287 lines
5.2 KiB
NASM
287 lines
5.2 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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; 64 bit IDT stuff
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IDT_ADDR equ 0x8000 ; bottom of IDT
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IDT_SIZE equ 0x1000 ; size of IDT
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IDT_SEGMENT equ 0x0008 ; gdt code segment to jump to for idt calls
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; magic numbers and things
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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
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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
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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:
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mov si, .msg
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call print
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.msg db "err ", 0x00
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ret
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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 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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; qemu -no-graphic overlaps serial and bios output without this
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mov al, 0x0D
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out dx, al
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mov al, 0x0A
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out dx, al
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; initialize IDT at IDT_ADDR
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call idt_init
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int3 ; silly test
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jmp KERNEL_LOAD_ADDR_ES * 0x10
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idt_init:
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; clear space for idt
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mov rdi, IDT_ADDR
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push di
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mov rcx, IDT_SIZE / 4
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xor rax, rax
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cld
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rep stosd ; clear double word for each rcx
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pop di
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push rdx ; dx contains the output device
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; nothing between here and the `pop rdx` will be able to print
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; breakpoint
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mov rdi, 0x8E ; present, no dpl, interrupt
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mov rsi, 0x03 ; breakpoint interrupt
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lea rdx, [int_hdl_dummy]
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call mk_idt_entry
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pop rdx ; get back dx output device
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lidt [idt_descriptor]
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ret
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int_hdl_dummy:
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mov rsi, .msg
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call lm_print
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iretq
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.msg db "INTdummy", 0x0D, 0x0A, 0x00
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; rdi = flags
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; rsi = entry number
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; rdx -> handler
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mk_idt_entry:
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mov rax, rdx
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mov rcx, rsi
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shl rcx, 4 ; offset = entry number * 16
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add rcx, IDT_ADDR
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mov word [rcx + 0], ax ; offset
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mov word [rcx + 2], IDT_SEGMENT ; code segment
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mov word [rcx + 5], di
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mov byte [rcx + 4], 0 ; IST (0 = none)
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shr rax, 16
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mov word [rcx + 6], ax ; offset
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shr rax, 16
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mov dword [rcx + 8], eax ; offset
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mov dword [rcx + 12], 0 ; reserved
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ret
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lm_print:
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.loop
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mov al, [rsi]
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test al, al
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jz .done
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out dx, al
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inc si
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jmp .loop
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.done:
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ret
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idt_descriptor:
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dw IDT_SIZE - 1 ; size
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dq IDT_ADDR ; start
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msg_a20_line db "a20 line", 0x0D, 0x0A, 0x00
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msg_disk_read db "disk", 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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