CSC 406: Computer System I, 2020 Fall, Assignment #3

Last revised 2020 October 22

Purpose:

To:

Go over the basics of assembly language
Go over how to use a debugger
Go over the layout of activation records/stack frames

Assignment

Please do the following:

Download the program called toAnalyzeCDM.zip from COL
Use an sftp program like filezilla to upload it to a ctilinux machine (like 140.192.36.184) Do not bother unzipping it on your local machine.
On the same cdmlinux machine unzip it with:
```
$ unzip toAnalyzeCDM.zip
```
Do
```
$ chmod u+x toAnalyze
```
to make tell Unix that it is an executable program

Analyze it with gdb: gdb toAnalyze. It has a structure like:

int kale (/* some number of args */)
{
  /* Some number of local vars */

  return( /* something */ );
}


int collardGreen (/* some number of args */)
{
  /* Some number of local vars */

  /* Some loop */

    /* Some code, including call(s) to kale() */

  return( /* something */ );
}


int main ()
{
  /* Some number of local vars, including call(s) to collardGreen() */
  return(0);
}

Answer the following:

(20 Points) Assembly language understanding (1):

The assembly language for kale() is:

(gdb) disass kale
Dump of assembler code for function kale:
   0x00000000004004ed <+0>:	push   %rbp
   0x00000000004004ee <+1>:	mov    %rsp,%rbp
   0x00000000004004f1 <+4>:	mov    %edi,-0x14(%rbp)
   0x00000000004004f4 <+7>:	mov    -0x14(%rbp),%eax
   0x00000000004004f7 <+10>:	add    $0x1,%eax
   0x00000000004004fa <+13>:	mov    %eax,-0x4(%rbp)
   0x00000000004004fd <+16>:	mov    -0x4(%rbp),%eax
   0x0000000000400500 <+19>:	add    $0x1,%eax
   0x0000000000400503 <+22>:	mov    %eax,-0x8(%rbp)
   0x0000000000400506 <+25>:	mov    -0x4(%rbp),%eax
   0x0000000000400509 <+28>:	imul   -0x8(%rbp),%eax
   0x000000000040050d <+32>:	pop    %rbp
   0x000000000040050e <+33>:	retq   
End of assembler dump.

Give a 1-2 sentence description of the purpose of each instruction.
I am more interested in the why than the what.

Instruction:	Purpose:
push %rbp	___________________________________________________________
mov %rsp,%rbp	___________________________________________________________
mov %edi,-0x14(%rbp)	___________________________________________________________
mov -0x14(%rbp),%eax	___________________________________________________________
add $0x1,%eax	___________________________________________________________
mov %eax,-0x4(%rbp)	___________________________________________________________
mov -0x4(%rbp),%eax	___________________________________________________________
add $0x1,%eax	___________________________________________________________
mov %eax,-0x8(%rbp)	___________________________________________________________
mov -0x4(%rbp),%eax	___________________________________________________________
imul -0x8(%rbp),%eax	___________________________________________________________
pop %rbp	___________________________________________________________
retq	___________________________________________________________

(10 Points) Assembly language understanding (2):
Write a C function that does what kale() does.
You won't be able to figure out the names of my parameters var(s) and local var(s); just make up your own name(s).

(20 Points) Activation Records (1):

Stop the program at its fifth call to kale(). When I did so I got the following:

(gdb) break kale
Breakpoint 1 at 0x4004f1
(gdb) run
Starting program: /home/instructor/Documents/Academic/DePaul/Classes/CSC373/202021-1Fal_406/Assign3/toAnalyze 

Breakpoint 1, 0x00000000004004f1 in kale ()
Missing separate debuginfos, use: debuginfo-install glibc-2.17-307.el7.1.x86_64
(gdb) c
Continuing.

Breakpoint 1, 0x00000000004004f1 in kale ()
(gdb) c
Continuing.

Breakpoint 1, 0x00000000004004f1 in kale ()
(gdb) c
Continuing.

Breakpoint 1, 0x00000000004004f1 in kale ()
(gdb) c
Continuing.

