lessons 8, 9, 10, entering 32-bit mode

08-32bit-print/32bit-print.asm

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+[bits 32] ; using 32-bit protected mode
+
+; this is how constants are defined
+VIDEO_MEMORY equ 0xb8000
+WHITE_OB_BLACK equ 0x0f ; the color byte for each character
+
+print_string_pm:
+    pusha
+    mov edx, VIDEO_MEMORY
+
+print_string_pm_loop:
+    mov al, [ebx] ; [ebx] is the address of our character
+    mov ah, WHITE_OB_BLACK
+
+    cmp al, 0 ; check if end of string
+    je print_string_pm_done
+
+    mov [edx], ax ; store character + attribute in video memory
+    add ebx, 1 ; next char
+    add edx, 2 ; next video memory position
+
+    jmp print_string_pm_loop
+
+print_string_pm_done:
+    popa
+    ret

08-32bit-print/README.md

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+*Concepts you may want to Google beforehand: 32-bit protected mode, VGA, video 
+memory*
+
+**Goal: Print on the screen when on 32-bit protected mode**
+
+32-bit mode allows us to use 32 bit registers and memory addressing
+, protected memory, virtual memory and other advangades, but we will lose
+BIOS interrupts and we'll need to code the GDT (more on this later)
+
+In this lesson we will write a print string routine by directly manipulating
+the VGA video memory instead of calling `int 0x10`. The VGA memory starts
+at address `0xb8000` and it has a text mode which is useful to avoid
+manipulating direct pixels.
+
+The formula for accessing a specific character on the 80x25 grid is:
+
+`0xb8000 + 2 * (row * 80 + col)`
+
+That is, every character uses 2 bytes (one for the ASCII, another for 
+color and such), and we see that the structure of the memory concatenates
+rows.
+
+Open `32bit-print.asm` to see the code. It will always print the string
+on the top left of the screen, but soon we'll write higher level routines
+to replace it.
+
+Unfortunately we cannot yet call this routine from the bootloader, because
+we still don't know how to write the GDT and enter protected mode. Once
+you have understood the code, jump to the next lesson.