123 lines
2.1 KiB
ArmAsm
123 lines
2.1 KiB
ArmAsm
.intel_syntax noprefix
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#include <mem.h>
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// Some info from here: http://os.phil-opp.com/multiboot-kernel.html
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.section .text
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.global _start
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.code32 // GRUB leaves us in 32 bit mode
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_start:
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cli
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// Check if long mode is available
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// Otherwise, there's no point in continuing
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call check_longmode
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// Enable long mode through process described in
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// AMD64 manual vol. 2 ch. 14
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// Enable Physical Address Extension
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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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// Load page directory into cr3
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mov eax, offset V2P(BootP4)
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mov cr3, eax
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// Enable long mode
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mov ecx, 0xC0000080
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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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// We are now in 32 bit long mode
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// Go to 64 bit mode by loading a GDT
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// and perforing a far jump
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lgdt [V2P(BootGDTp)]
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mov eax, 0x10
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mov ss, eax
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mov ds, eax
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mov es, eax
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.extern long_mode_start
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jmp 0x8:V2P(long_mode_start)
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.code64
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long_mode_start:
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// We are now in 64 bit long mode
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// but we are still running code in the identity mapped low memory kernel mirror
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// Next we jump to code in high memory (> 32 bits)
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movabs rax, offset .upper_memory
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jmp rax
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.upper_memory:
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// We are now in high memory
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// Set up a stack and set up a fake return address to stop backtracing
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movabs rsp, offset BootStack
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push 0
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mov rbp, rsp
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// Remove identity mapping
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mov rax, 0x0
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mov [V2P(BootP4)], rax
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// Call c kernel code
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.extern kmain
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call kmain
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hlt
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.code32
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// Check if CPUID is available
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// Method described in AMD64 manual vol. 3
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check_cpuid:
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pushfd
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pushfd
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xor dword ptr[esp], 0x00200000
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popfd
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pushfd
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pop eax
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xor eax, [esp]
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popfd
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and eax, 1<<21
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jz error
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ret
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// Check if long mode is available using cpuid
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// AMD manual vol. 3 Appendix E
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check_longmode:
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call check_cpuid
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mov eax, 0x80000000
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cpuid
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cmp eax, 0x80000001
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jb error
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mov eax, 0x80000001
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cpuid
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test edx, 1<<29
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jz error
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ret
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// Jump here if something went wrong
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error:
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// Print ERR! to screen
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mov [0xb8000], dword ptr 0x4f524f45
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mov [0xb8004], dword ptr 0x4f214f52
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jmp .
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