Download CO1 - Faculty of Engineering

ECP2046 – Computer Organization and Architecture
LAB: CO1
Duration: 3 hours
Introduction to the DEBUG Program
1 Objectives
i. To study and understand the mechanisms involed in program execution in a microprocessor unit
ii. To understand the concept of physical and logical addresses (Segment::Offset addressing).
2 Software
MS-DOS DEBUG program. Please refer to http://kipirvine.com/asm/debug/Debug_Tutorial.pdf
for additional examples of the program.
3 Introduction
The generic term x86 refers to the instruction set of the most commercially successful CPU architecture
in the history of personal computing. It is used in processors from Intel, AMD and others, and derived
from the model numbers of the first few generations of processors, backward compatible with Intel's
original 16-bit 8086 CPU.
3.1 X86 CPU registers
The main tools to write programs in x86 assembly are the processor registers. The registers are like
variables built in the processor. Using registers instead of memory to store values makes the process
faster and cleaner. Table 1 lists some of the registers in the x86 series of processors.
Table 1: List of registers in x86 CPU series
AX
BX
CX
DX
DS
ES
SS
CS
BP
SI
DI
IP
F
Accumulator
Base register
Counting register
Data register
Data Segment register
Extra Segment register
Battery segment register
Code Segment register
Base Pointers register
Source Index register
Destiny Index register
Next Instruction Pointer register
Flag register
Of all the registers listed above, AX, BX, CX, and DX are general purpose registers which will be used
extensively in the following exercises. These are 16-bit registers, which can also be used as 8-bit
registers. To use them as 8-bit registers, it is necessary to refer to them for example as: AH and AL,
which are the high and low bytes of the AX register. This nomenclature is also applicable to the BX,
CX, and DX registers.
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3.2
Physical and logical addresses
A physical (absolute) address is the actual physical location in memory. A logical address is the
address at which a memory location appears to reside from the perspective of an executing application
program.
Logical address may be different from the physical address due to the operation of a memory
management unit (MMU) between the CPU and the memory bus. MMU is a computer hardware
component responsible for handling accesses to memory requested by the central processing unit
(CPU). Its function includes translation of virtual addresses to physical addresses.
An example of the usage of logical address is the Segment:Offset addressing. Segment:Offset
addressing was introduced due to the limitation in the capability to address memory contents. Without
using Segment:Offset addressing, a CPU with a register of 16-bit long can only address 216 or 65,536
bytes (64 KBytes) of memory directly. This of course, significantly limits the size of usable program.
Using Segment:Offset addressing, the CPU is able to group two 16-bit registers together to refer to a
physical memory location beyond 64 KBytes. Of course, increasing the register size in the CPU will
also increase the addressing capability. However, such approach can be very costly.
The physical address for any combination of Segment and Offset pairs is found by using the formula:
Physical address = (Segment value × 1016) + Offset value
There is another easier way to compute physical address from the Segment:Offset values. Refer to the
following examples:
Example 1: The Physical address of the Segment:Offset pair, 0A00:0001 can be computed by inserting
a zero at the end of the Segment value ( which is the same as multiplying by 10 16 ) and then adding the
Offset value:
0A000
+ 0001
--------A001
--------Example 2: Segment:Offset = 023F:02FF
023F0
+ 02FF
--------26EF
--------The disadvantage of using Segment:Offset pairs is the fact that there are a large number of these pairs
that refer to the same exact memory locations. As an example, every Segment:Offset pair below refers
to the same physical address of 7C00 (or 0000:7C00):
0047:7790
004C:7740
007B:7450
0202:5BE0
0048:7780
0077:7490
007C:7440
0203:5BD0
0049:7770
0078:7480
01FF:5C10
0204:5BC0
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004A:7760
0079:7470
0200:5C00
07BB:0050
004B:7750
007A:7460
0201:5BF0
07BC:0040
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The number of different Segment:Offset pairs vary between memory locations, which can be up to
4,09610 pairs. For the physical addresses from FFF0 to FFFFF, the number of Segment:Offset pairs that
refers to the same physical address is fixed to 4,09610 pairs.
For Physical addresses 016 through FFEF (0 through 65,51910), the number of different pairs can be
computed by following 2 simple steps. First, divide the physical address by 1016 (which shifts the entire
hexadecimal digits one place to the right). Second, throw away any fractional remainder and add 1 to
the shifted value.
Below is an example of number of available Segment:Offset pairs for physical address 2B10:
Example 3: Physical address = 2B10
Step 1: 2B10 / 1016 = 2B1
Step 2: 2B1 + 1 = 2B2 (or 69010) Segment:Offset pairs
3.3 Using the DEBUG program
In this section, we will learn to use the DEBUG program to execute assembly language program on an
x86 compatible microprocessor. A summary of the common commands that may be used with DEBUG
is shown in Table 2 below.
