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Another great reference
Assembly Language Programs and Organizations of the IBM PC
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ASSEMBLY PROGRAMMING AND OS DESIGN
2.
Intro to Assembly
2.1
Data Types
2.2
Registers
2.3
DOS Int Calls
2.4
Basic Commands
2.4.1 DOS Hello World
2.5 BIOS Int Calls
2.5.1
BIOS Hello World
2.6 String Routines
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Another great reference |
Need a good reference? Assembly Language Step-by-Step: Programming with DOS and Linux, 2nd Edition |
Ex.6
Two equ 2
Exit equ 4Ch
org 100h
mov AX,Two
add AX,Two
mov AH,Exit
int 21h
As you can see, Two and Exit are just substitutes for actual values of 2 and 4ch respectively. These allow us to make code more readable as well as provide a way to define a value in one place that may be used thousands of times in our code.
The second method is to actually produce variables using the basic assembly data types.
db - byte
dw - word
dd - double word
This actually sets aside memory to store these values. For example:
Ex.7
org 100h
mov AX,2
add AX,2
mov [Result],AX ;save results in memory
mov AH,4Ch
int 21h
Result db 0 ;one byte of memory for result
Example 7 sets aside a byte of memory at the end of our code to store the result of the add operation. You may be wondering why we have brackets around Result. The easiest way to think of it is that any time you are saving to or reading from memory, the variable pointing to that memory must be in brackets. This will be explained in more detail later.
Labels are used to identify to the assembler where a location in the code is. A very common label that we will use in every program from here out is Main. Main will always be the main part of our code which will be executed as soon as we run it. Labels are declared by adding a colon onto the end of them.
Ex.8
org 100h
jmp Main
Result db 0
Main: mov AX,2
add AX,2
mov [Result],AX
mov AH,4Ch
int 21h
In example 8, we introduce the JMP command. This command simply jumps to a point in the code that you specify using a label. We move the variable Result up to the top and simply jump over it to the beginning of our code. This will be the standard structure of code from here on out in this text.
... Constants declared here
org Beginning Address
jmp Main
... Variables declared here
Main: ... Main program begins here
...
mov AH,4Ch
int 21h ;code complete
We already know everything we need to revisit Hello World, but first lets put everything together to finally output the result to 2+2.
Ex.9
Two
equ 2
Exit equ
4Ch
org 100h
jmp Main
Result db
0
Main: mov
AL,Two
add AL,Two
mov [Result],AL
mov DL,'0'
;ASCII values for numbers start with '0'
add DL,[Result] ;DL = '0'(48) + Result (4)
mov AH,02h
int 21h
mov AH,Exit
int 21h
Compile the above with NASMW -o ex9.exe ex9.asm. You can experiment
with the values you add. Note that any result above 9 is going to
output something other than a number.