1. Microprocessor and Assembly Language
Lecture-4-Introduction to Assembly Language Programming with Debug Utility
Muhammad Hafeez
Department of Computer Science
GC University Lahore

2. Today’s Agenda Introduction to Assembly Language Programming
Debug Utility

3. .COM and .EXE File
DOS allows two types of executable in its command shell
.COM --- Max. Program Size 64K
Directly converted into binary
If a .com file takes 10 bytes, actual size of file will be 10 bytes
Debug deals with .COM
Same Value in SS, DS, ES and SS
.EXE ---- Max. Program Size all available memory
Minimum Size 640 Bytes

4. .COM and .EXE File

5. Assembly Language Instructions
Data Movement Instructions
Arithmetic Instructions
Logic Instructions
Control Instructions

6. Data Movement Instructions
MOV Instruction
XCHG Instruction

7. MOV Instruction Move Byte/ Word to Syntax is,
Register-to-Register
Register-to-Memory
Memory-to-Register
Immediate [common error is trying to move larger constants than destination size]
Syntax is,
Size of the operands must be same
MOV Destination, Source
Source is from where we pick data
Destination is where we put data
CS and IP must never be destination

8. Legal Combinations of MOV Instruction
Destination
Source
General Register
Segment Register
Memory Location
Constant
Yes No

9. Legal Combinations of XCHG Instruction
Destination
Source
General Register
Memory Location
Yes No

10. Arithmetic Instructions
ADD Instruction
SUB Instruction
INC Instruction
DEC Instruction

11. ADD & SUB Instructions ADD Syntax SUB Syntax ADD Source, Destination
SUB Source, Destination

12. Legal Combinations of ADD & SUB Instructions
Destination
Source
General Register
Memory Location
Yes No
Constant

13. INC & DEC Instructions INC Syntax DEC Syntax INC Destination
DEC Destination
INC add 1 to the destination
DEC subtract 1 to the destination

14. NEG Instructions
NEG instruction negate the contents of memory or Register
Does this my taking 2’s complement of content
NEG Syntax
DEC Destination
NOTE: In all the Instructions Size/ Type of Operands must be same, however, assembler accepts MOV al,’C’ and MOV ax,’C’

15. Examples:
A = A -1
A = B
A = B + 2*A
B = B-1
Using Debug

16. Install DosBox
DOSBox is an Emulator which emulate old 8086/88 and 80286/386 environment on your current i3/i5/i7 processor.
Follow the steps given in online post ”Installing MASM on Windows 7”

17. Debug Utility
A debugger displays the content of memory quickly and easily, showing registers and variables.
You can step through a program one line at a time (called tracing).
Some debugging functions are:
Assemble short program
View a program’s source code, along with its machine code
View the CPU registers and flags
Trace or execute a program, watching variables for changes
Enter new values in memory
Fill a block of memory
load and write disk files

18. Debug Utility Debug assume all the numbers in HEX
C:\Masm>Debug [Press Enter]
R [to see the Registers]
R Register name to change the content
R AX
A Start Address [Assemble the Code]
A 0100
U Start Address End Address
U Start Address L #of bytes
U D
U 0100 LD

19. Debug Utility G [Run a program till breakpoint]
G=start Address End Address
G [press enter]
G= D
R [to see the Registers]
T [Trace, to run a program step by step]
T = Start Address # of instruction
T [press Enter]
T= [Run Two instructions]
T 3 [only number of instructions, assume IP=0100]

20. Debug Utility Common Errors with Register Usage MOV AH,FF3
MOV AX,12345
MOV DS,1240
MOV SI,DH
MOV BX,AH
MOV AL,CX
[Errors]

21. Data Manipulation in Debug Utility
F [fill the block of memory with data]
F [start address] [end address] [data]
F [start address] [L no of bytes] [data]
F 0 10 FF
F 0 LD EF F [alternate pattern]
D [Dump command, examine the content of memory]
D [start address] [end address]
D [start address] [L no. of bytes]
Variation of D

22. Data Manipulation in Debug Utility
D [dump will be shown from DS:0100 to 128 bytes]
D offset [D will assume DS as segment]
Fill and look up the Dump
Enter Command and Use Dump to examine machine code
E [Enter Data with E command]
E [start address] data
E Start Address
E DS:
E DS:0 ‘Adam Smith’
E CS:0100

23. Data Manipulation in Debug Utility
Alter data using E
E 106 [press enter]
P [Procedure], used to execute procedure
P [start address] [no. of instructions]
M [start address], [end address] [dest. Address]
C , Check block of memory for difference
C [start address] [end address] [compare address]

24. Debug Utility Debug assume all the numbers in HEX
C:\Masm>Debug [Press Enter]
R [to see the Registers]
R Register name to change the content
R AX
A Start Address [Assemble the Code]
A 0100
U Start Address End Address
U Start Address L #of bytes
U D
U 0100 LD

25. Debug Utility
D CS: D F
Either Enter program through E
E100
E 101 to put another char to print
Or Assemble with A
G to run the program U
HEX numbers back to mnemonic codes
See unassembled HEX numbers
Stack first CS is loaded then IP is loaded

26. Debug Utility
D CS: D F
Either Enter program through E
E100
E 101 to put another char to print
Or Assemble with A
G to run the program U
HEX numbers back to mnemonic codes
See unassembled HEX numbers
Stack first CS is loaded then IP is loaded

27. Debug Utility R to manipulate Registers Rax to change AX register
Stack first CS is loaded then IP is loaded

28. Debug Utility Save program on disk from debug utility
Three step process
-Nfilename.com Name of the File
How many bytes the program has taken
CX, BX register will be used to hold bytes to save
CX lower order of this number, BX high order of this number
Use only CX register to hold number of bytes, put no of bytes in CX -W

29. Debug Utility - Summary

30. Little Endian and Big Endian
If low byte is stored at lower memory address called little endian
Used in INTEL processors
If low byte is stored at high memory address called big endian
Used in Motorola processors

31. Questions
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