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LESSON3.DOC
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1994-10-30
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LESSON3 - INTERRUPTS LABELS AND DB
know, if we want to, let say do a program thats just run, the struc
I showed you before is not complete because, it will not exit,
we must tell dos we want to exit, then how will we do it :
simply by using the operator "int" which calls to interrupt
(0..0FFH) and instruct him what to do, so lets detailt of some int's :
009h - the keyboard interrupt
010h - the screen interrupt
013h - the sectors, (format, read, write) interrupt
016h - the keyboard interrupt
017h - the printer interrupt
021h - the main dos command
know each interrupt got many command and each one needs difrrent parameters
(if you want to know get the complete int list from your local BBS)
INT 21 - TERMINATE
ah - 4ch
al - errorlevel
so in order to initiate this interrupt we will do :
mov ah,4ch
mov al,0 ; if you want it can every value
int 21h ; and then the program will exit to dos
but how will the compiler know where to start, well we have to tell
him, the compiler will know to start excatly in the first, label -
label is a name which you can jump to him, the label will be like :
Name : - "start :", "now :", "rt :" all of those are labels, so this is
a basic (working) assembly program :
Sseg segment
db 10 dup (?) ; will be disscused late
ends
Dseg segment
ends
Cseg segment
assume cs:cseg,ds:dseg,ss:sseg
start : ; the compiler will start here
mov ah,4ch
mov al,0
int 21h ; terminate
ends ; the segment must finish before the label
end start ; close the label
end ; tell compiler script ends here
that was easy, it's the main assembly struc, but it can be much more complexed
now in the sseg (stack) there was "db" now what is that, well if (most
of the times) assembly registers are not enough so you (like pascal,c) can
define variables, how ????
well you have to operators - db,dw,dd,dt :
DB - define byte, the var will be 1 byte (8 bit)
DW - define word, the var will be 2 bytes (word, 16 bit)
DD - define double, the var will be 4 bytes (double word,32 bit)
DT - define ten, then var will be 10 bytes (not used)
the decleration goes like this :
name type [number] value.
examples : try db 1 - it will define "try" as 1 byte with starting value 1
tru dw 4 - it will define "tru" as 2 byte with starting value 4
ytu dw ? - it will define "ytu" as 2 byte with unknown starting
value
now lets say that you want to define an array (like pascal : a:array[1..10]of
like c : int c[100])
you do as follow : name type number dup(value).
lets say : gvr db 10 dup(2) - it will define "gvr" as 10 byte which each one
has value of 2.
but what if I want to define a messege or multiple values then what ??
then it should be like :
messege - mes db "this is a messege",10,13,"$"
now the numbers after the second """ are optional, it's needed only
for some interrupt.
multiple values (vectors, etc.) then :
vec db 23,54,76,34,234,14,23,123,245
db 124,34,245,23,52,34,52,43,13
db 213,213,123,123
the db can be dw,dd or dt, and you can do it for how much you want
but you must not pass the 64k limit.
but what if you want to write a messege then ???
you will search the int which do that :
int 21h - write string
ah - 9
ds:dx points to string (in the end "$" for termination)
so how will we do it ???
Sseg segment
db 10 dup (?)
ends
Dseg segment
msg db "dr. encryption",10,13,"$" ; for termination
ends
Cseg segment
assume cs:cseg,ds:dseg,ss:sseg
start : ; the compiler will start here
mov dx,offset msg ; ds is already pointing to dseg
mov ah,9
int 21h ; whop there he is
mov ah,4ch
mov al,0
int 21h ; terminate
ends ; the segment must finish before the label
end start ; close the label
end ; tell compiler script ends here
now what offset is, well offset will return the offset of the argument
so if you write : mov dx,offset msg then dx will be the offset of the msg
(offset is 16bit and so is segment ,so "mov al,offset ..." will be wrong)
the operator seg will return the segment of the argument, but if you change
ds and not return his original value the system will hang.
so then the stack comes to buisness.