From d826097f3b032247dd0396f8cf64b2fa775cea68 Mon Sep 17 00:00:00 2001
From: =?utf8?q?Duje=20Mihanovi=C4=87?= <duje.mihanovic@skole.hr>
Date: Sat, 21 Aug 2021 18:24:44 +0200
Subject: [PATCH] Read bytes from boot drive

---
 Makefile |  2 +-
 boot.s   | 82 +++++++++++++++-----------------------------------------
 disk.s   | 39 +++++++++++++++++++++++++++
 3 files changed, 62 insertions(+), 61 deletions(-)
 create mode 100644 disk.s

diff --git a/Makefile b/Makefile
index 9d95b1e..a85a94a 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
 AS = yasm # choose yasm or nasm here
 ASFLAGS = -f bin # compile a raw binary
 
-boot: boot.s print.s
+boot: boot.s print.s disk.s
 	$(AS) $(ASFLAGS) -o $@ boot.s
 
 clean:
diff --git a/boot.s b/boot.s
index 51be5d9..3a8adc7 100644
--- a/boot.s
+++ b/boot.s
@@ -2,70 +2,32 @@
 	org 7c00h ; boot sector is loaded at 0x7c00
 
 _start
-	mov di, printing_numbers
-	call print
+	mov [boot_drive], dl ; BIOS sets dl to number of boot drive, we don't want to forget this
 
-	or bl, 1
-	mov dx, 0DEADh ; number to print
-	call print_word
-	
-	mov di, space
-	call print
-	
-	xor bl, bl
-	mov dx, 0BEEFh
-	call print_word
-
-	mov di, waiting
-	call print
+	mov bp, 8000h ; Move the stack away
+	mov sp, bp
 
-	mov di, keystroke
-	mov bl, 1
+	push cs
+	pop es
+	mov bx, 9000h ; Load at 0000:9000
+	mov dh, 2 ; Load 5 sectors
+	mov dl, [boot_drive] ; Load from boot drive
+	call read_sectors
 
-.loop
-	mov ah, 0
-	int 16h
-	cmp ah, 1Ch
-	je .enterpressed
-	jmp .continue
-	cmp ah, 0Eh
-	je .backspace
-	jmp .continue
-.enterpressed
-	pusha
-	mov ah, 0Eh
-	mov al, 0Dh
-	int 10h
-	mov al, 0Ah
-	int 10h
-	popa
-	jmp .loop
-.backspace
-	pusha
-	mov ah, 03h
-	mov bh, 0
-	int 10h
-	dec dl
-	mov ah, 02h
-	int 10h
-	mov ah, 0Eh
-	mov al, ' '
-	int 10h
-	mov ah, 02h
-	int 10h
-	popa
-	jmp .loop
-.continue
-	mov [keystroke], al
-	call print
-	jmp .loop
+	mov dx, [9000h] ; Print value at 9000h, should be 0xDEAD
+	call print_word 
+	mov dx, [9000h + 512] ; Print value at 9200h, should be 0xBEEF
+	call print_word
 
-printing_numbers db "Printing some random hex numbers", 0
-waiting db "I await keystrokes", 0
-keystroke db "$", 0
-space db " ", 0
+	jmp $ ; Endless loop
 
 %include "print.s"
+%include "disk.s"
+
+boot_drive resb 1 ; Reserve 1 byte for boot drive number
+
+times 510-($-$$) db 0 ; fill with 0 until 0x1fc is reached
+dw 0AA55h ; BIOS MBR signature
 
-	times 510-($-$$) db 0 ; fill with 0 until 0x1fc is reached
-	dw 0AA55h ; BIOS MBR signature
+times 256 dw 0DEADh ; Fill in 2nd sector with 0xDEAD
+times 256 dw 0BEEFh ; Fill in 3rd sector with 0xBEEF
diff --git a/disk.s b/disk.s
new file mode 100644
index 0000000..b281c3a
--- /dev/null
+++ b/disk.s
@@ -0,0 +1,39 @@
+; ES:BX - where to load
+; DH - sector count
+; DL - drive number
+
+read_sectors:
+	xor bl, bl
+	mov di, disk_reading
+	call print
+	push dx
+	mov ah, 02h
+	mov al, dh
+	mov ch, 0
+	mov dh, 0
+	mov cl, 2
+	
+	int 13h
+
+	jc read_error
+
+	pop dx
+	cmp dh, al
+	jne read_error
+	ret
+	
+read_error:
+	xor bl, bl
+	mov di, disk_error
+	call print
+	mov dl, ah
+	xor dh, dh
+	call print_word
+	mov dl, al
+	call print_word
+	jmp $
+
+disk_reading db "Reading from disk...", 0
+disk_error db "Failed to read from disk! Take a look at this error code:", 0
+
+	ret
-- 
2.39.2