; cmpl $1, %edx // compare again jg.L6 // keep going if n > 1.L3: popl %ebp ret 95

96 Example 3: A for loop C code: int fact_for(int n) { int i; int result = 1; for (i=2; i <=n; i++) result *= i; return result; } The corresponding assembly code : fact_for: pushl %ebp movl %esp, %ebp movl 8(%ebp), %ecx movl $2, %edx movl $1, %eax cmpl $1, %ecx jle.L3 (continue if n > =2).L6: imull %edx, %eax addl $1, %edx cmpl %edx, %ecx jge.L6).L3: popl %ebp ret 96

97 Example 3: A for loop C code: int fact_for(int n) { int i; int result = 1; for (i=2; i <=n; i++) result *= i; return result; } The corresponding assembly code : fact_for: pushl %ebp movl %esp, %ebp movl 8(%ebp), %ecx // n into %ecx movl $2, %edx // 2 into %edx (this is i) movl $1, %eax // 1 into %eax (the result) cmpl $1, %ecx // compare n to 1 jle.L3 // done if n =2).L6: imull %edx, %eax // result = result * i addl $1, %edx // i++ cmpl %edx, %ecx // compare n to i jge.L6 // continue if n >=i (done if n < i).L3: popl %ebp ret 97

98 We will skip the sections and

99 Section 3.7 Procedure A procedure involves: – Passing data in the form of procedure parameters and return values – Passing control from one part of program to another. – Allocate space for the local variables of the procedure on entry and deallocate them on exit. 99

100 Section Stack Frame Structure Procedure P calls procedure Q – P: caller – Q: callee The stack is used for passing parameters, for local variables, and storing other values. The stack is organized into pieces called Stack Frames. Stack frame has 2 pointers – %ebp the frame pointer Most information is accessed relative to the frame pointer – %esp, the stack pointer Can move 100

101 Section Stack Frame Structure The %ebp frame pointer points to the saved %ebp register on the stack Usually %ebp does not change The first parameter is at 8(%ebp) because of the return address and saved %ebp. %ebp is used to address data on the caller’s stack (such as parameters) In our examples, %esp usually did not change during execution, but in general it will when space on the stack is allocated for – Saved registers – Local variables – Parameters of procedures that will be called, e.g., in Q, it calls R 101

102 Section Transferring Control InstructionDescription call LabelProcedure call – direct call *OperandProcedure call - indirect leavePrepare stack for return retReturn from call Three instructions used for supporting procedures call pushes the return address (current PC) on the stack and sets the PC to the label —Current PC holds the return address – the address of next instruction —direct and indirect call leave is equivalent to: mov %ebp, %esp popl %ebp The purpose of the first of these is to restore the stack pointer to the value it had after the initial push of %ebp We haven’t seen leave before because none of our procedures have needed to change %esp, so the first of these was not necessary. ret pops the return address and jumps to this address. 102

103 Practice Problem 9 Example Section sum and main - the following are excerpts of the disassembled code for the two functions: Beginning of function sum: : : 55 push %ebp … Return from function sum a4: c3 ret … call to sum from main dc: e8 b3 ff ff ff call e1: 83 c4 14 add $0x14, %esp Trace the registers %eip (PC) and %esp: 1)Before executing call, PC(%eip) = ________; % esp = 0xff9b960 2)When executing call, PC value (return address) is pushed into stack; %esp = _______; %eip = ________ 3)After return from call, %eip = _________; %esp = __________ 103

104 Practice Problem 9 Solution Example Section sum and main - the following are excerpts of the disassembled code for the two functions: Beginning of function sum: : : 55 push %ebp … Return from function sum a4: c3 ret … call to sum from main dc: e8 b3 ff ff ff call e1: 83 c4 14 add $0x14, %esp Trace the registers %eip (PC) and %esp: 1)Before executing call, PC(%eip) = 80483dc; % esp = 0xff9b960 2)When executing call, PC value (return address) is pushed into stack; %esp = 0xff9b95c; %eip = 0x )After return from call, %eip = 0x80483e1; %esp = 0xff9b

105 Section Register Usage Conventions The set of program registers acts as a single resource shared by all of the procedures. Only one procedure can be active at a given time caller-save registers: – %eax, %edx, %ecx – When Q is called by P, it can overwrite these registers without destroying any data required by P. callee-save registers: – %ebx, %esi, %edi – Q must save the values of any of these registers on the stack before overwriting them, and store them before returning. P might need these values for its further computation. – %ebp, %esp must be maintained according to the conventions described here. 105

106 Example Assembly Code Sequence: 1.Subl $12, %esp 2.movl %ebx, (%esp) 3.movl %esi, 4(%esp) 4.movl %edi, 8(%esp) 5.movl 8(%ebp), %ebx 6.movl 12(%ebp), %edi 7.movl (%ebx), %esi 8.movl (%edi), %eax 9.movl 16(%ebp), %edx 10.movl (%edx), %ecx Three registers (%ebx, %esi, %edi) are saved on the stack (Lines 2-4). The program modifies these and three other registers (%eax, %ecx, %edx) At the end of the procedure, the values of registers %edi, %esi, %ebx are restored (not shown), while the other three are left in their modified states. 106

