mirror of
https://github.com/gbowne1/ClassicOS.git
synced 2025-11-16 22:35:26 -08:00
multiple changes: BL1/BL2/kernel separation (build system, etc.) BL2 implementation. BL documentation
This commit is contained in:
vmttmv
292
bootloader/stage1.asm
Normal file
292
bootloader/stage1.asm
Normal file
@@ -0,0 +1,292 @@
|
||||
; ==============================================================================
|
||||
; boot.asm - First Stage Bootloader (CHS Based)
|
||||
; ==============================================================================
|
||||
|
||||
; Params for stage2
|
||||
%define s2_addr 1 ; stage2 disk offset, in sectors
|
||||
%define s2_laddr 0x7e00 ; stage2 load address
|
||||
%define s2_size 2048 ; stage2 size
|
||||
%define s2_nsect s2_size / 512 ; stage2 size in sectors
|
||||
|
||||
[BITS 16]
|
||||
global _start
|
||||
|
||||
_start:
|
||||
cli ; Disable interrupts
|
||||
|
||||
mov [bootdev], dl ; Save boot device number (from BIOS in DL)
|
||||
|
||||
; Setup stack safely below EBDA area (choose 0x0000:0x7A00)
|
||||
xor ax, ax ; AX = 0
|
||||
mov ss, ax ; Stack segment = 0x0000
|
||||
mov sp, 0x7A00 ; Stack offset = 0x7A00
|
||||
|
||||
; Initialize DS, ES for zero-based segments
|
||||
xor ax, ax
|
||||
mov ds, ax
|
||||
mov es, ax
|
||||
|
||||
; Query bios for disk parameters
|
||||
call get_disk_params
|
||||
|
||||
; Load second-stage bootloader (boot1.asm) to 0x7E00
|
||||
mov ax, 1 ; LBA of boot1.asm (starts at sector 1)
|
||||
call lba_to_chs
|
||||
mov al, s2_nsect ; Number of sectors to read
|
||||
mov bx, 0x7E00 ; Destination address offset ES = 0 (0x0000:0x7E00)
|
||||
call read_chs
|
||||
|
||||
; Enable A20 line
|
||||
call enable_a20
|
||||
jc a20_error ; Jump if A20 enable fails
|
||||
|
||||
; Setup Global Descriptor Table
|
||||
call setup_gdt
|
||||
|
||||
; Switch to protected mode and jump to second stage at 0x08:0x7E00
|
||||
call switch_to_pm
|
||||
|
||||
disk_error:
|
||||
mov si, disk_error_msg
|
||||
call print_string_16
|
||||
jmp halt
|
||||
|
||||
a20_error:
|
||||
mov si, a20_error_msg
|
||||
call print_string_16
|
||||
jmp halt
|
||||
|
||||
; ----------------------------------------------------------------
|
||||
; Verify Checksum Routine
|
||||
; Uses SI = start address, CX = byte count
|
||||
; Simple XOR checksum over bytes, expects result 0
|
||||
|
||||
verify_checksum:
|
||||
push ax
|
||||
push bx
|
||||
push di
|
||||
mov di, si
|
||||
xor al, al
|
||||
xor bx, bx
|
||||
|
||||
.verify_loop:
|
||||
lodsb
|
||||
xor bl, al
|
||||
loop .verify_loop
|
||||
|
||||
test bl, bl
|
||||
jz .checksum_ok
|
||||
stc ; Set carry on checksum error
|
||||
jmp .done
|
||||
|
||||
.checksum_ok:
|
||||
clc ; Clear carry on success
|
||||
|
||||
.done:
|
||||
pop di
|
||||
pop bx
|
||||
pop ax
|
||||
ret
|
||||
|
||||
get_disk_params:
|
||||
mov ah, 08h ; BIOS: Get Drive Parameters
|
||||
int 13h
|
||||
; TODO: error checking
|
||||
|
||||
; CL bits 0–5 contain sectors per track
|
||||
mov al, cl
|
||||
and al, 3Fh ; mask bits 0–5
|
||||
mov ah, 0
|
||||
mov [sectors_per_track], ax
|
||||
|
||||
; DH = maximum head number (0-based)
|
||||
mov al, dh
|
||||
inc ax ; convert to count (heads = maxhead + 1)
|
||||
mov [heads_per_cylinder], ax
|
||||
ret
|
||||
|
||||
; ----------------------------------------------------------------
|
||||
; CHS Disk Read Routine
|
||||
; AL = number of sectors
|
||||
; CL = starting sector (1-based)
|
||||
; Inputs:
|
||||
; AL = sector count
|
||||
; CH = cylinder
|
||||
; DH = head
|
||||
; CL = sector (1–63, with top 2 bits as high cylinder bits)
|
||||
|
||||
; ----------------------------------------------------------------
|
||||
; Convert LBA to CHS
|
||||
; Inputs:
|
||||
; AX = LBA sector number (0-based)
|
||||
; Outputs:
|
||||
; CH = cylinder
|
||||
; DH = head
|
||||
; CL = sector (1-63, top 2 bits are upper cylinder bits)
|
||||
|
||||
lba_to_chs:
|
||||
; Sector
|
||||
xor dx, dx
|
||||
mov bx, ax
|
||||
div word [sectors_per_track] ; divide lba with max sectors
