cfenollosa_os-tutorial/05-bootsector-functions-strings

Concepts you may want to Google beforehand: control structures, function calling, strings

Goal: Learn how to code basic stuff (loops, functions) with the assembler

We are close to our definitive boot sector.

In lesson 6 we will start reading from the disk, which is the last step before loading a kernel. But first, we will write some code with control structures, function calling, and full strings usage. We really need to be comfortable with those concepts before jumping to the disk and the kernel.

Strings

Define strings like bytes, but terminate them with a null-byte (yes, like C) to be able to determine their end.

mystring:
    db 'Hello, World', 0

Notice that text surrounded with quotes is converted to ASCII by the assembler, while that lone zero will be passed as byte 0x00 (null byte)

Control structures

We have already used one: jmp $ for the infinite loop.

Assembler jumps are defined by the previous instruction result. For example:

cmp ax, 4      ; if ax = 4
je ax_is_four  ; do something (by jumping to that label)
jmp else       ; else, do another thing
jmp endif      ; finally, resume the normal flow

ax_is_four:
    .....
    jmp endif

else:
    .....
    jmp endif  ; not actually necessary but printed here for completeness

endif:

Think in your head in high level, then convert it to assembler in this fashion.

There are many jmp conditions: if equal, if less than, etc. They are pretty intuitive but you can always Google them

Calling functions

As you may suppose, calling a function is just a jump to a label.

The tricky part are the parameters. There are two steps to working with parameters:

1. The programmer knows they share a specific register or memory address
2. Write a bit more code and make function calls generic and without side effects

Step 1 is easy. Let's just agree that we will use al (actually, ax) for the parameters.

mov al, 'X'
jmp print
endprint:

...

print:
    mov ah, 0x0e  ; tty code
    int 0x10      ; I assume that 'al' already has the character
    jmp endprint  ; this label is also pre-agreed

You can see that this approach will quickly grow into spaghetti code. The current print function will only return to endprint. What if some other function wants to call it? We are killing code reusage.

The correct solution offers two improvements:

• We will store the return address so that it may vary
• We will save the current registers to allow subfunctions to modify them without any side effects

To store the return address, the CPU will help us. Instead of using a couple of jmp to call subroutines, use call and ret.

To save the register data, there is also a special command which uses the stack: pusha and its brother popa, which pushes all registers to the stack automatically and recovers them afterwards.

Including external files

I assume you are a programmer and don't need to convince you why this is a good idea.

The syntax is

%include "file.asm"

Code!

Let's jump to the code. File boot_sect_print.asm is the subroutine which will get %included in the main file. It uses a loop to print bytes on screen.

The main file boot_sect_main.asm loads a couple strings, calls print and hangs. If you understood the previous sections, it's quite straightforward.

As a last goodie, we will learn how to print newlines. The familiar '\n' is actually two bytes, the newline char 0x0A and a carriage return 0x0D. Please experiment by removing the carriage return char and see its effect.