Re: 8086 Assembly Programming Help

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Date: Mon, 5 Apr 2004 17:43:11 +0000 (UTC)

"Steve, I-Net+" <aeroman10@yahoo.com> wrote in message
news:b0d144cb.0404041245.268c6ca4@posting.google.com...
> I am using DEBUG through MS-DOS to make and run a certain program.
> I need to write a program through DEBUG that will do the following:
>
> 1. Input two one digit numbers from keyboard [A and B]
> 2. Compare the numbers and display "First Larger" or "Second Larger"
> 3. Add the numbers and display the sum
> 4. Subtract the smaller from the larger and display the difference
> 5. Multiply the numbers and display the product
> 6. Divide the larger number by the smaller and display the quotient..
>
> Now my knowledge of assembly language is very limited, I havent taken
> a class to learn it yet.
> I know how to do the basic things like MOV, ADD, SUB but when it comes

You don't need much more for this task (div,mul..)

Calling interrupts goes like this:

mov ah, function_number ; (list of functions can be found in the link
bellow)
int xx ; xx stands for interrupt number (21H for DOS, 10H for video, 13H
for disk...)

Sometimes you need to pass a couple of other parameters in registers
al,dx,bx...
And that's it, easier than calling windows api.
For example, to print a string (masm format, for debug you'll use a
subcommand for code and a subcommand + db subsubcommand for data, and keep
track of all data and jump locations - forget all the labels and directives
(stuff with '.' in front) - read: locate masm/tasm/nasm) :

.model tiny ;one 64KB segment for code/data/stack

.data ;segment
    SomeString db "Goodbye world$" ; has to be terminated with $
(db goes for define byte)

.code ;segment
.startup
main:
    mov ah,9 ; DOS function print string
(9)
    mov dx, OFFSET SomeString ; dx carries the location of SomeString
    int 21H ; call DOS interrupt
number

    mov ah,4cH ; DOS function to exit program
(4c)
    int 21H

end main ; terminates the asm source
and points to the first instruction (label main:)

Ralf Brown's interrupt list:
http://www.ctyme.com/rbrown.htm
http://www-2.cs.cmu.edu/afs/cs/user/ralf/pub/WWW/files.html

To convert a number from binary integer to ascii:

    mov cx, 10 ; divisor
i2a:
    mov dx, 0 ; don't need dx (in this case)
    mov ax, 13579 ; dividend
    div cx ; dx:ax pair is implied source/destination
operand for word sized explicit operand (cx)
                               ; couple of ns later... ax holds the
quotient(1357); dx has modulo(9)
    or ax, 0 ; or instruction modifies flags (looking for
ax=0, nothing more to divide) (or 0,0 is the only combination that results
in 0)
    JNZ i2a ; Jump if Not Zero flag enabled (zero flag is
activated if the result of an instruction is 0)

This is how the results come out from the loop:
start: ax=13579
1. ax=1357, dx=9 ;last digit
2. ax=135, dx=7
3. ax=13, dx=5
4. ax=1, dx=3
5. ax=0, dx=1 ;first digit
                            ;or ax,0 = zero, JNZ jumps no more...

So if you look from bottom to the top you get what you're looking for in dx,
some minor adjustments required:
ASCII is an unsigned byte sized standard (number 0-9 -> ascii 48-57 decimal,
or 30-39 hex)
To convert dx:
add dx,30H ; if dx was 9 now it's 39(ascii 9)
mov location, dl ;store the ascii digit
                            ;ascii is byte sized, dl is always enough for
modulo of 10

You'll need cmp instruction for your other chores. It also modifies the
flags(by subtracting the operands but not storing the result!) so you can
use it and Jxx (JNZ...) instructions to control the flow of your program.

Newbie asm ( ! - based on tasm):
http://atrevida.comprenica.com/gameprog.html

Intel's instruction set (limited, but for this task it's just on cue, my
biggest problem when i began asm programming was ubiquity of information
available - just one more link... - click,click...)
http://www.manualy.sk/intel/

Debug.exe reference (you'll need a,e,g,t,p subcommands - at least):
http://www.microsoft.com/resources/documentation/WindowsServ/2003/enterprise/proddocs/en-us/Default.asp?url=/resources/documentation/WindowsServ/2003/enterprise/proddocs/en-us/debug_subcmds.asp

e to quickfix data
a to enter/assemble instructions (db sub subcommand for data - it's up to
you to worry about their location !)
g to execute
t to trace *each* instruction
p to trace instructions except calls,loops,movs (useful for avoiding
long/repetative parts of code not in the pivot of your attention)

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