Add the Reading from disk section

Author: Vanperdung

doc/1_Real_mode.md

@@ -67,4 +67,18 @@ The values of 0x08-0x11 are 00 01 11 11, which means the address of ISR (int 0x0
 Physical address = (0x1111 * 16) + 0x0001 = 0x11110 + 0x0001 = 0x11111
 ```
 
-To define a custom interrupt handler, we first define a routine with a label and then write the segment/offset of the routine to IVT.
+To define a custom interrupt handler, we first define a routine with a label and then write the segment/offset of the routine to IVT.f
+
+## Reading From The Disk
+
+The bootloader is appended the content of [boot5.txt](../examples/bootloader/boot5.txt) at the end of binary file (see [Makefile](../examples/bootloader/Makefile)).
+
+When we start up a QEMU with `boot5.bin`, it treats it as a hard disk.
+
+Whenever the CPU reads data from a hard disk, it must be one full sector (512 bytes). We need to make sure that our message (starting from the second sector 0x200 because we use the first sector for our bootloader code), is padded with zeros until the end of the sector.
+
+Disk access is done via [`Int13h/AH=02h`](http://www.ctyme.com/intr/rb-0607.htm).
+
+Note how we created an empty label called `buffer` at the very end of the bootloader code. Because the bootloader code is 512 bytes, this label is not loaded into the memory. That doesn't mean we cannot use the memory pointed by this label. Since the label is at the end, it points to 0x7e00 = 0x7c00 (start of the bootloader) + 0x200 (512 bytes).
+
+![RESULT](./img/1_Real_mode_final.png)

doc/img/1_Real_mode_final.png

@@ -0,0 +1,5 @@
+all:
+	rm -f boot5.bin
+	nasm -f bin -o boot5.bin boot5.asm
+	dd if=./boot5.txt >> boot5.bin
+	dd if=/dev/zero bs=512 count=1 >> boot5.bin

examples/bootloader/Makefile

@@ -0,0 +1,63 @@
+ORG 0
+BITS 16 ; 16-bit code, ensure the assembler only assemble 16-bit code
+
+_start:
+    jmp short start 
+    nop
+
+times 33 db 0 ; Fill the rest of sector with 0
+
+start: ; Entry point
+    jmp 0x07C0:second_stage
+
+second_stage:
+    cli ; Clear interrupts
+
+    mov ax, 0x07C0 ; Load boot sector address to AX register
+    mov ds, ax ; Load data segment to DS register
+    mov es, ax ; Load data segment to ES register
+
+    mov ax, 0x0000
+    mov ss, ax ; Load stack segment to SS register
+    mov sp, 0x07C0 ; Load stack pointer to SP register
+
+    sti ; Set interrupts
+
+    mov ah, 0x02 ; Read disk command
+    mov al, 0x01 ; Number of sectors to read
+    mov ch, 0x00 ; Cylinder number
+    mov cl, 0x02 ; Sector number
+    mov dh, 0x00 ; Head number
+    mov bx, buffer ; Buffer address
+    int 0x13 ; Call BIOS disk interrupt
+
+    jc error ; If carry flag is set, jump to error
+
+    mov si, buffer ; Load buffer address to SI register
+    call print ; Call print function
+
+    jmp $ ; Infinite loop
+
+error:
+    mov si, error_message ; Load error message address to SI register
+    call print ; Call print function
+    ret
+
+print: ; Print function
+    mov bx, 0 ; Page number
+.loop:
+    lodsb ; Load byte from DS:SI to AL and increment SI
+    cmp al, 0 ; Check if AL is null
+    je .done ; If AL is null, jump to done
+    mov ah, 0x0E ; BIOS teletype function
+    int 0x10 ; Call BIOS (http://www.ctyme.com/intr/rb-0106.htm)
+    jmp .loop ; Loop
+.done:
+    ret ; Return
+
+error_message: db 'Error reading disk', 0 ; Null-terminated string
+
+times 510-($-$$) db 0 ; Fill the rest of sector with 0
+dw 0xAA55 ; Boot signature
+
+buffer:

examples/bootloader/boot5.asm

@@ -0,0 +1 @@
+Hello world, this is my awesome message!