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; MIT License, Copyright (c) 2021 Marvin Borner
; This file includes definitions to reduce magic numbers
%include "src/entry/definitions.asm"
bits 16 ; Real mode is 16 Bit
org LOCATION ; Bootsector location
global _start
_start:
jmp .skip_bpb ; Some BIOSes override the BPB area
nop
times 87 db 0 ; Fill BPB area with 0
.skip_bpb:
; Clear registers (some BIOSes are weird)
xor bx, bx
mov ds, bx
mov es, bx
mov ss, bx
; Other registers may contain relevant data
mov sp, LOCATION ; Set stack (top) pointer - grows downwards
mov ax, SCREEN_CLEAR ; Clear screen command
int SCREEN_INT ; CLEAR!
; Print friendly welcome message
mov si, hello_msg
call print
call disk_support ; Is disk supported? Implicitly verifies that this is a 386+ architecture
mov esp, LOCATION ; Clear upper 16 Bit of esp, now that 16 Bit support is guaranteed
cli ; Disable interrupts
jmp load_stage ; Load and execute main stage
; Print function uses si as string pointer
print:
; a and b regs get partially destroyed - push for restoration
push bx
push ax
mov ah, SCREEN_OUT ; Set VGA command
xor bh, bh ; Clear b register (according to BIOS spec)
.putch:
lodsb ; Load next string byte (using ds:si)
test al, al ; Test loaded byte
jz .end ; End if al is zero (using previous test); NULL-termination
int SCREEN_INT ; WRITE!
jmp .putch ; Continue
.end:
; Restore ax/bx
pop ax
pop bx
ret
; Check if disk is supported using ID checks and LBA test
disk_support:
; BIOS puts the disk id into dl
cmp dl, 0x80
jb disk_error ; Error if below 0x80 - probably floppy disk
cmp dl, 0x8f
ja disk_error ; Error if above 0x8f - invalid
; Check if int 0x13 and LBA are supported
mov ah, DISK_EXT_CHECK ; Set needed interrupt values
mov bx, DISK_EXT_CHECK_REQ
int DISK_INT ; CHECK!
jc disk_error ; Carry means something went wrong
cmp bx, DISK_EXT_CHECK_RESP
jne disk_error ; Response is incorrect => error!
ret
; Read sectors from disk using dap information
disk_read:
mov si, dap
mov ah, DISK_READ
int DISK_INT
jc error_loop
ret
; Loads the main stage (main.c)
load_stage:
mov bx, loader
mov [dap.dest], bx
call disk_read
lgdt [gdt] ; Load GDT
; Set protected mode (32 Bit mode)
mov eax, cr0
or ax, 1 ; Set PE (Protection Enable) Bit
mov cr0, eax
jmp 0x08:protected_mode
bits 32
protected_mode:
; Set segment registers
mov ax, 0x10
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
mov ss, ax
call loader
jmp error_loop
bits 16
; Error handling
error_loop:
cli ; Disable interrupts
hlt ; Wait for interrupt; should wait forever
jmp error_loop ; Loop if returns anyway
disk_error:
mov si, disk_msg
call print
jmp error_loop
; Messages (NULL-terminated strings with newline/return)
hello_msg db "Hello world! Booting...", NEWLINE, RETURN, NULL
disk_msg db "Disk is invalid or unsupported", NEWLINE, RETURN, NULL
; Disk configuration data for DISK_READ interrupt (filled according to spec)
dap: ; Disk Address Packet
db 0x10 ; Disk address packet size
db 0 ; Always 0
.count:
dw 64 ; Number of sectors (512B each) to read ; 0x8000
.dest:
dw 0 ; Destination offset
dw 0 ; Destination segment
.lba:
dd 1 ; LBA number ; Inter stage is directly after first LBA (0)
dd 0 ; More storage bytes
; Global Descriptor Table (GDT)
; There are better explanations for the constants in definitions.asm
gdt:
dw .size - 1 + 8 ; GDT size
dd .start - 8 ; GDT start address
.start:
; Code
dw 0xffff ; Limit
dw 0x0000 ; Base (low 16 bits)
db 0x00 ; Base (mid 8 bits)
db GDT_PRESENT | GDT_DESCRIPTOR | GDT_EXECUTABLE | GDT_READWRITE ; Access
db GDT_GRANULARITY | GDT_SIZE | 0xf ; Granularity
db 0x00 ; Base (high 8 bits)
; Data
dw 0xffff ; Limit
dw 0x0000 ; Base (low 16 bits)
db 0x00 ; Base (mid 8 bits)
db GDT_PRESENT | GDT_DESCRIPTOR | GDT_READWRITE ; Access; Data isn't executable
db GDT_GRANULARITY | GDT_SIZE | 0xf ; Granularity
db 0x00 ; Base (high 8 bits)
.end:
.size: equ .end - .start
; The partition table gets inserted here (0x1b8 -> @440B)
times 0x1b8 - ($ - $$) db 0
times SECTOR_SIZE - 2 - ($ - $$) db 0 ; Fill until 512 (SECTOR_SIZE) - 2 bytes; -2 because of 2B signature
dw SECTOR_END_SIG ; Bootsector end signature
loader:
incbin "build/loader.bin"
; Limit to 0x8000 due to ext2 superblock start at 1024 on partition 1
times 0x8000 - ($ - $$) db 0
superblock:
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