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Diffstat (limited to 'asm/start.s')
| -rw-r--r-- | asm/start.s | 85 |
1 files changed, 85 insertions, 0 deletions
diff --git a/asm/start.s b/asm/start.s new file mode 100644 index 0000000..bb12a7c --- /dev/null +++ b/asm/start.s @@ -0,0 +1,85 @@ +# https://www.gnu.org/software/grub/manual/multiboot/multiboot.html#Boot-information-format +# http://wiki.osdev.org/Bare_Bones + +# Fill Multiboot Haeder, init stack and call kernel_main passing to params: +# eax - magic number +# ebx - multiboot structure + +# Declare constants used for creating a multiboot header. +.set ALIGN, 1<<0 # align loaded modules on page boundaries +.set MEMINFO, 1<<1 # provide memory map +.set GFXINFO, 1<<2 # provide gfx info +.set FLAGS, ALIGN | MEMINFO | GFXINFO # this is the Multiboot 'flag' field +.set MAGIC, 0x1BADB002 # 'magic number' lets bootloader find the header +.set CHECKSUM, -(MAGIC + FLAGS) # checksum of above, to prove we are multiboot + +# entry point for application processors at 0x7000 +.section .smp +.code16 +call smp_go # TODO: align later before going C + +# Declare a header as in the Multiboot Standard. We put this into a special +# section so we can force the header to be in the start of the final program. +# You don't need to understand all these details as it is just magic values that +# is documented in the multiboot standard. The bootloader will search for this +# magic sequence and recognize us as a multiboot kernel. + +.section .multiboot +.code32 +.align 4 +.long MAGIC +.long FLAGS +.long CHECKSUM + +.long 0 # we dont need this for ELF +.long 0 +.long 0 +.long 0 +.long 0 + +# we override this from grub anyway +.long 0 #gfx_stuff 0=enable! +.long 640 +.long 480 +.long 32 + +# Currently the stack pointer register (esp) points at anything and using it may +# cause massive harm. Instead, we'll provide our own stack. We will allocate +# room for a small temporary stack by creating a symbol at the bottom of it, +# then allocating 16384 bytes for it, and finally creating a symbol at the top. +.section .bootstrap_stack, "aw", @nobits +stack_bottom: +.skip 16384 # 16 KiB +stack_top: + +# The linker script specifies _start as the entry point to the kernel and the +# bootloader will jump to this position once the kernel has been loaded. It +# doesn't make sense to return from this function as the bootloader is gone. +.section .text +.global _start +.global stack_top +.global stack_bottom +.type _start, @function +_start: + + + # To set up a stack, we simply set the esp register to point to the top of + # our stack (as it grows downwards). + movl $stack_top, %esp + + sub $8, %esp #to align on 16byte before calling C + push %ebx #pass address of the multiboot information data structure + push %eax #pass eax, so kernel can check for magic number + + call kernel_main + + # should never be reached + + cli + hlt +.Lhang: + jmp .Lhang + +# Set the size of the _start symbol to the current location '.' minus its start. +# This is useful when debugging or when you implement call tracing. +.size _start, . - _start |