78 lines
3.8 KiB
ArmAsm
78 lines
3.8 KiB
ArmAsm
# Declare constants used for creating a 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_bottom:
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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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# Welcome to kernel mode! We now have sufficient code for the bootloader to
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# load and run our operating system. It doesn't do anything interesting yet.
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# Perhaps we would like to call printf("Hello, World\n"). You should now
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# realize one of the profound truths about kernel mode: There is nothing
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# there unless you provide it yourself. There is no printf function. There
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# is no <stdio.h> header. If you want a function, you will have to code it
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# yourself. And that is one of the best things about kernel development:
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# you get to make the entire system yourself. You have absolute and complete
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# power over the machine, there are no security restrictions, no safe
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# guards, no debugging mechanisms, there is nothing but what you build.
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# By now, you are perhaps tired of assembly language. You realize some
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# things simply cannot be done in C, such as making the multiboot header in
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# the right section and setting up the stack. However, you would like to
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# write the operating system in a higher level language, such as C or C++.
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# To that end, the next task is preparing the processor for execution of
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# such code. C doesn't expect much at this point and we only need to set up
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# a stack. Note that the processor is not fully initialized yet and stuff
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# such as floating point instructions are not available yet.
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# To set up a stack, we simply set the esp register to point to the top of
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# our stack (as it grows downwards).
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movl $stack_top, %esp
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# We are now ready to actually execute C code. We cannot embed that in an
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# assembly file, so we'll create a kernel.c file in a moment. In that file,
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# we'll create a C entry point called kernel_main and call it here.
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call kernel_main
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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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hlt
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.Lhang:
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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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