Writing a Simple Operating System — from Scratch

CHAPTER 4. ENTERING 32-BIT PROTECTED MODE 32

[bits 32]

; Define some constants

VIDEO_MEMORY equ 0xb8000

WHITE_ON_BLACK equ 0x0f

; prints a null -terminated string pointed to by EDX

print_string_pm:

pusha

mov edx , VIDEO_MEMORY ; Set edx to the start of vid mem.

print_string_pm_loop:

mov al, [ebx] ; Store the char at EBX in AL

mov ah, WHITE_ON_BLACK ; Store the attributes in AH

cmp al, 0 ; if (al == 0), at end of string , so

je done ; jump to done

mov [edx], ax ; Store char and attributes at current

; character cell.

add ebx , 1 ; Increment EBX to the next char in string.

add edx , 2 ; Move to next character cell in vid mem.

jmp print_string_pm_loop ; loop around to print the next char.

print_string_pm_done :

popa

ret ; Return from the function

Figure 4.1: A routine for printing a string directly to video memory (i.e.

without using BIOS).

The downside to our routine is that it always prints the string to the top-left of the
screen, and so will overwrite previous messages rather than scrolling. We could spend
time adding to the sophistication of this assembly routine, but let’s not make things too
hard for ourselves, since after we master the switch to protected mode, we will soon be
booting code written in a higher level language, where we can make much lighter work
of these things.

4.2 Understanding the Global Descriptor Table

It is important to understand the main point of this GDT, that is so fundamental to
the operation of protected mode, before we delve into the details. Recall from Section
XXX that the design rationale of segment-based addressing in the classical 16-bit real
mode was to allow the programmer to access (albeit slightly, by today’s standards) more
memory than a 16-bit offset would allow. As an example of this, suppose that the
programmer wanted to store the value ofaxat the address0x4fe56. Without segment-