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2003-05-04 01:19

Assembler Control
Trace: CALL
Trace: INT
Trace: small prog

Assembler Programming

This is an index page to resources to help you learn to program in Intel Assembler and debug your programs using DOS DEBUG.

It contains the following sections:

Your first Intel Assembler programs
Program Style for Full Marks
Understanding PUSH, POP, CALL, and INT
Program Debugging and Tracing

Your first Intel Assembler programs

Sample Programs

The sidebar contains links to many Intel assembler programs.  First is a one-page program named onepage.asm that you can select and download to make sure your assembler and linker are working correctly.  (If you can't assemble and run this simple program without any warnings or errors, something is wrong with your assembler or linker!)

The command lines used to assemble and link this program are given in the comments at the beginning of the program.  There is also a Web page describing them.  Either put the ASM and VAL programs in the same directory as your program, or adjust your DOS PATH variable to include where they reside.  Read the documentation for more details on how the ASM and VAL programs work.

Next is Alan's stars.asm program that prints numbers of asterisks based on one key of input, and noecho.asm that loops reading characters without echoing them to the screen.

The more complex tail.asm program displays the Command Tail in the Program Segment Prefix.  It shows how a set of high-level language statements (e.g. in C language) might be turned into assembler by a compiler (though this program was translated by hand, not by a compiler).

Next is a longer program (mostly comments!) named first.asm that uses Alan Pinck's I/O Package.  (This package contains some subroutines for inputting and outputting numbers.)  You will need Alan's I/O package in the current directory on your disk to assemble this example.  This program is a good test to see if the I/O package downloaded to your computer correctly.  Program addtwo.asm is another example that uses the I/O package.

The program series.asm also uses Alan's I/O package and is a fairly direct translation of LMC (Little-Man Computer) mnemonic instructions into Intel mnemonic instructions.  The original web page describing this algorithm is here:

Program Control Flow

Assembler has no high-level control flow statements.  To achieve the same effect, you must use conditional branching.  See the Assembler Control page for details.

Program Style for Full Marks

Programs submitted for marking in this course get maximum marks if they adhere to the following structure:

The first line of each program must contain its name.  This enables the instructor to find the program on your diskette, if the assignment requires you to submit the program on diskette.
The comments at the top of the source file for the program contain a copy of the Ian Allen Assignment Submission Label.
Assembler programs are written in four columns: Labels, Mnemonics, Operands, and Comments.  Programs that choose to put Labels in column 2 or Mnemonics in column 1 are unreadable and unmarkable.  Follow the example of the sample programs carefully!
Comment your code!  Unlike high-level languages where you can see by the names of the variables what manipulation is happening to your data, assembly language programming involves operations on registers whose names have no intrinsic meaning in the algorithm.  Use meaningful comments that relate to how the code expresses your algorithm.  Do not write comments that simply echo the action of the assembly-language statement:
    ADD    AL,20h  ; add 20h to AL  (pointless comment!)
Comments must relate to your algorithm:
    ADD    AL,20h  ; make lower-case  (good comment!)

Often, every line of an assembler program carries some form of simple in-line comment that explains how the operation relates to the algorithm being used.

Text editing your ASM Programs

The ASM and MASM assembler programs both insist that files be well-formed, with proper line end characters on every line, including the last line of the file.  You cannot create a text file using a word processor, unless you explicitly select "Save as text" when you save the file.  If you use Windows Notepad to build an ASM file, make sure that the last line of the file ends in a RETURN character.  Failure to do this will result in "premature end of file" errors of this type:

 C:> asm /s onepage ;
 End of file encountered on input file
     end start
 onepage.asm(35): ERROR#85 - Premature end of file

Edit the file to add the missing RETURN on the last line of the file.  Reading the file into DOS EDIT and writing it out will fix the problem, as will reading the file into VIM (an open source version of Unix VI) and writing it out again.

When in doubt, put a few blank lines at the end of your program source files!

If you see programming as a career, you will do well to pick and learn a proper text editor that has cut buffers, key macros, and global search-and-replace capabilities.  (I use VIM on both Unix and Windows.)  Keyboard and editing skills should not stand in your way of a good job!


The course makes extensive use of the DOS DEBUG command.

