The midterm exam will be in class this Friday, October 17.
You are responsible for understanding:
Here's an example midterm exam, for you to get a feeling for my exam style. Answers are listed at the bottom of the page.
1.) Convert each assembly code fragment to C++, and each C++ code fragment to assembly:
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C++ |
Assembly |
|---|---|
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return new int[10]; |
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start: push rdi call print_int pop rdi add rdi,1 cmp rdi,10 jle start |
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int scale2(int x,int y) { return x*7 + y*114; }
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(Score: 18 points, six per blank. Each piece of code is separate.)
2.) If “mov eax,DWORD[rcx+4*rdi]” is accessing an array element,
The pointer to the start of the array is ___________________.
The array index is ____________________.
Array element [3] is stored at [ _____________________ ].
Each array element has ______________ bits.
(Score: 12 points.)
3.) Fill in the blanks to make this code return 0x12.
sub rax,rax
mov _______, [ptr]
ret
ptr:
___ 0xffff12
(Score: 6 points.)
4.) Amazingly, this C++ code returns zero. Why? _____________________
int x=2048, y=2048, z=1024;
return x*y*z;
How could you detect this sort of problem? ____________________________________
How could you get the correct answer? ____________________________________
(6 points. )
5.) What does this code return? ____________
push 123
push 456
pop rax
pop rdx
ret
(4 points.)
6.) True or false!
___ You can always store any value into any scratch register.
___ In 32-bit mode, the 64-bit registers such as rax run half as fast.
___ Push and pop should be used to preserve the value of scratch registers like r9.
___ A value stored in a preserved register will not change after calling a function.
(8 points.)
7.) This code works as is, but why do I need the “pop”? _____________________________________
call myfunction
back:
pop rdx ; ???
ret
myfunction:
mov rax,17
jmp back
How could you make this code cleaner? Sketch your changes above.
(6 points.)
8.) Write some assembly to call the C function “void scratchy(long *data);” with one long of valid memory allocated from the stack or heap.
ANSWER KEY & Suggested Readings
1.)
mov rdi,40 ; 4 bytes / int, 10 ints
extern malloc
call malloc
; my return value rax == malloc' return value rax
void print_ints_to(int param) {
for (int i=param;i<=10;i++) print_int(i);
}
global scale2
scale2:
; edi: parameter 1; esi: parameter 2
imul edi,7
imul esi,114
mov eax,edi
add eax,esi
ret
2.)
rcx is the start of this array
rdi is the array index
[rcx+4*3]
32 bits per DWORD
3.)
al
(This one is subtle; you want the low 8 bits of the answer.)
4.)
Detect with "jo"
Use a 64-bit long to get correct answer
5.) 456
6.)
True, scrach registers can be freely used.
False, in 32-bit mode rax does not even exist
True, r9 is a scratch register
True, functions will not change preserved registers.
7.)
You need the pop to balance "call myfunction". It'd be cleaner if myfunction did "ret" instead.
8.)
mov rdi,8
extern malloc
call malloc
mov rdi,rax
extern scratchy
call scratchy
; also call free!
For this year's exam, I'm trying to lean more toward conceptual problems than implementation problems: more why, than how. This means you need to understand both why and how!