CS31 Weekly Lab Week 4

IA32 assembly and gdb disassembly

Create a week04 subdirectory in your weeklylab subdirectory and copy over some files:

    cd cs31/weeklylab
    mkdir week04
    cd week04
    cp ~kwebb/public/cs31/week04/* .

    Makefile conditions.c simpleops.c

Week 4 lab goals:

  1. Disassemble binary executables with objdump, and gdb.
  2. Recap ALU condition codes.
  3. Explore real PC hardware.

Using gcc to generate IA32 assembly

Let's try out gcc to build IA32 assembly files and .o files and look at the results. Open up simpleops.c.

We are going to look at how to use gcc to create an assembly version of this file, and how to create a object .o file, and how to examine its contents. We are also going to look at how to used gdb to see the assembly code and to step through the execution of individual instructions.

If you open up the Makefile you can see the rules for building .s, .o and executable files from simpleops.c. We will be compiling the 32-bit version of instructions, so we will use the -m32 flag.

gcc -m32 -S simpleops.c   # just runs the assembler to create a .s text file
gcc -m32 -c simpleops.s   # compiles to a relocatable object binary file (.o)
gcc -m32 -o simpleops simpleops.o  # creates a 32-bit executable file
To see the machine code and assembly code mappings in the .o file:
objdump -d simpleops.o
You can compare this to the assembly file:
cat simpleops.s

gdb disassemble and instruction stepping

Next, let's try disassembling code using gdb. With gdb we can execute individual IA32 instructions, examine register values, and disassemble functions. Do a 'make clean' then a 'make' to rebuild an IA32 version of the simpleops executable file.

gdb ./simipleops
(gdb) break main
(gdb) run
In gdb you can disassemble code using the disass command:
(gdb) disass main
You can set a break point at a specific instruction:
(gdb) break *0x5655550f   # set breakpoint at specified address
(gdb) cont
(gdb) disass
And you can step or next at the instruction level using ni or si (si steps into function calls, ni skips over them):
(gdb) ni	  # execute the next instruction then gdb gets control again
(gdb) ni
(gdb) ni
(gdb) ni
(gdb) ni
(gdb) disass
You can print out the values of individual registers like this:
(gdb) print $eax
You can also view all register values:
(gdb) info registers
You can also use the display command to automatically display values each time a breakpoint is reached:
(gdb) display $eax
(gdb) display $edx

ddd, a graphical debugging interface

Next, we're going to try running this in ddd instead of gdb, because ddd has a nicer interface for viewing assembly, registers, and stepping through program execution:

ddd ./simpleops

The gdb prompt is in the bottom window. There are also menu options and buttons for gdb commands, but I find using the gdb prompt at the bottom easier to use.

You can view the assembly code by selecting the View->Machine Code Window menu option. You will want to resize this part to make it larger.

You can view the register values as the program runs (choose Status->Registers to open the register window).

See Tia's GDB Guide for more information about using gdb and ddd. (see the "using gdb to debug assembly code and examine memory and register values" section).

Also see figure 3.30 on p.255 of the textbook.

Condition codes

Recall the CPU condition codes we talked about in class:

ZF: If the computed result is zero.

SF: If the computed result's most significant bit is set (i.e., it's negative if interpreted as signed).

CF: If the computed result caused an overflow, when interpreted as an unsigned operation.

OF: If the computed result caused an overflow, when interpreted as a signed operation.

Assume %eax holds the value 5, and %ecx holds 7. Suppose we executed the following instructions:

  1. subl $5, %eax
  2. cmpl %ecx, %eax

For each of these instructions, which flags should be set?

Experimental verification

Now that think we know which flags would be set, let's see if we're correct. Open conditions.c in your text editor. We'll use this simple C file to generate the above instructions (or, at least some very similar ones).

Run make, and let's open ddd:

ddd ./conditions

Let's open the register status information window (described in the ddd section above) and put a breakpoint at main:

break main

From here, we can step through individual instructions (via the ni command). Let's stop when we get to the second sub instruction (sub $0x5, %eax). We can see that at this point, %eax holds the value 5, as expected. Now, execute the instruction with ni. Are the condition codes (in the eflags register) what you expected them to be?

Now do the same with the cmp instruction that comes a few instructions after the sub we just examined.

The condition codes for your ALU should work the same way!

Computer Teardown

Log out, we're taking a field trip! You've been assigned to a groups, and each group is assigned a computer that you can take apart. The goal is to find as many parts of a computer as you can. We have tools available to remove parts from your computer, and here are a few links that may be helpful:

Try to identify some of the following:

Before leaving lab, please clean-up all spare parts, screws, etc. that you have removed by putting them in the boxes. You do not need to put the cases in the boxes, just all loose parts. CPU thermal glue is toxic, so just to be extra safe I'd recommend washing your hands after lab today.