ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
    ASM1.ASM - print a string
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    Well, here's the classic example for the first program in just about
every language.  It prints a message to the screen by using a DOS function.
More specifically, it uses function 9 of interrupt 21h.  Here's the mock
specification for the function:

þ-
|IN:     ah = 9      ;ah tells INT 21h which function you want
|        DS:DX = FAR pointer to the string to be printed.
|                    ;the string must terminate with a dollar sign ($)
|
|OUT:    Prints the string to the screen
þ-

    Other than that function, there's nothing new that can't easily be
figured out.  The directive SEG, as you might have guessed, returns the 
segment that the specified label is in.  OFFSET returns the offset from 
the begining of the segment to the specified label.  Together, you can
form a FAR pointer to a specified label.

    Another thing you might wonder about is why I put the SEG Message into
AX and THEN Put it in DS.  

    The answer is:  You have to.  An immediate value cannot be put into a 
segment register, but a register or an indexed value can.  For instance:

These are legal:

:   mov     DS,AX
:   mov     DS,[TheSegment]

But these are not:

:   mov     DS,50
:   mov     DS,0a000h
    
    One last piece of info: in the lines:

:Message     db  "This was printed using function 9 " 
:            db  "of the DOS interrupt 21h.$"

    The DB is just a data type.  Its the same as a CHAR in C, which is 1 byte
    in length.

    Other common data types are:

    DW  same as an INT in C - 2 bytes
    DD  same as a double int or long int or a FAR pointer - 4 bytes


    Well, that's pretty much it for this short section...  Try playing around
with the 'print' function... Ya learn best by playing with it.


One last side note:
    I COULD have put the message in the CODE segment instead, by doing this:
    
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

    DOSSEG
    .MODEL SMALL
    .STACK 200h
    .CODE

Message     db  "Hey look! I'm in the code segment!$" 
            
START:
    mov     ax,cs   ;since CS already points to the same segment as Message,
    mov     ds,ax   ;I don't have to explicitly load the segment that message
                    ;is in..

    mov     dx,offset Message
    mov     ah,9
    int     21h

    mov     ax,4c00h    ;Returns control to DOS
    int     21h         ;MUST be here! Program will crash without it!

END START

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

    The advantage to having all your data in the CODE segment is that DS and
ES can be pointing anywhere and you can still access your data via a segment
override!  

    Example:
        say I'm in the middle of copying one section of the screen memory to
    another and I need to access the variable "NumLines" I'd do it like this:

ÄÄÄÄÄÄÄÄ

    mov ax,[CS:NumLines]    ;this is in IDEAL mode
            ^^^
ÄÄÄÄÄÄÄÄ    Code Segment override

    Pretty flexable, eh?


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³ ASM1.ASM ³
ÀÄÄÄÄÄÄÄÄÄÄÙ

    DOSSEG
    .MODEL SMALL
    .STACK 200h
    .DATA

Message     db  "This was printed using function 9 " 
            db  "of the DOS interrupt 21h.$"
                  
    .CODE
    
START:
    mov     ax,seg Message  ;moves the SEGMENT that `Message' is in into AX
    mov     ds,ax           ;moves ax into ds (ds=ax)
                            ;you cannot do this -> mov ds,seg Message

    mov     dx,offset Message   ;move the OFFSET of `Message' into DX
    mov     ah,9        ;Function 9 of DOS interupt 21h prints a string that
    int     21h         ;terminates with a "$".  It requires a FAR pointer to
                        ;what is to be printed in DS:DX

    mov     ax,4c00h    ;Returns control to DOS
    int     21h         ;MUST be here! Program will crash without it!

END START