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[GLOBAL loader] ; making entry point visible to linker
[EXTERN kmain] ; kmain is defined in kmain.c
STACKSIZE equ 0x4000 ; that's 16k.
; setting up the Multiboot header - see GRUB docs for details
MODULEALIGN equ 1<<0 ; align loaded modules on page boundaries
MEMINFO equ 1<<1 ; provide memory map
FLAGS equ MODULEALIGN | MEMINFO ; this is the Multiboot 'flag' field
MAGIC equ 0x1BADB002 ; 'magic number' lets bootloader find the header
CHECKSUM equ -(MAGIC + FLAGS) ; checksum required
section .text
align 4
MultiBootHeader:
dd MAGIC
dd FLAGS
dd CHECKSUM
section .setup
loader: ;here, we load our false GDT, used for having the kernel in higher half
lgdt [trickgdt]
mov cx, 0x10;
mov ds, cx;
mov es, cx;
mov fs, cx;
mov gs, cx;
mov ss, cx;
jmp 0x08:higherhalf
section .text
higherhalf: ; now we're running in higher half
mov esp, stack+STACKSIZE ; set up the stack
push eax ; pass Multiboot magic number
add ebx, 0xE0000000 ; update the MB info structure so that it is in the new seg
push ebx ; pass Multiboot info structure
call kmain ; call kernel proper
cli ; disable interupts
hang:
hlt ; halt machine should kernel return
jmp hang
[section .setup] ; this is included in the .setup section, so that it thinks it is at 0x00100000
trickgdt: ; our false GDT
dw gdt_end - gdt - 1 ; gdt limit
dd gdt ; gdt base
gdt:
dd 0, 0 ; null GDT entry
db 0xFF, 0xFF, 0, 0, 0, 10011010b, 11001111b, 0x20 ; kernel code segment
db 0xFF, 0xFF, 0, 0, 0, 10010010b, 11001111b, 0x20 ; kernel data segment
gdt_end:
[section .bss]
align 32
stack:
resb STACKSIZE ; reserve 16k stack on a quadword boundary
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