1. Chapter 3 Machine-Level Representation of Programs
Guobao Jiang

2. Problem 3.1 (P138)
Assume the following values are stored at the indicated memory addresses and registers:
Fill in the following table showing the values for the indicated operands:
Address
Value
0x100
0xFF
0x104
0xAB
0x108
0x13
0x10C
0x11
Register
Value
%eax
0x100
%ecx
0x1
%edx
0x3
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3. M[Imm + R[Eb] + R[Ei]*S]
Address
Value
0x100
0xFF
0x104
0xAB
0x108
0x13
0x10C
0x11
Register
Value
%eax
0x100
%ecx
0x1
%edx
0x3
Imm(Eb, Ei, S)
M[Imm + R[Eb] + R[Ei]*S]
Operand
Value
%eax
0x104
$0x108
(%eax)
4(%eax)
9(%eax, %edx)
260(%ecx, %edx)
0xFC(, %ecx, 4)
(%eax, %edx, 4)
0x Register
0xAB Absolute address
0x Immediate
0xFF Address 0x100
0xAB Address 0x104
0x Address 0x10C
0x Address 0x108
0xFF Address 0x100
0x Address 0x10C
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4. Problem 3.2 (P142)
You are given the following information. A function with prototype
void decode1(int *xp, int *yp, int *zp)
is compiled into assembly code. The body of the code is as follows: …
Parameters xp, yp and zp are stored at memory locations with offsets 8, 12, and 16, respectively, relative to the address in register %ebp
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5. Problem 3.2+ 1 movl 8(%ebp), %edi 2 movl 12(%ebp), %ebx
int x = *xp;
int y = *yp;
int z = *zp;
*yp = x;
*zp = y;
*xp = z;
1 movl 8(%ebp), %edi
2 movl 12(%ebp), %ebx
3 movl 16(%ebp), %esi
4 movl (%edi), %eax
5 movl (%ebx), %edx
6 movl (%esi), %ecx
7 movl %eax, (%ebx)
8 movl %edx, (%esi)
9 movl %ecx, (%edi)
Write C code for decode1 that will have an effect equivalent to the assembly code above.
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6. Problem 3.3 (143)
Suppose register %eax holds values x and %ecx holds value y. Fill the table …
Expression
Result
leal 6(%eax), %edx
leal (%eax, %ecx), %edx
leal (%eax, %ecx, 4), %edx
leal 7(%eax, %eax ,8), %edx
leal 0xA (, %ecx, 4), %edx
leal 9(%eax, %ecx, 2), %edx
6 + x
x + y
x + 4y
7 + 9x
10 + 4y
9 + x + 2y
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7. Problem 3.4 (P145)
Assume the following values are stored at the indicated memory addresses and registers:
Fill in the following table showing the effects of the following instructions …
Address
Value
0x100
0xFF
0x104
0xAB
0x108
0x13
0x10C
0x11
Register
Value
%eax
0x100
%ecx
0x1
%edx
0x3
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8. Instruction Destination Value addl %ecx, (%eax) subl %edx, 4(%eax)
Address
Value
0x100
0xFF
0x104
0xAB
0x108
0x13
0x10C
0x11
Register
Value
%eax
0x100
%ecx
0x1
%edx
0x3
Instruction
Destination
Value
addl %ecx, (%eax)
subl %edx, 4(%eax)
imull $16, (%eax, %edx, 4)
incl 8(%eax)
decl %ecx
subl %edx, %eax
0x11 * 16(dec)= (16+1) * 16 = = 0x 110
0x x100
0x xA8
0x10C x110
0x x14
%ecx x0
%eax xFD
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9. Problem 3.5 (P145)
Suppose we want to generate assembly code for the following C Function:
int shift_left2_rightn(int x, int n) {
x <<= 2;
x >>= n;
return x; }
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10. Problem 3.5+
Parameters x and n are stored at memory locations with offsets 8 and 12, respectively, relative to the address in register %ebp.
1 movl 12(%ebp), %ecx Get n
2 movl 8(%ebp), %eax Get x
3 ______________ x <<= 2
4 ______________ x >>= n
Note: the right shift should be performed arithmetically.
%ecx ? Shifts amounts between 0 and 31 are allowed
sall $2, %eax
subl %cl, %eax
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11. Problem 3.6 (P147)
In the compilation of the loop for (i = 0; i < n; i++) v += i; we find the following assembly code line: xorl %edx, %edx Explain why this instruction would be there, even though there are no EXCLUSIVE-OR operators in our C code. What operation in the C program does this instruction implement ?
To set register %edx to 0, exploiting the program that x^x =0 for any x. It corresponds to the C statement i = 0.
