152 lines
4.3 KiB
ArmAsm
152 lines
4.3 KiB
ArmAsm
; Copyright 2022 Mattia Giambirtone & Contributors
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;
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; Licensed under the Apache License, Version 2.0 (the "License");
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; you may not use this file except in compliance with the License.
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; You may obtain a copy of the License at
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;
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; http://www.apache.org/licenses/LICENSE-2.0
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;
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; Unless required by applicable law or agreed to in writing, software
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; distributed under the License is distributed on an "AS IS" BASIS,
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; WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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; See the License for the specific language governing permissions and
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; limitations under the License.
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; Welcome to TSBL: The Simple Bootloader
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[org 0x7c00] ; Address where the code expects to be loaded in. The BIOS always loads us here
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[bits 16] ; All x86 CPUs start in 16 bit (aka "real") mode, so we tell nasm to emit 16-bit code
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; Code copied from the limine bootloader
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jmp skip_bpb ; Workaround for some BIOSes that require this stub
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nop
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; Some BIOSes will do a funny and decide to overwrite bytes of code in
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; the section where a FAT BPB would be, potentially overwriting
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; bootsector code.
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; Avoid that by filling the BPB area with dummy values.
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; Some of the values have to be set to certain values in order
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; to boot on even quirkier machines.
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; Source: https://github.com/freebsd/freebsd-src/blob/82a21151cf1d7a3e9e95b9edbbf74ac10f386d6a/stand/i386/boot2/boot1.S
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bpb:
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times 3-($-$$) db 0
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bpb_oem_id: db "TSBL "
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bpb_sector_size: dw 512
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bpb_sects_per_cluster: db 0
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bpb_reserved_sects: dw 0
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bpb_fat_count: db 0
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bpb_root_dir_entries: dw 0
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bpb_sector_count: dw 0
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bpb_media_type: db 0
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bpb_sects_per_fat: dw 0
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bpb_sects_per_track: dw 18
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bpb_heads_count: dw 2
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bpb_hidden_sects: dd 0
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bpb_sector_count_big: dd 0
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bpb_drive_num: db 0
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bpb_reserved: db 0
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bpb_signature: db 0
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bpb_volume_id: dd 0
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bpb_volume_label: db "TSBL "
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bpb_filesystem_type: times 8 db 0
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kernel_offset: equ 0x1000 ; Address in memory where kernel will be loaded
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reserved_sectors: equ 4 ; How many disk sectors we reserve for our second stage
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skip_bpb:
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cli
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cld
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; This isn't needed inside the qemu emulator, but
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; real hardware is unlikely to start up with the
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; segment registers zeroed, so we do it here
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xor si, si
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mov ds, si
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mov ss, si
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mov es, si
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; We save the value of the current boot drive
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mov [boot_drive], dl
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; Now we setup the stack by setting the
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; base pointer to a location that's far
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; enough from where the code for the BIOS
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; is located
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mov sp, 0x7c00
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mov bp, sp
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sti
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call bios_cls
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mov si, loading_stage2_msg
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call bios_println
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mov bx, stage2
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mov cl, 2
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mov dh, reserved_sectors
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mov dl, [boot_drive]
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call load_disk
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jmp stage2
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; Variables needed in the boot
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; sector
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boot_drive: db 0
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loading_stage2_msg: db "TSBL - INFO: Stage 1 loaded, loading stage 2", 0
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; We include just the bare minimum needed to load the
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; second stage of our bootloader
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%include "src/boot/util/disk.s"
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%include "src/boot/util/io.s"
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; Padding and magic number
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times 510 - ($ - $$) db 0
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dw 0xaa55
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[bits 16]
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stage2:
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; Here we're no longer limited by the size of the
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; boot sector, so we can perform the more complex
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; part of the boot process
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mov si, stage2_loaded_msg
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call bios_println
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call load_kernel
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call switch_to_protected_mode
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load_kernel:
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; Loads the kernel into memory
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mov bx, kernel_offset
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mov cl, reserved_sectors + 1
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mov dh, 4
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mov dl, [boot_drive]
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call load_disk
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mov si, kernel_loaded_msg
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call bios_println
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ret
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[bits 32]
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BEGIN_32BIT: ; After the switch we will get here
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call enableA20
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call kernel_offset
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cli
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hlt
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jmp $
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; Now we include the necessary files to switch to
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; 32-bit (aka protected) mode
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%include "src/boot/gdt.s"
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%include "src/boot/switch32.s"
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%include "src/boot/util/enablea20.s"
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; Here we define our variables used in the second stage
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kernel_loaded_msg: db "TSBL - INFO: Kernel loaded, switching to protected mode", 0
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stage2_loaded_msg: db "TSBL - INFO: Stage 2 loaded, loading kernel", 0
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times (512 * reserved_sectors) - ($ - $$) db 0 ; Pads the section to exactly reserved_sectors
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