173 lines
5.0 KiB
NASM
173 lines
5.0 KiB
NASM
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; vulcanOS Kernel ;
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; Developed by Marco 'icebit' Cetica ;
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; (c) 2019-2021 ;
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; Released under GPLv3 ;
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; https://github.com/ice-bit/vulcanos ;
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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EXTERN long_mode_start
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[BITS 32] ; Ensure we are in protected mode
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section .text
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kernel_loader:
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mov esp, kernel_stack + KERNEL_STACK_SZ ; Define stack pointer
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; Now we need to switch to Long Mode(i.e., 64bit mode)
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; to do that, we first need to check if we actually are
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; being loaded by a multiboot compliant bootloader. Then,
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; we will execute CPUID instruction in order to obtain details
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; about out CPU(i.e., if the CPU supports long mode).
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call check_multiboot
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call check_cpuid
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call check_long_mode
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; After that, we need to setup paging
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call setup_page_tables
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call enable_paging
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; We also need to load a GDT
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lgdt[gdt64.pointer]
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jmp gdt64.code_segment:long_mode_start
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push eax ; Set multiboot header register
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check_multiboot:
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; To prove that we are being loaded by a multiboot
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; bootloader, we can read the EAX register to see if
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; it contains the magic number
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cmp eax, 0xE85250D6
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jne .multiboot_panic
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ret
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.multiboot_panic:
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; We handle this error by showing a special code
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mov al, "M"
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jmp error ; Call print routine
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check_cpuid:
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; To check if CPUID is available we need to flip
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; ID bit of the flag register, if this is possibile
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; then CPUID instruction is available. In order to achieve
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pushfd ; Push flag register into the stack
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pop eax ; Pop the stack
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mov ecx, eax ; Make a copy of the stack
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xor eax, 1 << 21 ; Flip ID bit(21th) from the eax register
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push eax
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popfd ; Restore flag register into the stack
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pushfd
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pop eax
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push ecx
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popfd
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cmp eax, ecx ; If values are the same, then CPUID is not available
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je .cpuid_panic
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ret ; Otherwise return
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.cpuid_panic:
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mov al, "C"
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jmp error ; Call print routine
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check_long_mode:
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; Check if CPU supports extended processor info
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mov eax, 0x80000000 ; Store magic number
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cpuid ; CPUID instruction takes EAX by default
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; CPUID will set EAX a value greater than the magic number
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; if the processor supports extended processor info
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cmp eax, 0x80000001 ; Magic number + 1
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jb .long_mode_panic
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; Otherwise we can use extended processor info
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; to check if long mode is available
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mov eax, 0x80000001
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cpuid
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; CPUID, with this magic number, will set LM bit(29th)
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; if and only if long mode is available
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test edx, 1 << 29
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jz .long_mode_panic
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ret
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.long_mode_panic:
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mov al, "L"
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jmp error ; Call print routine
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setup_page_tables:
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mov eax, page_table_l3
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or eax, 0b11
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mov [page_table_l4], eax
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mov eax, page_table_l2
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or eax, 0b11
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mov [page_table_l3], eax
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.loop:
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mov eax, 0x200000 ; 2MiB
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mul ecx
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or eax, 0b10000011
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mov [page_table_l2 + ecx * 8], eax
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mov ecx, 0 ; Counter
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inc ecx ; Increment counter
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cmp ecx, 512 ; Check if whole table is being mapped
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jne .loop ; if not, repeat
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ret
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enable_paging:
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; Pass page table location to the cpu
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mov eax, page_table_l4
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mov cr3, eax
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; Enable PAE
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mov eax, cr4
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or eax, 1 << 5
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mov cr4, eax;
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; Enable long mode
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mov ecx, 0xC0000000
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rdmsr
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or eax, 1 << 8
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wrmsr
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; Enable paging
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mov eax, cr0
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or eax, 1 << 31
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mov cr0, eax
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ret
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error: ; Prints "ERR: X" where X stands ofr the given error code
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; This routine will just print the error code in video memory
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; Let's understand what those hex codes actually mean:
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; 0xB8000 is the begin of the VGA text buffer, each byte will
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; contain one character. 0x4F is the color code; it means
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; white text on read background. 0x52 is the ASCII code for
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; the letter 'R', while 0x45 stands for the letter 'E',
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; 0x3A stands for the symbol ':' and 0x20 is just a space.
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; Do note that addressing is in little endian ordering and
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; that one space is begin overwritten by the ASCII byte.
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; previous NEW CONTENT
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mov dword [0xB8000], 0x4F524F45 ; [] ER
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mov dword [0xB8004], 0x4F3A4F52 ; [ER] R:
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mov dword [0xB8008], 0x4F204F20 ; [ERR:] <space>
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mov byte [0xB800a], al ; Print the error code
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hlt ; Halt the CPU
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KERNEL_STACK_SZ equ 4096 ; Stack size(4KiB)
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section .bss
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align 4096
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page_table_l4:
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resb 4096
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page_table_l3:
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resb 4096
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page_table_l2:
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resb 4096
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kernel_stack:
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resb KERNEL_STACK_SZ ; Reserver 4KiB for kernel's stack
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section .rodata
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gdt64:
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dq 0 ; zero entry
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.code_segment: equ $ - gdt64
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dq (1 << 4) | (1 << 44) | (1 << 47) | (1 << 53) ; Code segment
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; executable, descriptor, present 64bit flags
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.pointer:
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dw $ - gdt64 - 1
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dd gdt64 |