Terminal Driver

In this homework assignment, you will be writing a terminal driver in C. A terminal driver has basically two jobs: (1) print characters to the screen and (2) receive input from the keyboard. In the first homework assignment, we used the BIOS to do all of our I/O for us, but in this project, we will be doing it ourselves.

This repo contains a skeleton for your project with a demo program in assembly that prints a bunch of characters to the screen. You can write your program in either C or assembly.

Whatever language you decide to use, your program should do the following things:

1. A main or _start function that calls all of your other functions to demonstrate them.
2. A function puts that prints a NULL-terminated string to the terminal.
   • Calling puts sequentially will print strings one after another. In other words, puts should not overwrite text that was previously written to the screen.
   • Printing a '\n' character should advance to the next line on the screen.
   • When the cursor gets to the bottom of the screen, advancing to a new line should scroll the existing text on the screen up one line.

For extra credit:

3. Interrupt handler for the keyboard that (a) reads the key's scancode, (b) translates the scancode to an ASCII character, and (c) puts the ASCII character into an input buffer.
4. getc reads a character from the keyboard input buffer.

Printing Characters

See the OSDev page for help on how to do this. Basically, all you have to do is write ASCII values to video memory, which starts at address 0xB8000. There's a demo program in C on the OSDev page that prints a string to the upper left corner of the screen. That would be a good place to start.

Coding in C

If you want to write your project in C, you need to do the following steps to make it compile:

1. Create a new C file called term.c in the src directory. Put an empty function in it called main:

    int main() {
        for(;;);  // Loop forever
    }
   

2. Add your new C source file to the Makefile. On line 10 of the Makefile there is a list of source files to compile. Right now, there's only one file in the list: terminaldemo.o. You need to add your new C file to the list of objects to compile. Make a new line after terminaldemo.o:

    OBJS = \
        terminaldemo.o \
        term.o \
   

Makefiles are picky about indentation. Don't use spaces to indent in a Makefile, use tabs.

3. In terminaldemo.s, put extern main right above the _start function:

    section .text
   
    global _start
    extern main   ; DECLARE MAIN
    _start:
        call main ; CALL MAIN
   

This tells the assembler that there is a function called main in a different source file. If you don't put that line in, you'll get error when you try to compile.

4. Call your main function from terminaldemo.s. You probably want to make the call the first instruction in the program.

Searching Google

Googling for answers tends to be difficult because OS development is an arcane topic. Try adding "osdev" to your Google search. For example: "osdev XXX" where XXX is the term you're looking for. This will bring up answers from the OSDev wiki and forums, which tend to be pretty useful.