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Set up a separate long-mode GDT

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This commit is contained in:
Thomas Oltmann 2025-07-05 20:41:51 +02:00
parent 0bfcd7afd2
commit c24a6014f7
1 changed files with 31 additions and 8 deletions
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39
lboot.S
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@ -43,7 +43,7 @@ _start: cli
jmp hang
.set COM1, 0x3F8
.set COM1, 0x3F8
.macro com1_write offset=0, byte
mov $COM1+\offset, %dx
mov $\byte, %al
@ -67,7 +67,7 @@ enable_a20: // TODO more thorough implementation
// unreal: Enter unreal mode
unreal: push %ds
lgdt gdt_ptr
lgdt gdt16_ptr
mov %cr0, %eax
or $0x01, %al
@ -244,7 +244,7 @@ long:
mov %eax, %cr3
// Enable long mode
.set IA32_EFER, 0xC0000080
.set IA32_EFER, 0xC0000080
mov $IA32_EFER, %ecx
rdmsr
or $0x100, %eax
@ -255,14 +255,16 @@ long:
or $0x80000001, %eax
mov %eax, %cr0
lgdt gdt64_ptr
ret
// hang: sleep indefinitely
hang: hlt
jmp hang
// gdt: Protected mode / Unreal mode 16-bit GDT
gdt: // entry 0: null descriptor
// gdt16: Protected mode / Unreal mode 16-bit GDT
gdt16: // entry 0: null descriptor
.word 0
.word 0
.byte 0
@ -283,9 +285,30 @@ gdt: // entry 0: null descriptor
.byte 0b10010010
.byte 0x8F
.byte 0
.set gdt_size, .-gdt
gdt_ptr: .word gdt_size-1
.long gdt
.set gdt16_size, .-gdt16
gdt16_ptr: .word gdt16_size-1
.long gdt16
// gdt64: Long mode 64-bit GDT
gdt64: // entry 0: null descriptor
.quad 0
// entry 1: code segment
.word 0
.word 0
.byte 0
.byte 0x98
.byte 0x60
.byte 0
// entry 2: data segment
.word 0
.word 0
.byte 0
.byte 0x90
.byte 0x00
.byte 0
.set gdt64_size, .-gdt64
gdt64_ptr: .word gdt64_size-1
.quad gdt64
// Messages to print
msg_start: .asciz "Netboot via fernlader v1 ...\r\n"