back to nasm
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nub31
106
src/boot.asm
Normal file
106
src/boot.asm
Normal file
@@ -0,0 +1,106 @@
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global _start
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extern kernel_main
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%define FLAGS 0b10
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%define MAGIC 0x1BADB002
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%define CHECKSUM -(MAGIC + FLAGS)
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section .multiboot
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align 4
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dd MAGIC
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dd FLAGS
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dd CHECKSUM
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section .bss
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align 4096
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pml4_table:
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resb 4096
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pdpt_table:
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resb 4096
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pd_table:
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resb 4096
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section .bss
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align 16
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stack_bottom:
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resb 16384
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stack_top:
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section .data
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align 8
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gdt64:
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dq 0x0000000000000000
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.code:
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dq 0x00AF9A000000FFFF
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.descriptor:
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dw .end - gdt64 - 1
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dq gdt64
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.end:
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section .text
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bits 32
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_start:
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mov esp, stack_top
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; Multiboot will place a magic value in eax
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; If this magic value is not present, then we might not have multiboot info in ebx,
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; therefore we throw an error
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cmp eax, 0x2BADB002
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jne error
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; Check if cpuid is available by flipping the 22-nth youngest bit
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; in the eflags register and checking if the cpu flipped it back
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pushfd
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pop eax
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mov ecx, eax
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xor eax, 1 << 21
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push eax
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popfd
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pushfd
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pop eax
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; If cpuid is available, eax should be different than ecx
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xor eax, ecx
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jz error
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; Finally restore eflags register to the original value
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push ecx
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popfd
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; Check if extended cpuid is available by calling cpuid with 0x80000000,
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; If cpuid is available, eax will be greater than 0x80000000 after the call
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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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; Check if long mode is available by calling cpuid with 0x80000001
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; this will place the extended features of the cpu in edx
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; The 30-nth youngest bit tells us if long mode is supported or not
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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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; todo(nub31): setup paging
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; todo(nub31): enter long mode
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; Load global descriptor table which is set up for 64 bit
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lgdt [gdt64.descriptor]
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call kernel_main
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jmp error
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error:
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cli
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mov byte [0xb8000], 'B'
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mov byte [0xb8002], 'O'
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mov byte [0xb8004], 'O'
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mov byte [0xb8006], 'T'
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mov byte [0xb8008], ' '
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mov byte [0xb800a], 'E'
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mov byte [0xb800c], 'R'
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mov byte [0xb800e], 'R'
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mov byte [0xb8010], 'O'
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mov byte [0xb8012], 'R'
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hang:
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hlt
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jmp hang
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108
src/boot.s
108
src/boot.s
@@ -1,108 +0,0 @@
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.intel_syntax noprefix
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.set ALIGN, 1<<0
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.set MEMINFO, 1<<1
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.set FLAGS, ALIGN | MEMINFO
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.set MAGIC, 0x1BADB002
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.set CHECKSUM, -(MAGIC + FLAGS)
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.section .multiboot
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.align 4
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.long MAGIC
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.long FLAGS
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.long CHECKSUM
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.section .bss
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.align 4096
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pml4_table:
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.zero 4096
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pdpt_table:
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.zero 4096
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pd_table:
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.zero 4096
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.section .bss
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.align 16
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stack_bottom:
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.skip 16384
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stack_top:
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.section .data
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.align 8
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gdt64:
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.quad 0x0000000000000000
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gdt64_code:
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.quad 0x00AF9A000000FFFF
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gdt64_descriptor:
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.word gdt64_end - gdt64 - 1
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.long gdt64
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gdt64_end:
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.section .text
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.code32
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.global _start
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_start:
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mov esp, stack_top
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// Multiboot will place a magic value in eax
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// If this magic value is not present, then we might not have multiboot info in ebx,
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// therefore we throw an error
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cmp eax, 0x2BADB002
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jne error
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// Check if cpuid is available by flipping the 22-nth youngest bit
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// in the eflags register and checking if the cpu flipped it back
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pushfd
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pop eax
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mov ecx, eax
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xor eax, 1 << 21
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push eax
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popfd
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pushfd
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pop eax
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// If cpuid is available, eax should be different than ecx
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xor eax, ecx
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jz error
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// Finally restore eflags register to the original value
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push ecx
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popfd
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// Check if extended cpuid is available by calling cpuid with 0x80000000,
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// If cpuid is available, eax will be greater than 0x80000000 after the call
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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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// Check if long mode is available by calling cpuid with 0x80000001
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// this will place the extended features of the cpu in edx
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// The 30-nth youngest bit tells us if long mode is supported or not
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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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// todo(nub31): setup paging
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// todo(nub31): enter long mode
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// Load global descriptor table which is set up for 64 bit
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lgdt [gdt64_descriptor]
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call kernel_main
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jmp error
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error:
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cli
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mov byte ptr [0xb8000], 'B'
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mov byte ptr [0xb8002], 'O'
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mov byte ptr [0xb8004], 'O'
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mov byte ptr [0xb8006], 'T'
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mov byte ptr [0xb8008], ' '
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mov byte ptr [0xb800a], 'E'
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mov byte ptr [0xb800c], 'R'
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mov byte ptr [0xb800e], 'R'
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mov byte ptr [0xb8010], 'O'
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mov byte ptr [0xb8012], 'R'
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hang:
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hlt
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jmp hang
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