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12. Problem 3.7 (P151) < < unsigned short >= short char != char
In the following C code, we have replaced some of the comparison operators with “__” and omitted the data types in the casts.
char ctest(in a, int b, int c) {
char t1 = a ___ b;
char t2 = b ___( ) a;
char t3 = ( ) c __ ( )a;
char t4 = ( ) a __ ( )c;
char t5 = c ____ b;
char t6 = a _____ 0;
return t1 + t2 + t3 + t4 + t5 + t6 ; }
cmpl %ecx, %esi
setl %al
cmpl %ecx, %esi
setb -1(%ebp)
cmpw %cx, 16(%ebp)
setge -2(%ebp)
movb %cl, %dl
<
< unsigned
short >= short
char != char
>
>
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13. Problem 3.8 (P155)
In the following excerpts from a disassembled binary, some of the information has been replaced by x’s. Answer the following questions:
A. What is the target of the jbe instruction below?
8048d1c: 76 da jbe xxxxxxx
8048d1e: eb jmp d44
8048dle
+da
8048df8
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14. Problem 3.8+ 80489D2 B. What is the address of the mov instruction ?
xxxxxxx: eb jmp d44
xxxxxxx: c7 45 f mov $0x10, 0xfffffff8 (%ebp)
C. 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 ?
: e9 cb jmp xxxxxxx
: nop
8048CF0
+cb
80489d2
80489D2
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15. Problem 3.8++
D. Explain the relation between the annotation on the right and the byte coding on the left. Both lines are part of the encoding of the jmp instruction.
80483f0: ff 25 e0 a2 04 jmp *0x804a2e0
80483f5: 08
Answer: An indirect jump is denoted by instruction code ff 25. The address from which the jump target is to be read is encoded explicitly by the following 4 bytes. Since the machine is little endian, these are given in reverse order as e0 a
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16. Problem 3.9 (P157) When given the C code void cond(int a, int *p) {
if (p && a > 0)
*p += a; }
GCC generates the following assembly code:
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17. A. Write a goto vision in C that …
1 movl 8(%ebp), %edx
2 movl 12(%ebp), %eax
3 testl %eax, %eax
4 je .L3
5 testl %edx, %edx
6 jle .L3
7 addl %edx, (%eax)
8 .L3:
A. Write a goto vision in C that …
B. Explain why the assembly code contains two conditional branches, even though the C code has only one if statement.
void cond (int a, int *p){
if (p == 0)
goto done;
if (a <= 0)
*p += a;
done: }
The first conditional branch is part of the implementation of the || expression. If the test for p being nonnull fails, the code will skip the test of a > 0
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18. Problem 3.10 (P160) For the C code int dw_loop(int x, int y, int n) {
do {
x += n;
y *= n;
n--;
} while ((n > 0) & (y < n));
return x; }
GCC generates the following assembly code:
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19. 1 movl 8(%ebp), %esi 2 movl 12(%ebp), %ebx 3 movl 16 (%ebp), %ecx
put x in %esi
movl 8(%ebp), %esi
2 movl 12(%ebp), %ebx
3 movl 16 (%ebp), %ecx
p2align 4, ,7
5 .L loop:
6 imull %ecx, %ebx
7 addl %ecx, %esi
8 decl %ecx
9 testl %ecx, %ecx
10 setg %al
11 cmpl %ecx, %ebx
12 setl %dl
13 andl %edx, %eax
14 testb $1, %al
15 jne .L6
put y in %ebx
put n in %ecx
y * = n
Register usage
Register
Variable
Initially
%esi
%ebx
%ecx x y n
x + = n
n--
test n
n > 0
compare y:n
y < n
(n > 0) & (y < n)z
test least significant bit
if !=0, goto loop
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20. Problem 3.11 (P163) For the following C code:
int loop_while (int a, int b) {
int i = 0;
int result = a;
while (i < 256){
result += a;
a -= b;
i += b; }
return result;
GCC generates the following assembly code:
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21. int loop_while_goto(int a, int b){
int i=0;
int result =a;
loop:
result += a;
a -= b;
i + =b;
if (i <= 255)
goto loop:
return result }
put a in %eax
movl 8(%ebp), %eax movl 12(%ebp), %ebx xorl %ecx, %ecx movl %eax, %edx .p2align 4, , 7 .L5: loop: addl %eax, %edx subl %ebx, %eax addl %ebx, %ecx cmpl $255, %ecx jle .L5
put b in %ebx
i= 0
result = a
result += a
a -= b
Register usage
Register
Variable
Initially
%eax
%ebx
%ecx
%edx a b i
result
i += b
compare i:255
if <= goto loop
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22. Problem 3.12 (P165) Consider the following assembly code:
movl 8(%ebp), %ebx
movl 16(%ebp), %edx
xorl %eax, %eax
decl %edx
js .L4
movl %ebx, %ecx
imull 12(%ebp), %ecx
.p2align 4, ,4
.L6:
addl %ecx, %eax
subl %ebx, %edx
jns .L6
.L4:
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23. int loop(int x, int y, int n){ int result = 0; int i;
for (i = ___; i___; i = ___){
result += ___; }
return result;
Which registers hold program values result and i ?