Table 2: DEBUG program command sets
Command
assemble
compare
dump
enter
fill
go
hex
input
load
move
name
output
proceed
quit
register
search
trace
unassembled
write
allocate expanded memory
deallocate expanded memory
map expanded memory pages
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Syntax
A [address]
C range address
D [range]
E address [list]
F range list
G [=address] [addresses]
H value1 value2
I port
L [address] [drive] [firstsector] [number]
M range address
N [pathname] [arglist]
O port byte
P [=address] [number]
Q
R [register]
S range list
T [=address] [value]
U [range]
W [address] [drive] [firstsector] [number]
XA [#pages]
XD [handle]
XM [Lpage] [Ppage] [handle]
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3.4 Procedure:
1. Click the Start button on Windows’ taskbar, select Run, type “cmd” in the corresponding dialog
box and press Enter () to invoke MS-DOS command prompt.
2. In the command prompt, type “debug” and press Enter to invoke the DEBUG program.
3. The DEBUG prompt (-) will be displayed, indicating that DEBUG has been executed and is now
waiting for a command.
4. Type in “?” (without the double quotes) at the DEBUG prompt and press Enter. This will display a
summary of the available commands and their syntax, as shown in Table 2.
5. These commands allow DEBUG to load data, examine or modify the state of the microprocessor’s
internal registers and memory, and execute a program. The following exercises will focus only on
some of the commands.
6. The exercises will mainly focus on some of the commands. These commands and their functions
are detailed in Table 3.
7. To quit from the DEBUG program, type “Q” (with the double quotes) in the DEBUG prompt and
press Enter. This will terminate DEBUG and return to the command prompt.
Table 3: Commands and their functions
Command
Register
Quit
Dump
Enter
Unassemble
Assemble
Trace
Go
Function
Examine or modify the contents of an internal register
Exit from the DEBUG program
Dump the contents of memory to the display
Examine or modify the contents of memory
Unassemble the machine code into its equivalent assembler instructions
Assemble the instruction into machine code and store in memory
Trace (single-step) the assembled instructions
Execute the instructions down through the breakpoint address
4 Exercise 1
In this exercise, we will assemble a program which adds together both 8-bit numbers at AL (lower byte
of AX register) and BL (lower byte of BX register), subtract 24H from the result and multiply it with
04H. The instructions used in this program are describe in Table 4 below.
Table 4: Description of instructions used in Exercise 1
Instruction
ADD AL, BL
SUB AL, 24
MOV CL, 04
MUL CL
4.1
Descriptions
Add BL to AL, store result in AL
Subtract 2416 from AL, store result in AL
Store 416 in CL
Multiply CL to AL, store result in AX
Procedure:
1. Invoke the DEBUG program.
2. Using the Register (R) command, change the contents of register AX to 0035H and BX to
0068H as follows:
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-R AX
AX 0000
:0035
-R BX
BX 0000
:0068
-R IP
IP
3. Using the Assemble (A) command, assemble the instructions to perform required arithmetic
operation as follows:
-A CS:0100
XXXX:0100 ADD
XXXX:0102 SUB
XXXX:0104 MOV
XXXX:0106 MUL
XXXX:0108 
AL,BL
AL,24
CL,04
CL
*XXXX indicates that the contents of CS may vary from machine to machine.
4. Using the Unassemble (U) command, the instructions previously assembled into the memory
can be verified as shown below:
-U CS:0100 0106
5. This command will display the instructions assembled between address CS:0100 to CS:0106 in
3 columns: 1st column shows the logical addresses of the instructions; 2nd column shows the
machine codes of each instruction; 3rd column shows the assembled instructions.
6. List down the output of the Unassemble command in Table 5.
Table 5: Output of the Unassemble command
Address
Machine Codes
Instruction
[1 Mark]
7. The Trace (T) command can then be used to step through the assembled instructions by
executing one instruction at a time and the contents of the internal registers after the execution
of each instruction are automatically displayed.
8. Before executing the instructions, use the Register (R) instruction to check the contents of the
microprocessor’s internal registers and record down the contents of AX, BX, CX and IP
registers in Row 1 of Table 6.
[1 Mark]
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9. Execute the first assembled instruction (ADD AL, BL) by using the Trace (T) command as
follows:
-T =CS:0100
10. Identify the contents of AX, BX,CX and IP registers after executing the instruction and list
down their contents in Row 2 of Table 6.
[1 Mark]
11. Execute the next instruction (SUB AL, 24) by issuing the Trace (T) command as follows:
-T

This form of the Trace command is used to execute the next instruction.
