Read from the disk

taikiy · Feb 9, 2023 · fc44421 · fc44421
1 parent 8912251
commit fc44421
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Showing 3 changed files with 35 additions and 35 deletions.
diff --git a/Makefile b/Makefile
@@ -3,5 +3,8 @@ all:
 	dd if=./message.txt >> ./boot.bin
 	dd if=/dev/zero bs=$(shell echo $$((512 - `stat message.txt | cut -d ' ' -f 8`))) count=1 >> ./boot.bin
 
+run:
+	qemu-system-x86_64 -hda ./boot.bin
+
 clean:
 	rm -rf ./boot.bin
diff --git a/boot.asm b/boot.asm
@@ -11,20 +11,6 @@ times 33 db 0
 init:
     jmp 0x7c0:start
 
-handle_zero:
-    mov ah, 0eh
-    mov al, 'A'
-    mov bx, 0x00
-    int 0x10
-    iret
-
-handle_one:
-    mov ah, 0eh
-    mov al, 'B'
-    mov bx, 0x00
-    int 0x10
-    iret
-
 start:
     ; setup the data segment
     cli                         ; Disable interrupts. We don't want interrupts messing with registers
@@ -36,26 +22,29 @@ start:
     mov sp, 0x7c00
     sti                         ; Enables interrups
 
-    ; Add a custom interrupt handler for Int0h (address 0x00).
-    ; word[0x00] will use `ds` as the segment by default, which at this point of the code points at
-    ; 0x7c0. We explicitly specify [ss:0x00], which points at 0x00, set in the code above.
-    mov word[ss:0x00], handle_zero  ; First two bytes of RAM - offset
-    mov word[ss:0x02], 0x7c0        ; Second two bytes of RAM - segment
-
-    mov word[ss:0x04], handle_one
-    mov word[ss:0x06], 0x7c0
+    ; Disk interrupt preparation
+    mov ah, 0x02                ; READ SECTOR command
+    mov al, 0x01                ; Read 1 sector
+    mov ch, 0x00                ; Cylinder number 0
+    mov cl, 0x02                ; Sector number 2
+    mov dh, 0x00                ; Head number 0
+                                ; We don't set DL. BIOS sets it to the booted disk
+    mov bx, buffer              ; Data read will be buffered at ES:BX
+    int 0x13
 
-    int 0                       ; same as mov ax, 0x00; div ax
-    int 1
+    jc error                    ; if the carry flag is set (error), jump
 
-    mov si, message             ; SI = Source Index
+    mov si, buffer
     call print
-    
+
     cld                         ; Clears direction flag
+    cli                         ; Disables interrupts
+    hlt                         ; This hangs the computer
 
-    ; this hangs the computer
-    cli
-    hlt
+
+error: 
+    mov si, error_message
+    call print
 
 print:
     ; setup INT10h params
@@ -75,8 +64,12 @@ print:
 .done:
     ret
 
-message:
-    db 'Hello, World!', 0
+error_message:
+    db 'Failed to load sector', 0
 
 times 510 - ($ - $$) db 0       ; Pad the boot sector to 510 bytes
-dw 0xAA55                       ; Boot signature. 55AA (2 bytes) in the little-endian
+dw 0xAA55                       ; Boot signature. 55AA (2 bytes) in the little-endian
+
+;; Everything under here is at the second sector from 0x7c00 (0x7e00)
+
+buffer:
diff --git a/doc/real_mode_development.md b/doc/real_mode_development.md
@@ -1,8 +1,10 @@
 # Real Mode Development
 
-##
+## Writing a bootloader
 
-BIOS operates in _Real Mode_. Real mode (aka. read address mode) is an operating mode of all x86-compatible CPUs. All code in real mode is required to be 16 bits. Addresses in real mode correspond to real locations in memory. It uses a 20-bit _segmented memory_ address space (= 1MB of addressable memory) and unlimited direct software access to all addressable memory, I/O addresses and peripheral hardware. It does not provide memory protection, multitasking, or code privilege levels. [(wiki)](https://en.wikipedia.org/wiki/Real_mode)
+Bootloader is a set of CPU instructions (usually written in assembly) that is loaded by the BIOS when a PC is booted. Bootloader's code sits at 0x7c00 when loaded into the memory, and must be 1-sector (= 512 bytes) long. The end of the bootloader code is marked by 2-byte signature `0x55AA`.
+
+BIOS operates in _Real Mode_. Real Mode (aka. read address mode) is an operating mode available to all x86-compatible CPUs. All code in real mode is required to be 16 bits. Addresses in real mode correspond to real locations in memory. It uses a 20-bit _segmented memory_ address space (= 1MB of addressable memory) and unlimited direct software access to all addressable memory, I/O addresses and peripheral hardware. It does not provide memory protection, multitasking, or code privilege levels. [(wiki)](https://en.wikipedia.org/wiki/Real_mode)
 
 ## 1. Printing a string to the screen
 
@@ -117,10 +119,12 @@ To define a custom interrupt handler, we first define a routine with a label, an
 
 We add [`message.txt`](../message.txt) file and append the content to `boot.bin` using `dd` command (see [`Makefile`](../Makefile)). When we start up a QEMU with `boot.bin`, it treats it as a hard disk.
 
-Whenever CPU reads data from a hard disk, it must be one full block (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
+Whenever 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).
+
 ## Notes
 
 - `lodsb` is one of x86 memory segmentation instructions. It uses DS (Data Segment) and SI (Source Index) registers. The real memory address is `DS * 16 + SI`.