60 lines
2.5 KiB
ArmAsm
60 lines
2.5 KiB
ArmAsm
# Declare some constants for the multiboot header.
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.set ALIGN, 1<<0 # Align loaded modules on page boundaries
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.set MEMINFO, 1<<1 # Provide memory map
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.set FLAGS, ALIGN | MEMINFO # This is the multiboot 'flag' field
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.set MAGIC, 0x1BADB002 # Magic number lets bootloader find the header
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.set CHECKSUM, -(MAGIC + FLAGS) # Checksum of above, to prove we are multiboot
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# Declare a header as in the Multiboot Standard. We put this into a special
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# section so we can force the header to be in the start of the final program.
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# You don't need to understand all these details as it is just magic values that
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# is documented in the multiboot standard. The bootloader will search for this
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# magic sequence and recognize us as a multiboot kernel.
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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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# Currently the stack pointer register (esp) points at anything and using it may
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# cause massive harm. Instead, we'll provide our own stack. We will allocate
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# room for a small temporary stack by creating a symbol at the bottom of it,
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# then allocating 16384 bytes for it, and finally creating a symbol at the top.
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.section .bootstrap_stack, "aw", @nobits
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stack_buttom:
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.skip 16384 # 16 KiB
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stack_top:
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# The linker script specifies _start as the entry point to the kernel and the
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# bootloader will jump to this position once the kernel has been loaded. It
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# doesn't make sense to return from this function as the bootloader is gone.
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.section .text
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.global _start
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.type _start, @function
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_start:
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# Set up some space for a call stack. The stack grows downwards, so esp gets set to the top of the stack.
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movl $stack_top, %esp
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# Very early initialization done here.
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call kearly
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# Global initialization done here.
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call _init
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# Here we go...
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call kmain
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# In case the function returns, we'll want to put the computer into an
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# infinite loop. To do that, we use the clear interrupt ('cli') instruction
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# to disable interrupts, the halt instruction ('hlt') to stop the CPU until
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# the next interrupt arrives, and jumping to the halt instruction if it ever
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# continues execution, just to be safe. We will create a local label rather
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# than real symbol and jump to there endlessly.
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cli
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.Lhang:
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hlt
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jmp .Lhang
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# Set the size of the _start symbol to the current location '.' minus its start.
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# This is useful when debugging or when you implement call tracing.
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.size _start, . - _start
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