What is the initial value of i ?
What is the test condition on i ?
How does i get updated?
The C expression describing how to increment result in the loop body does not change value from one iteration of the loop to the next. The compiler detected this and moved its computation to before the loop. What is the expression?
n-1
>= 0
i - x
y * x
%eax %edx
n-1
i to be nonnegative
instruction 4: decl %edx
1,6,7 cause x*y to be stored in register %ecx
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24. Problem 3.13 (P169)
In the C code, the case labels did not span a contiguous range, and some cases had multiple labels.
int switch2(int x){
int result =0;
switch (x){
/*Body of switch statement omitted*/ }
return result;
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25. setting up jump table access 3 .long .L10
Jump table for switch2
1 .L11:
2 .long .L4
setting up jump table access long .L10
1 movl 8(%ebp), %eax long .L5
2 addl $2, %eax long .L6
3 cmpl $6, %eax long .L8
4 ja .L long .L8
5 jmp *.L11 (, %eax, 4) long .L9
Use the foregoing information to answer the following questions:
What were the values of the case labels in the switch statement body ?
What cases had multiple labels in the C code ?
In jump table line 6 and 7 have the save destination. These correspond to case labels 2 and 3.
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26. Problem 3.14 (P172)
The following code fragment occurs often in the compiled vision of library routines:
call next
2 next:
popl %eax
To what value does register %eax get set?
Explain why there is no matching ret instruction to this call.
What useful purpose does this code fragment serve?
%eax is set to the address of the popl instruction.
Not true subroutine all, since the control flows the same ordering as the instructions and the return address is popped from the stack.
This is the only way in IA32 to get the value of the program counter into an integer register.
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27. Problem 3.15 (P174)
Registers %edi, %esi, and %ebx are callee save. The procedure must save them on the stack before altering their values and restore them before returning. The other three registers are caller save. They can be altered with out affecting the behavior of the caller
1 pushl %edi 2 pushl %esi 3 pushl %ebx 4 movl 24(%ebp), %eax 5 imull 16(%ebp), %eax 6 movl 24(%ebp), %ebx 7 leal 0(,%eax,4), %ecx 8 addl 8(%ebp), %ecx 9 Movl %ebx, %edx Explain this apparent inconsistency in the saving and restoring of register states ?
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28. Problem 3.16 (P177) Given the C function 1 int proc(void) 2 {
2 {
int x, y;
scanf(“%x %x”, &y, &x);
return x - y;
6 }
GCC generates the following assembly code:
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29. Register Value %esp %ebp 0x800040 0x800060 proc: push1 %ebp
movl %esp, %ebp
subl $24, %esp
addl $-4, %esp
leal (%ebp), %eax
pushl %eax
leal -8(%ebp), %eax
pushl $.LC0 pointer to string “%x %x”
call scanf
movl -8(%ebp), %eax
movl -4(%ebp), %edx
subl %eax, %edx
movl %edx, %eax
movl %ebp, %esp
popl %ebp
ret
Assume that procedure proc starts executing with the following register values:
Register
Value
%esp
%ebp
0x800040
0x800060
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30. A. What value does %ebp get set to on line 3 ?
Suppose proc calls scanf (line 11), and that scanf reads values 0x46 and 0x53 from the standard input. Assume that the string “%x %x” is stored at memory location 0x300070
A. What value does %ebp get set to on line 3 ?
B. At what addresses are local variables x and y?
C. What is the value of %esp after line 10 ?
D. Draw a diagram of the stack frame for proc right after scanf returns. Include as much information as you can about the addresses and the contents of the stack frame elements.
E. Indicate the regions of the stack frame that are not used by proc (these wasted areas are allocated to improve the cache performance).
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31. Register Value %esp %ebp 0x800040 0x800060
A. What value does %ebp get set to on line 3 ?
proc:
push1 %ebp
movl %esp, %ebp
subl $24, %esp
addl $-4, %esp
leal (%ebp), %eax
pushl %eax
leal -8(%ebp), %eax
pushl $.LC0 pointer to string “%x %x”
call scanf
movl -8(%ebp), %eax
movl -4(%ebp), %edx
subl %eax, %edx
movl %edx, %eax
movl %ebp, %esp
popl %ebp
ret
Assume that procedure proc starts executing with the following register values:
0x800040
C. What is the value of %esp after line 10 ?
0x800014
B. At what addresses are local variables x and y stored ?
X :0x %edx
Y : 0x %eax
Register
Value
%esp
%ebp
0x800040
0x800060
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32. 0x80003C
0x800038
0x800034
0x800030
0x80002C
0x800028
0x800024
0x800020
0x80001C
0x800018
0x800014
0x800060
0x53
0x46
0x800038
0x800034
0x300070
%ebp x y
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33. Q&A ?
Thank you!
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