1. Lecture 1. Program Surgery
Wonder of Execution
The Visual C++ Debugger
Data type, integer, short
Addresses, content, pointer
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2. Program Surgery * Means how to debug a program:
we need to master the Visual C++ debugger, and understand
how data (one byte, two bytes..) is stored in the memory,
the address to store the data
the characteristics of function call (call a function) and the program code.
In USA, a programmer does two things – make the program faster and debug it.
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3. Short Type - data and address
Short - occupy 2 bytes 12
1234
Here, a memory location of 1234 contains a value of is address and 12 is the content. 13
Memory address
1236 14
1238
As it occupies two bytes, the address is
incremented by 2. Say 1234, = 1236
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4. Integer Type - data and address
Integer - occupy 4 bytes 12
1234
Here, a memory location of 1234 contains a value of is address and 12 is the content. 13
Memory address
1238 14
1242
As it occupies four bytes, the address is
incremented by 4. Say 1234, = 1238
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5. Difficult to add two numbers in Computer
To add two numbers such as a and b from human viewpoints is very simple.
It is simply to add them. However, from computer viewpoint, it is quite complicated involving a lot of steps and intermediate utility.
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6. Explanation No need to memorise
c = a + b in a programming language, the steps for a computer
Look in an "index" to see in which "sheet" it has jotted down the current value of a.
Look in the "index" to see in which "sheet" it has jotted down the current value of b.
Copy the two values to a working pad, one on top of the other.
Add the two numbers, digit by digit, paying attention to carries.
Look in the "index" to see in which "sheet" it keeps the current value of c
Copy the result to the sheet where it keeps the value of c.
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7. An "abstraction”
No need to memorise
It is a notion that describes multiple instances of objects but that is clear that the instances
For example, driving a car, you don’t need to know how combustion engine works, but you know how to control.
For example, c = a + b, you don’t need to tell when to get a, b and add together to produce c, (internally, CPU controls the electronic gate to do c = a + b, step by step)
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8. Compilers and Debuggers
For the following simple loop in C++
for (i = 0; i < 4; i++) { sum += array[i]; }
The Visual C++ compiler will translate as follows:
It is difficult to understand
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9. the location of the variable array
is given by the number , which we now highlight in the original machine code:
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10. Debugger – a tool to determine the contents of variables
A tool that allows us to stop the program and inspect its variables in the middle of its execution is called a debugger.
It helps us (user) determine anything goes wrong.
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11. Steps to debug a simple program
Look at the memory location
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12. Visual Environment
Visual C++ is a very large utility and has many components that we will not be using in this course
Demonstration
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13. The value i will be incremented by one
Visual C++ debugger
Press F10 will move by one
Demonstration: how to set break point F9, F10, and dump memory data
The value i will be incremented by one
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14. Variable and Addresses
Global (access by all sub-routines), local (only to its routine)
int c  This is a variable occupying 4 bytes
char c  This is a variable occupying 1 byte, the remaining three are 0xcccccc
&c, the address of variable c (usually 4 bytes (unsigned )) that contains name of variable
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15. Memory location, address
Note that in Visual, the above occupy 4 bytes, it means that some are not used.
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16. int var (var is a variable and occupies 4 bytes )
Pointer
int var (var is a variable and occupies 4 bytes )
int *var (*var is a pointer that points to an integer)
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17. Example 1
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18. & means address of varpt
Example 2
Pointer to an integer
int var = 108; //define var as an integer and assign a value of 108
int *varpt; //define varpt as a pointer
varpt = &var; //content of varpt is address of var
& means address of varpt
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19. Explanation of variable and address
address 0x0012ff78, value = 0x0012ff7c
address 0x0012ff7c, value = 108, 0x c
looking from this direction
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20. Example 3
Point to location 0x0012FF7C
Value is 0x61 = ‘a’
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21. Same result, different expression
Array and Pointer *
char a[11]=“ ”; //reserve a[11] as ‘\0’
a[0] =“1”;
a[1] =“2”;
char *ptr; //point to a single byte, not an integer (4 bytes)
ptr = &a; (or ptr = a in C) //now the content of ptr is the
address of variable a
*ptr -> // (a[0])
*(ptr + 1) ->2 // (a[1])
array[i] * (array + i) &array[i] array + i array[i + j] * (array + i + j) &array[i + j] array + i + j
Same result, different expression
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22. Example –look at the result
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23. Example – look at the ptr++
The pointer will be incremented by one, together with + i, it increments by 2
Result
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24. Multi-Dimensional Arrays – pointer of pointer
array2[6][2] or *(*(array2 + 6) +2) 5 6 1 2 3 4 1 2
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25. Example int a[2][3] = {0, 1, 2, 3, 4, 5}; a[0][0] = “0”; a[0][1] = “1”
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26. Example (1), note the value
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27. Example (2), the value of a[0][2]
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28. Naughty Pointers
The value pointed by pointer is modified. (for example, you write a program that uses pointer to modify the content of your program, it will destroy the program and is not recommended.)
However, if you can master pointer, you can write a very elegant program. (writing pointer is the essential skill)
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29. Summary : Integer Integer : 4 bytes such as int a = 3;
In hex 0x
Integer : 4 bytes such as int a = 3;
The memory location (address of a is 0x0065FDF4)
0x0065FDF4:03
0x0065FDF5:00
0x0065FDF6:00
0x0065FDF7:00
0x0065FDF4
0x0065FDF7
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30. Summary: Short Short: two bytes Short a = 3; 0x0065FDF4: 03
Not used
Short: two bytes
Short a = 3;
0x0065FDF4: 03
0x0065FDF3: 00
Short b = 4;
0x0065FDF0: 04
0x0065FDF1: 00
As short uses 2 bytes, remaining two bytes in memory 0x0065FDF2 (0xCC) and 0x0065FDF3 (0xCC) are not used,
0xCC means
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31. Summary: Pointer
address of a (means &a) is 0x0065FDF4, content is 0x03
address of pointer is 0x0065FDF0, content 0x0065FDF4 (you have to look at from right hand), address of variable a
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