107 A procedure example of Section Here is the assembly code generated: swap_add: pushl %ebp movl %esp, %ebp pushl %ebx // movl 8(%ebp), %edx // movl 12(%ebp), %ecx // movl (%edx), %ebx // movl (%ecx), %eax // movl %eax, (%edx) // movl %ebx, (%ecx) // addl %ebx, %eax // popl %ebx popl %ebp ret C Code: int swap_add(int *xp, int *yp) { int x = *xp; int y = *yp; *xp = y; *yp = x; return x + y; } 107

108 A procedure example of Section Here is the assembly code generated: swap_add: pushl %ebp movl %esp, %ebp pushl %ebx movl 8(%ebp), %edx // xp into %edx movl 12(%ebp), %ecx // yp into %ecx movl (%edx), %ebx // x, i.e. %ebx = *xp movl (%ecx), %eax // y, i.e. %eax = *yp movl %eax, (%edx) // *xp = *yp movl %ebx, (%ecx) // *yp = *xp addl %ebx, % eax // %eax = x +y for return popl %ebx popl %ebp ret C Code (swap_add.c) int swap_add(int *xp, int *yp) { int x = *xp; int y = *yp; *xp = y; *yp = x; return x + y; } 108

109 A procedure example of Section Here is the assembly code generated: caller: pushl %ebp movl %esp, %ebp subl $24, %esp // movl $534, -4(%ebp) // movl $1057, -8(%ebp) // leal -8(%ebp), %eax // movl %eax, 4(%esp) // leal -4(%ebp), %eax // movl %eax, (%esp) // call swap_add.R1 movl -4(%ebp), %edx // subl -8(%ebp), %edx // imull %edx, %eax // leave // ret C Code (caller.c) int caller() { int arg1 = 534; int arg2 = 1057; int sum = swap_add(&arg1, &arg2); int diff = arg1 - arg2; return sum*diff; } 109

110 A procedure example of Section Here is the assembly code generated: caller: pushl %ebp movl %esp, %ebp subl $24, %esp //allocate 6 double words on the stack movl $534, -4(%ebp) // 534 on stack movl $1057, -8(%ebp) // 1057 on the stack leal -8(%ebp), %eax // &1057 into %eax movl %eax, 4(%esp) // &1057 on stack leal -4(%ebp), %eax // &534 on into %eax movl %eax, (%esp) // &534 on stack call swap_add.R1 movl -4(%ebp), %edx // arg1 into %edx subl -8(%ebp), %edx // arg1 – arg2 into %edx imull %edx, %eax // diff * return value in %eax leave // restore the stack pointer ret C Code (caller.c) int caller() { int arg1 = 534; int arg2 = 1057; int sum = swap_add(&arg1, &arg2); int diff = arg1 - arg2; return sum*diff; } 110

111 A procedure example of Section Practice Problem 1 : Keep track of register values: %eax, %ebx, %ecx, %edx, %ebp, %esp 111

112 A procedure example of Section Why did the compiler reserve 6 words = 24 bytes on the stack when it only needed 4? Answer: Convention: the total number of stack bytes used by a function should be a multiple of 16, e.g., 16, 32, 48. This counts the 4 bytes for the return address and the 4 bytes for the saved %ebp If only 4 words were reserved, this would be 16+8=24 bytes To get this up to 32, we need to add 8 more bytes, or 2 more words. This does not reduce the speed of execution. It does use a small amount of extra memory. 112

113 A recursive procedure example of Section C code: int rfact(int n) { int result; if (n <1 ) result =1; else result = n * rfact(n-1); return result; } Here is the assembly code generated: rfact: pushl %ebp movl %esp, %ebp pushl %ebx subl $4, %esp movl 8(%ebp), %ebx movl $1, %eax testl %ebx, %ebx jle.L3 leal -1(%ebx), %eax movl %eax, (%esp) call rfact.R1 imull %ebx, %eax.L3: addl $4, %esp popl %ebx popl %ebp ret 113

114 A Recursive Procedure Example of Section C code: int rfact(int n) { int result; if (n <1 ) result =1; else result = n * rfact(n-1); return result; } Here is the assembly code generated: rfact: pushl %ebp movl %esp, %ebp pushl %ebx subl $4, %esp // reserve 4 extra bytes on the stack movl 8(%ebp), %ebx // n into %ebx movl $1, %eax // 1 into %eax testl %ebx, %ebx // test n jle.L3 // jump if n <=0 (same as n <1) leal -1(%ebx), %eax // %eax = n-1 movl %eax, (%esp) // move n-1 on the stack call rfact // call rfact with parameter n-1.R1 imull %ebx, %eax // n * return value into %eax for return.L3: addl $4, %esp // restore %esp popl %ebx // restore %ebx popl %ebp ret 114

115 A procedure example of Section Practice Problem 2 : Keep track of register values: %eax, %ebx, %ebp, %esp 115


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