97 lines
2.4 KiB
NASM
97 lines
2.4 KiB
NASM
global start
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extern x86_start
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section .text
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bits 32
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start:
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; our stack, located in bss, linker.ld puts bss at the end of the binary
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mov esp, stack_top
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; multiboot information pointer
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push ebx
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call check_multiboot
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call set_up_page_tables
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; load the new gdt
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lgdt [GDTR.ptr]
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jmp GDTR.gdt_cs:x86_start
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check_multiboot:
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cmp eax, 0x36d76289
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jne .no_multiboot
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ret
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.no_multiboot:
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mov al, "0"
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jmp error
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; minimal page tables (first 8MB identity mapped)
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; core page tables will be loaded in rust
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set_up_page_tables:
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; map P2 table recursively
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mov eax, p2_table
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or eax, 0b11 ; present + writable
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mov [p2_table + 1023 * 4], eax
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; identity map first P2 entry to a huge page
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mov eax, 0x0 ; 0MB -> 4MB (first page)
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or eax, 0b10000011 ; huge + present + writable
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mov [p2_table], eax
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mov eax, 0x400000 ; 4MB -> 8Mb (second page)
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or eax, 0b10000011 ; huge + present + writable
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mov [p2_table + 4], eax
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mov eax, 0x800000 ; 8MB -> 12Mb (third page)
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or eax, 0b10000011 ; huge + present + writable
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mov [p2_table + 8], eax
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mov eax, p2_table
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mov cr3, eax
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ret
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error:
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mov dword [0xb8000], 0x4f524f45
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mov dword [0xb8004], 0x4f3a4f52
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mov dword [0xb8008], 0x4f204f20
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mov byte [0xb800a], al
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cli
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HALT:
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hlt
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jmp HALT
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section .bss
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align 4096
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p2_table:
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resb 4096
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stack_bottom:
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resb 4096 * 4
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stack_top:
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; minimal boot gdt (cs & ds)
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; core gdt will be loaded in rust
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section .gdt
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GDTR:
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; http://tuttlem.github.io/2014/07/11/a-gdt-primer.html
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.gdt_top:
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DD 0, 0
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.gdt_cs: equ $ - .gdt_top; the code segment Aka KERNEL CODE
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DW 0xffff ; Limit ( bits 0 -15 )
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DW 0x0 ; Base ( bits 0 -15 )
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DB 0x0 ; Base ( bits 16 -23 )
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DB 0x9A ; [ Access Flags: 0x9A=10011010b = (present)|(Privilege Ring 0=00b)|(1)|(code => 1)|(expand down => 0)|(readable)|(0) ]
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DB 0xCF ; [ Flags: C=1100b = (granularity)|(32bit)|(!64bit)|(0) ] / [ Limits: (bits 16-19): F=1111b ]
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DB 0x0 ; Base ( bits 24 -31 )
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.gdt_ds: equ $ - .gdt_top; the data segment Aka KERNEL DATA
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DW 0xffff ; Limit ( bits 0 -15 )
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DW 0x0 ; Base ( bits 0 -15 )
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DB 0x0 ; Base ( bits 16 -23 )
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DB 0x92 ; [ Access Flags: 0x92=10010010b = (present)|(Privilege Ring 0=00b)|(1)|(data => 0)|(expand down => 0)|(readable)|(0) ]
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DB 0xCF ; [ Flags: C=1100b = (granularity)|(32bit)|(!64bit)|(0) ] / [ Limits: (bits 16-19): F=1111b ]
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DB 0x0 ; Base ( bits 24 -31 )
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.gdt_bottom:
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.ptr:
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DW .gdt_bottom - .gdt_top - 1 ; length of the structure minus 1
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DD .gdt_top ; pointer to top of gdt
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