Day 2

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Interrupts

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Interrupts are exactly like they sound, they interrupt your program, do their thing, and
are actually called by your program like an API or static library.

The INT Instruction
The INT instruction calls an interrupt, it works like the following:

int 21h  ; a 'h' after a number means hexadecimal notation.
; 21h is the number for a DOS Interrupt Service routine.
; there are many functions that can be called with interrupt 21h

The number that follows the INT instruction (21h in the above example) is the service to
call. The actual function number goes in AH, which brings me to something rather important:

I've said that all registers are 16bit, that's true except for one thing:
The High and Low 8bits of AX,BX,CX,and DX can be accessed by substituting the X for
either H for High and L for Low, so the lower 8bits of the registers are AL,BL,CL,and DL, and the high
8bits are AH,BH,CH,DH. The MOV instruction works the same for accessing the 8bit parts of the registers
as well as the registers as their 16bit whole selves.

Please note that if you touch AH or AL for example, you'll be modifying AX's value. So watch out.

A Character on the Screen
Here, we'll put together a program that outputs a letter to the screen and assemble it to the
final COM program (a DOS EXE,sort of).

;;---CODE START---;;
mov ah,2  ; 2 is the function number of output char in the DOS Services.
mov dl,'E' ; DL takes the letter to be outputted. note that "mov dl,'E'" will put 'E's ASCII code in DL.
int 21h    ; will call DOS Services, DOS will notice that the function # is 2 and will then take the ASCII code
; in DL and output it to the screen at the currect cursor location.

mov ah,4Ch   ; 4Ch is the function number for exit program in DOS Services.
int 21h      ; function 4Ch doesn't care about anything in the registers.
;;---CODE END---;;

Put that in a file called first.asm, and open a DOS Prompt, type:

a86 first.asm Hit ENTER

It should output 2 files, keep the COM file and delete the other. Open a new DOS Prompt if you closed the old one.
Type:

first Always hit enter after every line

A capital E should be outputted and the program should end.

This Day In Review

Tomorrow, we'll get to see pointers in action!

Hope you understood everything!,
-Mike H a.k.a GbaGuy

Intro - Day 3
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