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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:
■-
|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 ($)
|
|OUT: Prints the string to the screen
■-
Other than that function, there's nothing new that can't easily be
figured out. The directive SEG, as you might have guessed, returns the
segment that the specified label is in. OFFSET returns the offset from
the begining of the segment to the specified label. Together, you can
form a FAR pointer to a specified label.
Another thing you might wonder about is why I put the SEG Message into
AX and THEN Put it in DS.
The answer is: You have to. An immediate value cannot be put into a
segment register, but a register or an indexed value can. For instance:
These are legal:
: mov DS,AX
: mov DS,[TheSegment]
But these are not:
: mov DS,50
: mov DS,0a000h
One last piece of info: in the lines:
:Message db "This was printed using function 9 "
: db "of the DOS interrupt 21h.$"
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:
────────────────────
DOSSEG
.MODEL SMALL
.STACK 200h
.CODE
Message db "Hey look! I'm in the code segment!$"
START:
mov ax,cs ;since CS already points to the same segment as Message,
mov ds,ax ;I don't have to explicitly load the segment that message
;is in..
mov dx,offset Message
mov ah,9
int 21h
mov ax,4c00h ;Returns control to DOS
int 21h ;MUST be here! Program will crash without it!
END START
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The advantage to having all your data in the CODE segment is that DS and
ES can be pointing anywhere and you can still access your data via a segment
override!
Example:
say I'm in the middle of copying one section of the screen memory to
another and I need to access the variable "NumLines" I'd do it like this:
────────
mov ax,[CS:NumLines] ;this is in IDEAL mode
^^^
──────── Code Segment override
Pretty flexable, eh?
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│ ASM1.ASM │
└──────────┘
DOSSEG
.MODEL SMALL
.STACK 200h
.DATA
Message db "This was printed using function 9 "
db "of the DOS interrupt 21h.$"
.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