Here are some links to descriptions of how to use the DOS DEBUG command:

Saving output from DEBUG

Some of the assignments require you to save the output of DEBUG in a file.  If you're running in a DOS Window under Windows 9x, you can always use the mouse to copy text and paste it into another application such as Notepad, Wordpad, or Write.  (Do not save the window as a graphic using Print Screen - the resulting file is huge and unnecessary.  Save the text only, using cut-and-paste into another application such as Write, Wordpad, or Notepad, and print from there.)

When you print some screen text or programs, you must use  a Courier or Terminal fixed-width font so that the text lines up.  Do not print with a variable-width font such as Times, Tahoma, or Arial!

If you're running in pure-DOS mode, without Windows, study the examples below under the heading DEBUG Scripts.  You can enter a few DEBUG commands into a text file, and have DEBUG read the file and execute the commands while you redirect the output into an output file.  Then, you can print the output files (using a Courier or Terminal fixed-width font).

Understanding PUSH, POP, CALL, and INT

The PUSH, POP, CALL, and INT instructions all use the stack (the memory area pointed at by SS:SP).  The following DEBUG scripts and their annotated output files show what happens.  Each of the input files was run through DEBUG to produce the corresponding output file, to which explanatory comments were added by hand:

C:> debug <push_pop.txt >push_pop_out.txt
C:> debug <call_push.txt >call_push_out.txt
C:> debug <int_push.txt >int_push_out.txt
Read these Output files for annotated examples of the workings of PUSH/POP, CALL, and INT:
DEBUG Input Annotated DEBUG Output
push_pop.txt  push_pop_out.txt
call_push.txt  call_push_out.txt
int_push.txt  int_push_out.txt

Homework, test, and exam questions typically show one of these types of instructions and ask you to describe which registers and what memory is affected after the instruction executes (see the Homework questions!).

Program Debugging and Tracing

The DOS DEBUG command lets you single-step through your programs to debug them. You can also use it to load raw disk sectors from disks and examine them (as done in Project 4).

DEBUG scripts

One of the handy things you can do with DEBUG is prepare small text files of DEBUG commands (and even include assembler instructions) and run them through DEBUG using command line redirection to get a quick idea of how something works:

C:> debug  <inputcommands  >outputfile

See the script file debug_script.txt and its corresponding output file debug_script_out.txt.  I've edited the output to add comments explaining what I was doing in the script file, and how to create the script file.

If you're trying to dump disk sectors (as you do in Project 4 in this course), you may find these kinds of DEBUG scripts, used with command line redirection, necessary for saving your DEBUG output into files for later printing: Script file: debug_dump.txt Script output: debug_dump_out.txt

Tracing COM file programs

DEBUG is able to load and let you single-step through ".COM" format executables.  The quickest way to get the executable loaded into DEBUG is to give the program you wish to debug as a command argument.  All .COM programs start at segment offset 0100h, so that's where you'll find your first instructions, ready to be traced:

C:> debug
-u 0100 
1454:0100 BE8100        MOV     SI,0081
1454:0103 803C0D        CMP     BYTE PTR [SI],0D
1454:0106 7421          JZ      0129
1454:0108 803C20        CMP     BYTE PTR [SI],20
1454:010B 7511          JNZ     011E
1454:010D BA2F01        MOV     DX,012F
1454:0110 B409          MOV     AH,09
1454:0112 CD21          INT     21
1454:0114 83C601        ADD     SI,+01
1454:0117 803C20        CMP     BYTE PTR [SI],20
1454:011A 74F8          JZ      0114
1454:011C EBE5          JMP     0103
1454:011E 8A14          MOV     DL,[SI]

With your program in memory, you are now ready to debug it by single-steping through it using the "Trace" or "Proceed" commands, watching the flow of control and the values of the registers.

If you want to supply command line arguments to a program being debugged, simply add them to your DOS command line when you call DEBUG:

C:> debug argument1 argument2 arg3 ...etc...

Using "Trace" vs. "Proceed" in DEBUG

Don't use "Trace" to trace an INT instruction; use "Proceed" instead.  If you trace the INT instruction, you will end up tracing the call to the interrupt service routine in DOS that actually performs the interrupt service, and that can be many hundreds or thousands of instructions!  The "Proceed" instruction will not trace the interrupt service routine; it will simply let it execute (without tracing each instruction) and it will pause when the service routine returns to your program.


Web Author: Ian! D. Allen      Updated: 2003-05-04 01:19

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