12. Identify and list down the contents of AX, BX, CX and IP registers in the next row of Table 6.
[1 Mark]
13. Repeat Step (9) and (10) to execute the two remaining instructions and record down the
contents of AX, BX, CX and IP in Table 6.
Table 6: Contents of IP, AX, BX and CX after each Trace command
Row No.
1
2
3
4
5
6
IP
AX
BX
CX
[1 Mark]
5 Exercise 2
Dump is one the commands from the DEBUG program. It displays the contents of a block of memory.
The Memory locations near the beginning of Segment C000 should display information about the kind
of video card installed on your PC. The example below shows the video card information of a system
obtained from the dump command using logical address C000:0010.
-d C000:0010
C000:0010 24
C000:0020 4D
C000:0030 52
C000:0040 2F
C000:0050 29
C000:0060 50
C000:0070 40
C000:0080 E8
12
20
4F
56
00
43
00
26
FF
43
58
42
87
49
12
56
FF
4F
2F
45
DB
52
10
8B
00
4D
4D
20
87
2B
00
D8
00
50
47
42
DB
10
80
E8
00
41
41
49
87
01
00
C6
00-60
54-49
2D-47
4F-53
DB-87
10-00
00-38
56-74
00
42
31
20
DB
00
37
22
00
4C
30
28
87
18
34
8C
00
45
30
56
DB
00
2D
C8
00
20
20
31
87
00
32
3D
20
4D
56
2E
DB
00
00
00
49
41
47
32
87
00
FF
C0
42 $.......`.... IB
54 M COMPATIBLE MAT
41 ROX/MGA-G100 VGA
20 /VBE BIOS (V1.2
DB )...............
03 PCIR+...........
FF @.......874-2...
74 . &V....Vt"..=..t
The dump command will be used in the following exercise.
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5.1
Procedure
1. Invoke the DEBUG program.
2. Use the dump command to display the video card information on your system.
[1 Mark]
3. Use the dump command to verify that logical addresses 0007:7B90 and 07C0:0000 is pointing
to the same physical address by looking at the memory contents.
[1 Mark]
4. Compute the physical address of logical address 0C10:0010. Verify your answer using the
dump command by looking at the memory contents. Assuming XXXX is your physical address,
type:
d 0000:XXXX
to access the memory location.
[2 Marks]
5. Compute the number of logical address available for physical address 6B00. Provide your
answer in hexadecimal and decimal forms.
[2 Marks]
6. Compute the number of Segment:Offset pairs that point to the same physical location as the
Segment:Offset pair 0030:0000. Include Segment:Offset pair 0030:0000 in your calculation.
Provide your answer in hexadecimal and decimal forms.
[2 Marks]
Hint: Verification can be done by comparing the contents of memory locations to see whether
they are the same or not.
6
Exercise 3
In this exercise, you are required to complete the assembly program for 8-bit unsigned integer
multiplication.
Figure 1: Flow chart for 8-bit unsigned integer multiplication
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6.1
Procedure
1. Assume the following:
AH
AL
BL
CX
=
=
=
=
Register A
Multiplier, Q
Multiplicand, M
Count
2. Create the following assembly program for 8-bit unsigned integer multiplication of multiplier =
25H and multiplicand = 24H by completing the missing parts.
[6 Marks]
ORG
CLC
MOV
MOV
MOV
MOV
0100
AH,
AL,
BL,
CX,
00
_____
_____
_____
MOV DL, AL
AND DL, _____
JZ 0113
ADD _____, BL
RCR _____, 1
CLC
LOOP 010A
MOV DX, AX
3. Verify your answer by keying in the assembly program into the DEBUG program and stepping
through the program. List down the register content change for each step until the program
reaches the end by completing the table below:
Table 7: Contents of the registers in each step
Steps
1
2
3
.
.
.
.
BL
AH
AL
CX
DL
[3 Marks]
4. State the final value in register DX.
[1 Mark]
5. Compute the product of 25H and 24H (Show manual calculation). Compare if whether the
calculated value is the same as the one in register DX.
[2 Marks]
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7
Report writing guidelines
Your report must contain the following sections:
1. Report cover
Please follow the report cover format in the FOE lab webpage. You can download the template
at http://foe.mmu.edu.my/lab/form/student_lab-report_cover-1%202010.doc
2. Brief introduction of the experiment
Describe the motivations/objectives of the experiment and the DEBUG program. Please use
your own words.
[5 marks]
3. Experiment results and discussions
Describe your observations in each exercise of this experiment. All questions in the exercises (if
any) must be answered. Distribution of marks is indicated in each exercise.
[25 marks]
4. Conclusions
Conclude your lab report with a brief summary on knowledge and skills that you have acquired
from this experiment.
[5 marks]
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