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/*
* Copyright (c) 2026, Apollo Telephone Laboratories.
* Provided under the BSD-3 clause.
*/
#define BOOT_SEG 0x7C0 /* Segment we reside on startup */
#define BOOT_RELOC_SEG 0x7A0 /* Relocation segment */
#define RDAR_MAGIC ">RDAR!"
#define RDAR_MAGIC_LEN 6
#define BOOT_PATH "s2.bin"
#define BOOT_PATH_LEN 6
.text
.code16
_start:
ljmp $BOOT_SEG,$main
main:
cli
cld
/* Set segment registers */
xor %ax, %ax
mov %ax, %fs
mov %ax, %gs
mov %ax, %ss
mov %ax, %es
mov $BOOT_SEG, %ax
mov %ax, %ds
/* Save the drive */
mov %dl, drive_num
/*
* The second stage bootloader expects to be loaded to 0x7C00 but since we are
* already executing at this address we'll need to relocate ourselves first before
* we clobber it.
*/
mov $BOOT_SEG, %ax
mov %ax, %ds
mov $BOOT_RELOC_SEG, %ax
mov %ax, %es
xor %si, %si
xor %di, %di
mov $0x200, %cx
cld
rep movsb
jmp $BOOT_RELOC_SEG,$1f
1: mov $0x7C00, %sp
/* Load two sectors */
movw $2, dap_blkcnt
movl $1, dap_lba_low
call disk_read_lba
mov $BOOT_RELOC_SEG, %ax
mov %ax, %ds
/* Get stage2 */
call find_stage2
/* Obtain the data offset */
mov $BOOT_SEG, %ax
mov %ax, %ds
add $40, %si
movw (%si), %si
mov $BOOT_SEG, %ax
mov %ax, %ds
mov %ax, %es
xor %di, %di
mov $0x200, %cx
cld
rep movsb
/* Jump to stage 2 */
xor %ax, %ax
mov %ax, %ds
ljmp $0x0000,$0x7C00
/*
* Write a character to the boot console
*
* %al: Character to write
*/
putc:
mov $0x0E, %ah
xor %bh, %bh
int $0x10
ret
/*
* Write a string to the boot console
*
* %si: Pointer to string to write
*/
puts:
lodsb
or %al, %al
jz 1f
call putc
jmp puts
1: ret
/*
* Locate stage2 on the disk
*
* Stage 2 data descriptor is returned in %SI
*/
find_stage2:
push %ds
push %es
/* Verify that magic is correct */
mov $BOOT_SEG, %ax
mov %ax, %ds
xor %bx, %bx /* Tracks %SI */
xor %si, %si
mov $BOOT_RELOC_SEG, %ax
mov %ax, %es
mov $rdar_magic, %di
mov $RDAR_MAGIC_LEN, %cx
rep cmpsb
jne .bad_magic
/* Iterate every data descriptor */
clc /* Carry set if found */
1: movw %ds:6, %cx /* Add data descriptor size */
mov %bx, %si
mov %si, %dx
add %cx, %bx
mov $rdar_magic, %di
mov $RDAR_MAGIC_LEN, %cx
rep cmpsb
jne .not_found
/* Does the path match? */
mov %dx, %si
add $8, %si
mov $stage2_path, %di
mov $BOOT_PATH_LEN, %cx
rep cmpsb
jne 1b
mov %dx, %si
stc
pop %es
pop %ds
jnc .not_found
ret
.not_found:
pop %ds
mov $not_found_msg, %si
call puts
1: cli
hlt
jmp 1b
.bad_magic:
/* Restore %DS */
mov $BOOT_RELOC_SEG, %ax
mov %ax, %ds
mov $bad_magic_msg, %si
call puts
1: cli
hlt
jmp 1b
/*
* Read sectors from the disk
*/
disk_read_lba:
pusha
mov $dap, %si
mov $0x42, %ah
mov drive_num, %dl
int $0x13
jc .error
popa
ret
.error:
mov $disk_err_msg, %si
call puts
1: cli
hlt
jmp 1b
/* Global stores */
rdar_magic: .ascii RDAR_MAGIC
bad_magic_msg: .ascii "bad RDAR magic\r\n\0"
not_found_msg: .ascii "stage 2 not found\r\n\0"
disk_err_msg: .ascii "\r\n!!!disk error!!!\r\n\0"
stage2_path: .ascii BOOT_PATH
drive_num: .byte 0
dap:
.byte 0x10
.byte 0x00
dap_blkcnt:
.word 0
dap_destptr:
.word 0x7C00
dap_destseg:
.word 0x0000
dap_lba_low:
.long 0
dap_lba_high:
.long 0
.org 510
.byte 0x55
.byte 0xAA
|
