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Josh Holtrop 2020-10-19 22:12:34 -04:00
parent 8ae84716d6
commit 05de14800b
1 changed files with 40 additions and 40 deletions
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src/boot.S
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@ -25,54 +25,54 @@ doesn't make sense to return from this function as the bootloader is gone.
.global _start .global _start
.type _start, @function .type _start, @function
_start: _start:
/* /*
The bootloader has loaded us into 32-bit protected mode on a x86 The bootloader has loaded us into 32-bit protected mode on a x86
machine. Interrupts are disabled. Paging is disabled. The processor machine. Interrupts are disabled. Paging is disabled. The processor
state is as defined in the multiboot standard. The kernel has full state is as defined in the multiboot standard. The kernel has full
control of the CPU. The kernel can only make use of hardware features control of the CPU. The kernel can only make use of hardware features
and any code it provides as part of itself. There's no printf and any code it provides as part of itself. There's no printf
function, unless the kernel provides its own <stdio.h> header and a function, unless the kernel provides its own <stdio.h> header and a
printf implementation. There are no security restrictions, no printf implementation. There are no security restrictions, no
safeguards, no debugging mechanisms, only what the kernel provides safeguards, no debugging mechanisms, only what the kernel provides
itself. It has absolute and complete power over the itself. It has absolute and complete power over the
machine. machine.
*/ */
/* /*
To set up a stack, we set the esp register to point to the top of the To set up a stack, we set the esp register to point to the top of the
stack (as it grows downwards on x86 systems). This is necessarily done stack (as it grows downwards on x86 systems). This is necessarily done
in assembly as languages such as C cannot function without a stack. in assembly as languages such as C cannot function without a stack.
*/ */
mov $stack_top, %esp mov $stack_top, %esp
/* /*
This is a good place to initialize crucial processor state before the This is a good place to initialize crucial processor state before the
high-level kernel is entered. It's best to minimize the early high-level kernel is entered. It's best to minimize the early
environment where crucial features are offline. Note that the environment where crucial features are offline. Note that the
processor is not fully initialized yet: Features such as floating processor is not fully initialized yet: Features such as floating
point instructions and instruction set extensions are not initialized point instructions and instruction set extensions are not initialized
yet. The GDT should be loaded here. Paging should be enabled here. yet. The GDT should be loaded here. Paging should be enabled here.
C++ features such as global constructors and exceptions will require C++ features such as global constructors and exceptions will require
runtime support to work as well. runtime support to work as well.
*/ */
/* /*
Enter the high-level kernel. The ABI requires the stack is 16-byte Enter the high-level kernel. The ABI requires the stack is 16-byte
aligned at the time of the call instruction (which afterwards pushes aligned at the time of the call instruction (which afterwards pushes
the return pointer of size 4 bytes). The stack was originally 16-byte the return pointer of size 4 bytes). The stack was originally 16-byte
aligned above and we've pushed a multiple of 16 bytes to the aligned above and we've pushed a multiple of 16 bytes to the
stack since (pushed 0 bytes so far), so the alignment has thus been stack since (pushed 0 bytes so far), so the alignment has thus been
preserved and the call is well defined. preserved and the call is well defined.
*/ */
push $0 push $0
push $0 push $0
push $0 push $0
push %ebx push %ebx
call hos_main call hos_main
cli cli
1: hlt 1: hlt
jmp 1b jmp 1b
/* /*
Set the size of the _start symbol to the current location '.' minus its start. Set the size of the _start symbol to the current location '.' minus its start.