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ProgramsDocumentation.md

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How to write a program for MascOS

NOTE: For now this file is a remainder for me on how to create programs for MascOS. Many features may be buggy or just bad.

Program structure

You first need to know some stuff first, such as how a program gets loaded and how it gets terminated. You can use Assembly to create your fabulous code. If you're program is a .COM executable file it must have the [org 0x100] directive, if instead it's a .BIN executable then remove that directive.

The general layout of a program is this:

[bits 16]
[org 0x100]
[cpu 8086]


; We are here when we jump from the kernel
ProgramEntryPoint::
    ; Code....
    ; Your cool code here


    ; Stop the program
    int 0x20

Load and execute a program in memory

It's surprisingly simple, here's how:
you need to call the LoadProgram label, and give it the pointer to the file name, it can be outside the root directory too, it doesn't change anything, it just needs to point to an 11 byte valid FAT12 file name. After calling the label it will load the program in memory and directly jumps to it, you don't need to do anything else.

Important note: LoadProgram WILL RETURN if the operation failed, setting the carry flag. So ALWAYS handle the case that it failed to execute your program.

Here's a code example:

ProgramFileName: db "SOMENAMEBIN"

; Loads a program
lea si, ProgramFileName
call LoadProgram

; if we are here it failed the operation(carry flag is set)
cli
hlt

The location in memory of the program is decided by the os, but like in MS DOS the actual file is loaded at offset 0x100, this is why the org directive must be set to 0x100. Don't use the memory area before that address.

Exit program

When you have done your thing it would be cool to exit from the program, right? To do that you can just call int 0x20, nothing more nothing less. It is a much cleaner way than doing it the old way, which involed doing a far jump to a specific location in the kernel, you can see that this was prone to bugs, right? Here's an example if you really need it:

; Exit and go back to shell
int 0x20

This interrupt doesn't expect any input.

Alternatively you can use MS DOS interrupt 0x21 with ah set to 0x4c.

; Exit and go back to shell
mov ah, byte 0x4c
int 0x21

Interrupts

MS DOS interrupt 0x21(int 0x21) is present, but the kernel also adds custom interrupts to the IVT(Interrupt Vector Table), so external programs can utilize VGA functions, disk functions and more, which allows to print strings, load files in memory, find a file and much more. To see the complete list of interrupts you can head to the interrupts documentation.