|
||||
add dl, 1 ; take the remainder, sectors start at 1
|
||||
mov cl, dl ; sector is in cl
|
||||
|
||||
; Head
|
||||
mov ax, bx
|
||||
mov dx, 0
|
||||
div word [sectors_per_track] ; divide lba with max sectors
|
||||
mov dx, 0
|
||||
div word [heads_per_cylinder] ; divide quotient with heads
|
||||
mov dh, dl ; take the remainder, head is in dh
|
||||
|
||||
; Cylinder
|
||||
mov ch, al ; take the quotient, cylinder is in ch
|
||||
ret
|
||||
|
||||
read_chs:
|
||||
pusha
|
||||
push dx
|
||||
|
||||
.retry:
|
||||
mov ah, 0x02 ; BIOS: Read sectors
|
||||
mov dl, [bootdev] ; Boot device
|
||||
; Assume CH, DH, CL already set before this call
|
||||
int 0x13
|
||||
jc .error
|
||||
pop dx
|
||||
popa
|
||||
ret
|
||||
|
||||
.error:
|
||||
dec cx
|
||||
jz disk_error
|
||||
xor ah, ah
|
||||
int 0x13
|
||||
jmp .retry
|
||||
|
||||
; ----------------------------------------------------------------
|
||||
enable_a20:
|
||||
; Try fast A20 gate method
|
||||
in al, 0x92
|
||||
or al, 0x02
|
||||
and al, 0xFE ; Clear bit 0 to avoid fast A20 bugs
|
||||
out 0x92, al
|
||||
|
||||
; Verify A20
|
||||
call check_a20
|
||||
jnc .done ; Success
|
||||
|
||||
; Fallback: use keyboard controller method
|
||||
call .fallback
|
||||
|
||||
.done:
|
||||
ret
|
||||
|
||||
.fallback:
|
||||
mov al, 0xAD ; Disable keyboard
|
||||
out 0x64, al
|
||||
call .wait_input_clear
|
||||
|
||||
mov al, 0xD0 ; Command: read output port
|
||||
out 0x64, al
|
||||
call .wait_output_full
|
||||
in al, 0x60
|
||||
or al, 0x02 ; Set A20 enable bit
|
||||
mov bl, al
|
||||
|
||||
call .wait_input_clear
|
||||
mov al, 0xD1 ; Command: write output port
|
||||
out 0x64, al
|
||||
call .wait_input_clear
|
||||
mov al, bl
|
||||
out 0x60, al
|
||||
|
||||
call .wait_input_clear
|
||||
mov al, 0xAE ; Enable keyboard
|
||||
out 0x64, al
|
||||
ret
|
||||
|
||||
.wait_input_clear:
|
||||
in al, 0x64
|
||||
test al, 0x02
|
||||
jnz .wait_input_clear
|
||||
ret
|
||||
|
||||
.wait_output_full:
|
||||
in al, 0x64
|
||||
test al, 0x01
|
||||
jz .wait_output_full
|
||||
ret
|
||||
|
||||
check_a20:
|
||||
in al, 0x64 ; Read keyboard controller status
|
||||
test al, 0x02 ; Check if input buffer is full
|
||||
jnz check_a20 ; Wait until empty
|
||||
|
||||
mov al, 0xD0 ; Command: read output port
|
||||
out 0x64, al
|
||||
|
||||
.wait_output:
|
||||
in al, 0x60 ; Read output port value
|
||||
test al, 0x02 ; Check A20 gate bit (bit 1)
|
||||
jnz .a20_enabled ; If set, A20 enabled
|
||||
|
||||
xor al, al ; Clear carry to indicate failure
|
||||
stc ; Set carry for failure, jc will jump
|
||||
|
||||
ret
|
||||
|
||||
.a20_enabled:
|
||||
clc ; Clear carry flag to indicate success
|
||||
ret
|
||||
|
||||
; ----------------------------------------------------------------
|
||||
gdt_start:
|
||||
dq 0x0000000000000000 ; Null descriptor
|
||||
dq 0x00CF9A000000FFFF ; 32-bit code segment (selector 0x08)
|
||||
dq 0x00CF92000000FFFF ; 32-bit data segment (selector 0x10)
|
||||
dq 0x00009A000000FFFF ; 16-bit code segment for real mode (selector 0x18)
|
||||
|
||||
gdt_descriptor:
|
||||
dw gdt_end - gdt_start - 1
|
||||
dd gdt_start
|
||||
gdt_end:
|
||||
|
||||
setup_gdt:
|
||||
lgdt [gdt_descriptor]
|
||||
ret
|
||||
|
||||
; ----------------------------------------------------------------
|
||||
switch_to_pm:
|
||||
cli
|
||||
mov eax, cr0
|
||||
or eax, 1
|
||||
mov cr0, eax
|
||||
jmp 0x08:0x7E00 ; jump to S2
|
||||
|
||||
; ----------------------------------------------------------------
|
||||
print_string_16:
|
||||
.loop:
|
||||
lodsb
|
||||
or al, al
|
||||
jz .done
|
||||
mov ah, 0x0E
|
||||
int 0x10
|
||||
jmp .loop
|
||||
.done:
|
||||
ret
|
||||
|
||||
disk_error_msg db "Disk error!", 0
|
||||
a20_error_msg db "A20 error!", 0
|
||||
|
||||
halt:
|
||||
cli
|
||||
hlt
|
||||
|
||||
bootdev db 0
|
||||
sectors_per_track dw 0
|
||||
heads_per_cylinder dw 0
|
||||
|
||||
times 510 - ($ - $$) db 0
|
||||
dw 0xAA55
|
||||
Reference in New Issue
Block a user