Add disk read features and bochs support

Makefile

@@ -9,6 +9,7 @@ BUILD_DIR = build
 # Floppy image
 #
 floppy_image: $(BUILD_DIR)/main_floppy.img
+$(BUILD_DIR)/main_floppy.img: bootloader kernel
 	dd if=/dev/zero of=$(BUILD_DIR)/main_floppy.img bs=512 count=2880
 	mkfs.fat -F 12 -n "NBOS" $(BUILD_DIR)/main_floppy.img
 	dd if=$(BUILD_DIR)/bootloader.bin of=$(BUILD_DIR)/main_floppy.img conv=notrunc
@@ -19,7 +20,8 @@ floppy_image: $(BUILD_DIR)/main_floppy.img
 #
 
 bootloader: $(BUILD_DIR)/bootloader.bin
-	$(BUILD_DIR)/bootloader.bin: always
+
+$(BUILD_DIR)/bootloader.bin: always
 	$(ASM) $(SRC_DIR)/bootloader/boot.asm -f bin -o $(BUILD_DIR)/bootloader.bin
 
 

bochs_config

@@ -0,0 +1,7 @@
+megs: 128
+romimage: file=/usr/share/bochs/BIOS-bochs-latest, address=0xfffe0000
+vgaromimage: file=/usr/share/bochs/VGABIOS-lgpl-latest,
+floppya: 1_44=build/main_floppy.img, status=inserted
+boot: floppya
+mouse: enabled=0
+display_library: sdl, options="gui_debug"

debug.sh

@@ -0,0 +1 @@
+bochs -f bochs_config

run.sh

@@ -1,2 +1,2 @@
 qemu-system-i386 \
-    -fda build/main_floppy.img
+    -fda ./build/main_floppy.img

src/bootloader/boot.asm

@@ -3,6 +3,34 @@ bits 16
 
 %define ENDL 0x0D,0x0A
 
+;
+; FAT12 header record
+;
+
+jmp short start
+nop
+
+bdb_oem:                            db 'MSWIN4.1'
+bdb_bytes_per_sector:               dw 512
+bdb_sectors_per_cluster:            db 1
+bdb_reserved_sectors:               dw 1 
+bdb_fat_count:                      db 2
+bdb_dir_entries_count:              dw 0E0h
+bdb_total_sectors:                  dw 2880
+bdb_media_descriptor_type:          db 0F0h
+bdb_sectors_per_fat:                dw 9
+bdb_sectors_per_track:              dw 18
+bdb_heads:                          dw 2
+bdb_hidden_sectors:                 dd 0
+bdb_large_sector_count:             dd 0
+
+ebr_drive_number:                   db 0
+                                    db 0
+ebr_signature:                      db 0x29
+ebr_volume_id:                      db 0x01,0x02,0x03,0x04
+ebr_volume_label:                   db 'InfoSecIITR'
+ebr_system_id:                      db 'FAT12   '
+
 start:
     jmp main
 
@@ -40,15 +68,141 @@ main:
     ; Setup the program stack
     mov ss, ax
     mov sp, 0x7C00
+
+    ; Read something from floppy disk
+    ; BIOS should set `dl` to drive number
+    mov [ebr_drive_number], dl
+
+    mov ax, 1                           ; LBA = 1, second sector from disk
+    mov cl, 1                           ; 1 sector to read
+    mov bx, 7E00h                       ; data should be after the bootloader
+    call disk_read
+
+    ; Print the boot message
     mov si, msg_boot
     call puts
 
     hlt
 
+floppy_error:
+    mov si, msg_read_fail
+    call puts
+    jmp wait_key_and_reboot
+
+wait_key_and_reboot:
+    mov ah, 0
+    int 016h                            ; wait for key press
+
+    jmp 0FFFFh:0                        ; jump to the beginning of bios.
+                                        ; This will reboot the system 
+
+    hlt
+
 .halt:
-    jmp .halt
+    cli                                 ; disable interrupts
+                                        ; THis way CPU cant get out of halt state
+    hlt
+
+; Converts an LBA address to CHS address
+; Parameters:
+;   - ax : LBA address
+; Returns:
+;   - cx [0-5] : sector number
+;   - cx [6-15]: cyliinder
+;   - dh       : head
+
+lba_to_chs:
+
+    push ax
+    push dx
+
+    xor dx, dx
+    div word [bdb_sectors_per_track]    ; `ax` = lba / sectors per track
+                                        ; `dx` = lba % sectors per track
+    inc dx                              ; `dx` = lba % sectors per track + 1 = sector
+    mv  cx, dx
+
+    xor dx, dx
+    div word [bdb_heads]                ; `ax` = (lba / sectors per track) / heads
+                                        ; `dx` = (lba / sectors per track) % heads
+    mov dh, dl
+    mov ch, al
+    shl ah, 06
+    or  cl, ah
+
+    pop ax
+    mov dl, al
+    pop ax
+
+
+; Reads sectors from disk
+; Parameters:
+;   - ax : LBA address
+;   - cl : number of sectors to read
+;   - dl : drive number
+;   - es:bx : memory address where to store read data
+
+disk_read:
+
+    push ax                             ; Save registers on the stack
+    push bx
+    push cx
+    push dx
+    push di
+
+    push cx                             ; save `cl` -> number of sectors per thread
+    call lba_to_chs
+    pop ax                              ; number of sectors per thread
+    
+    mov ah, 02h
+    mov di, 3                           ; loop count variable
+
+; floppy being unreliable the call may not always be sucessful. 
+; Hence operations are performed multiple times.
+.retry:                                 
+    pusha                               ; push all registers on stack
+    stc                                 ; store carry flag
+    int 013h                            ; if the call is sucessful carry flag is set to 0
+    jnc .done                           ; jmp if call not succeful
+
+    popa
+    call disk_reset
+
+    dec di
+    test di, di
+    jnz .retry
+
+; If operation failed every time
+.fail:
+    jmp floppy_error
+
+.done:
+    popa
+
+    push di
+    push dx
+    push cx
+    push bx
+    push ax                             ; restore modified registers
+    ret
+    
+;
+; Resets disk controller
+; Parameters:
+;   -dl : drive number
+;
+disk_reset:
+    pusha
+    mov ah, 0
+    stc
+    int 13h
+    
+    jc floppy_error
+    popa
+    ret
 
 msg_boot: db "Hello World!",ENDL,0
+msg_read_fail: db "Read from disk failed",ENDL,0
 
 times 510-($-$$) db 0
 dw 0AA55h