Assembly Basics, hello world!
===This is the level 0 intro!===
There’s only two handfuls of assembly instructions you really need to know. Many others aren’t used as often. Most instructions are made up of three or four characters with an operand, a comma, then another operand.
To put some data into a register you use the MOV instruction.
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mov ax,1 ; put 1 into ax mov bx,2 ; put 2 into bx mov cx,3 ; put 3 into cx mov dx,4 ; put 4 into dx |
Push puts data at the top of a stack. Pop moves the data from the top of the stack into a specified register. To exchange two registers, ax and cx:
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push cx ; put cx on top of the stack push ax ; put ax on top of the stack pop cx ; move data from the stack into cx pop ax ; move data from the stack into ax |
But this could be done easier with the XCHG instruction:
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xchg ax,cx |
To move data from one place to another, or copy, use MOV. To make the value of ax the same as what’s in bx:
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mov ax, bx ; $ax=$bx in other languages |
To call a BIOS function that does some pre determined stuff such as printing something, you use INT. Most functions have multiple methods you can call so you’ll need to figure out which routine you want and put that number into the ah register first:
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mov ah,9 ; nine is the print subroutine int 21h ; call the interrupt |
The EQU instruction creates absolute symbols and aliases by assigning an expression or value to the declared variable name. Everytime you call your custom alias, it gets replaced with the expression on the right hand side:
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H22 : EQU 22H |
===The obligatory Hello World example:===
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.stack .data MyString DB "Hello Kitty$" ; because i'm tired of Hello World! CODE .model small .stack .code start: mov dx,OFFSET MyString ; DX contains the offset of MyString mov ax,SEG MyString ; AX contains the segment of MyString mov ds,ax ; DS:DX points to the message mov ah,9 ; nine is the print subroutine int 21h ; call the interrupt mov ax,4c00h ; this will terminate the program nicely int 21h; call the interrupt END start |
The END keyword informs the assembler that there are no more source statements for that section. By itself, it should be the very last instruction. You don’t have to call it “start”, you could name it whatever you want as long as you don’t use a word that the assembler will think is some sort of instruction. You can have multiple sections and allow jump points based on register comparisons also.