Breakpoint 1, 0x00000000004004f1 in kale ()
(gdb) stepi
0x00000000004004f4 in kale ()
(gdb) stepi
0x00000000004004f7 in kale ()
(gdb) stepi
0x00000000004004fa in kale ()
(gdb) stepi
0x00000000004004fd in kale ()
(gdb) stepi
0x0000000000400500 in kale ()
(gdb) stepi
0x0000000000400503 in kale ()
(gdb) stepi
0x0000000000400506 in kale ()
(gdb) stepi
0x0000000000400509 in kale ()
(gdb) stepi
0x000000000040050d in kale ()
(gdb) info reg
rax            0x14	20
rbx            0xc	12
rcx            0x400590	4195728
rdx            0x7fffffffdd28	140737488346408
rsi            0x3	3
rdi            0x3	3
rbp            0x7fffffffdbe0	0x7fffffffdbe0
rsp            0x7fffffffdbe0	0x7fffffffdbe0
r8             0x7ffff7dd5e80	140737351868032
r9             0x0	0
r10            0x7fffffffd720	140737488344864
r11            0x7ffff7a2f460	140737348039776
r12            0x400400	4195328
r13            0x7fffffffdd10	140737488346384
r14            0x0	0
r15            0x0	0
rip            0x40050d	0x40050d <kale+32>
eflags         0x206	[ PF IF ]
cs             0x33	51
ss             0x2b	43
ds             0x0	0
es             0x0	0
fs             0x0	0
gs             0x0	0

Write the activation record (a.k.a. stack frame) for kale() when %rip gets to 0x000000000040050d.
Under Value put the numeric value held at that address.
Under Purpose put one of the following:

not part of kale()'s activation record
argument to kale()
the address in collardGreen() to which rip should return
the stored rbp address for collardGreen()
local variable to kale()
in the activation record of kale(), but not used

	Address:	Value:	Purpose:
	rbp + 0x10	___________	___________
	rbp + 0xC	___________	___________
	rbp + 0x8	___________	___________
	rbp + 0x4	___________	___________
rbp -->	rbp + 0x0	___________	___________
	rbp - 0x4	___________	___________
	rbp - 0x8	___________	___________
	rbp - 0xC	___________	___________
	rbp - 0x10	___________	___________
	rbp - 0x14	___________	___________
	rbp - 0x18	___________	___________

(10 Points) Assembly language understanding (3):
What are the value(s) that collardGreen() obtains as argument(s) from main()?
In which register(s) are they passed?

(10 Points) Assembly language understanding (4):
How many local variables does collardGreen() have?
Where are they on the stack?
Give an offset from rbp from within collardGreen()'s activation record. (Include arguments passed in registers that are subsequently placed on the stack as local variables, too.)

(20 Points) Debugger usage (1):

collardGreen() has a loop. Inside of collardGreen() what are the values of both its arguments and local variables the first time, second time, third time and fourth time rip is 0x0000000000400553? At the top of the table give the offset from rbp (the hexadecimal number added to rbp to get the address of the variable) of the parameter or local variable. (I may have tried to fool you the the number of variables.)

In the body of the table write the values that that variable has when you hit address local variables.

Call:	rbp + _____	rbp + _____	rbp + _____	rbp + _____	rbp + _____	rbp + _____
1	________	________	________	________	________	________
2	________	________	________	________	________	________
3	________	________	________	________	________	________
4	________	________	________	________	________	________

(5 Points) Debugger usage (2):
What value does collardGreen() return to main()?

(5 Points) Assembly language understanding (5):
collardGreen() calls kale() twice. kale() starts at address 0x00000000004004ed. If you look at the machine code for collardGreen()'s second call to kale(), however, you'll see that the actual number in the function call is 0xFFFFFFA7
```
000000000040050f <collardGreen>:
  . . .
  40053f:	89 c7                	mov    %eax,%edi
  400541:	e8 a7 ff ff ff       	callq  4004ed <kale>
  400546:	0f af c3             	imul   %ebx,%eax
  . . .
	
```
1. To what number did the CPU add with 0xFFFFFFA7 to get the address of kale(), 0x00000000004004ed?
2. Do this addition. Compute 0x0040,04ED.