213 lines
3.8 KiB
NASM
213 lines
3.8 KiB
NASM
; bootler
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; legacy bootloader in nasm-flavoured asm
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; 2026 Andromeda
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; GPL licensed
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LOAD_ADDR equ 0x7C00
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KERNEL_START equ 2 ; first sector on disk to load kernel from; 1 indexed
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KERNEL_SIZE equ 1 ; length of kernel in sectors
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KERNEL_LOAD_ADDR_ES equ 0x1000 ; kernel to be loaded at es * 0x10 + 0x0000
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PAGE_TABLE_LOAD_ADDR equ 0x1000 ; start of page table; 4 * pt size
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PAGE_TABLE_SIZE equ 0x1000 ; size of page table in bytes
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A20_LINE_ENABLE equ 0x2401 ; magic number in ax to enable a20 line with 0x15
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BIOS_INT_DISK_OP equ 0x13
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BIOS_INT_MEMORY_OP equ 0x15
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CR0_PE equ 1 << 0 ; Protected Mode Enabled
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CR0_PG equ 1 << 31 ; Paging
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CR4_PAE equ 1 << 5 ; Physical Address Extension
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CR4_PGE equ 1 << 7 ; Page Global Enabled
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MSR_IA32_EFER equ 0xC0000080 ; Extended Feature Enable model-specific Register
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[bits 16]
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[org LOAD_ADDR]
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jmp 0x0000:.cs_reset ; in case it loads us at 0x7C00:0x0000
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.cs_reset:
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xor ax, ax
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mov ss, ax
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mov sp, LOAD_ADDR ; stack builds down, loader faces up
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mov ds, ax
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mov es, ax
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mov fs, ax
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mov gs, ax
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cld
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; activate a20
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mov ax, A20_LINE_ENABLE
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int BIOS_INT_MEMORY_OP
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jc error_a20_line
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mov si, msg_a20_line
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call print
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; prep to read disk
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mov al, KERNEL_SIZE ; size in sectors, min. 1
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mov ch, 0 ; cylinder
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mov cl, KERNEL_START ; start sector, 1-indexed
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mov dh, 0 ; head
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; dl set by bios
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mov bx, KERNEL_LOAD_ADDR_ES ; load addr es:bx
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mov es, bx
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xor bx, bx
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; read disk
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mov ah, 0x02
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int BIOS_INT_DISK_OP
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jc error_disk_read
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mov si, msg_disk_read
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call print
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; zero es
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push ax
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xor ax, ax
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mov es, ax
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pop ax
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mov edi, PAGE_TABLE_LOAD_ADDR
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jmp lm_load
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; expects:
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; es:edi -> 16kb for page table
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; ss:esp -> stack
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; heavily based on https://wiki.osdev.org/Entering_Long_Mode_Directly
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lm_load:
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; clear page table
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push di
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mov ecx, PAGE_TABLE_SIZE
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xor eax, eax
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cld
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rep stosd ; clear space for all 4 tables
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pop di
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; PML4TE0
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lea eax, [es:di + PAGE_TABLE_SIZE]
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or eax, 11b
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mov [es:di], eax
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; PDPT0E0
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lea eax, [es:di + 2 * PAGE_TABLE_SIZE]
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or eax, 11b
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mov [es:di + PAGE_TABLE_SIZE], eax
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; PDT0E0
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lea eax, [es:di + 3 * PAGE_TABLE_SIZE]
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or eax, 11b
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mov [es:di + 2 * PAGE_TABLE_SIZE], eax
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; PT0
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push di
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lea di, [di + 3 * PAGE_TABLE_SIZE]
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mov eax, 0x11
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.set_pt_entry
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mov [es:di], eax
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add eax, 0x1000
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add di, 8
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cmp eax, 0x200000 ; size of page
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jb .set_pt_entry
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pop di
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; disable interrupts from PIC by masking all interrupts
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mov al, 0xFF ; mask
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out 0xA1, al
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out 0x21, al
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; PAE, PGE
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mov eax, CR4_PAE | CR4_PGE
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mov cr4, eax
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; cr3 -> PML4TE0
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mov edx, edi
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mov cr3, edx
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; highs LME bit in EFER
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mov ecx, MSR_IA32_EFER
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rdmsr
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or eax, 0x00000100
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wrmsr
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; activate long mode
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mov ebx, cr0
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or ebx, CR0_PG | CR0_PE
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mov cr0, ebx
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cli
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lgdt [gdt_descriptor]
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jmp 0x08:lm_start
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error_a20_line:
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mov si, .msg
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call print
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jmp done
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.msg db "err a20 line", 0x00
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error_disk_read:
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mov si, .msg
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call print
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jmp done
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.msg db "err disk read", 0x00
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done:
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hlt
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jmp done
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print:
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pushad
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.loop
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lodsb
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test al, al
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je .done
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mov ah, 0x0E
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int 0x10
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jmp .loop
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.done
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popad
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ret
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; TODO make real idt
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empty_idt:
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dw 0 ; length
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dd 0 ; base
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; TODO make readable (yoinked from https://wiki.osdev.org/Entering_Long_Mode_Directly)
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gdt_data:
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dq 0x0000000000000000
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dq 0x00209A0000000000
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dq 0x0000920000000000
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gdt_descriptor:
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dw $ - gdt_data - 1 ; size
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dd gdt_data ; start
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[bits 64]
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; long mode entry
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lm_start:
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mov ax, 0x10
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mov ds, ax
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mov es, ax
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mov fs, ax
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mov gs, ax
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mov ss, ax
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; set up serial device
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; TODO poll and whatnot (in rust??)
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mov dx, 0x3F8
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jmp KERNEL_LOAD_ADDR_ES * 0x10
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msg_a20_line db "a20 line", 0x0D, 0x0A, 0x00
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msg_disk_read db "disk read", 0x0D, 0x0A, 0x00
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times 510-($-$$) db 0 ; 2 bytes less now
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db 0x55
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db 0xAA
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