convert boot.s to c++
parent
0f5bbed9a9
commit
56f5c483ee
GPG Key ID:
1B74EE6C49C96795
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#include <array>
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#include <cstdint>
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extern void kernel_main();
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struct multiboot2
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{
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static constexpr std::uint32_t MAGIC = 0xE85250D6;
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static constexpr std::uint32_t FLAGS = 0;
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static constexpr std::uint32_t LENGTH = 16;
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static constexpr std::uint32_t CHECKSUM = -(MAGIC + FLAGS + LENGTH);
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alignas(8)
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std::uint32_t magic = MAGIC;
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std::uint32_t flags = FLAGS;
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std::uint32_t length = LENGTH;
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std::uint32_t checksum = CHECKSUM;
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} __attribute__((packed));
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struct multiboot2_tag
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{
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alignas(8)
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std::uint16_t id;
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std::uint16_t flags;
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std::uint32_t length;
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std::uint32_t data[];
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} __attribute__((packed));
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__attribute__((section(".multiboot2")))
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multiboot2 multibootHeader;
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__attribute__((section(".multiboot2")))
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multiboot2_tag multibootTagInfoRequest = {
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1, 0, sizeof(multiboot2_tag) + sizeof(std::uint32_t),
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{4}
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};
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__attribute__((section(".multiboot2")))
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multiboot2_tag multibootTagEnd = {
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0, 0, sizeof(multiboot2_tag), {}
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};
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alignas(16)
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std::array<std::uint8_t, 16384> stack;
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extern "C"
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__attribute__((naked))
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void _start()
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{
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asm volatile(R"(
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mov %%eax, multiboot_magic
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mov %%ebx, multiboot_ptr
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mov %0, %%esp
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)" :: "i" (stack.data() + stack.size()));
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extern std::uint32_t __init_array_start;
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extern std::uint32_t __init_array_end;
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auto it = &__init_array_start;
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while (it < &__init_array_end) {
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auto fn = reinterpret_cast<void (*)()>(*it);
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fn();
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++it;
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}
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kernel_main();
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asm volatile("cli");
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for (;;)
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asm volatile("hlt");
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}
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@ -1,127 +0,0 @@
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/* Declare constants for the multiboot header. */
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.set MAGIC, 0xE85250D6
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.set FLAGS, 0x0
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.set LENGTH, 16
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.set CHECKSUM, -(MAGIC + FLAGS + LENGTH)
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.section .multiboot2
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.align 8
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.int MAGIC
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.int FLAGS
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.int LENGTH
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.int CHECKSUM
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/* info request */
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.align 8
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.hword 1, 0
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.int 12
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.int 4
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/* end tag */
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.align 8
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.hword 0, 0
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.int 8
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/*
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The multiboot standard does not define the value of the stack pointer register
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(esp) and it is up to the kernel to provide a stack. This allocates room for a
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small stack by creating a symbol at the bottom of it, then allocating 16384
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bytes for it, and finally creating a symbol at the top. The stack grows
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downwards on x86. The stack is in its own section so it can be marked nobits,
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which means the kernel file is smaller because it does not contain an
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uninitialized stack. The stack on x86 must be 16-byte aligned according to the
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System V ABI standard and de-facto extensions. The compiler will assume the
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stack is properly aligned and failure to align the stack will result in
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undefined behavior.
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*/
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.section .bss
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.align 16
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stack_bottom:
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.skip 16384 # 16 KiB
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stack_top:
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/*
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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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*/
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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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/*
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The bootloader has loaded us into 32-bit protected mode on a x86
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machine. Interrupts are disabled. Paging is disabled. The processor
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state is as defined in the multiboot standard. The kernel has full
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control of the CPU. The kernel can only make use of hardware features
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and any code it provides as part of itself. There's no printf
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function, unless the kernel provides its own <stdio.h> header and a
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printf implementation. There are no security restrictions, no
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safeguards, no debugging mechanisms, only what the kernel provides
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itself. It has absolute and complete power over the
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machine.
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*/
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mov %eax, multiboot_magic
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mov %ebx, multiboot_ptr
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/*
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To set up a stack, we set the esp register to point to the top of the
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stack (as it grows downwards on x86 systems). This is necessarily done
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in assembly as languages such as C cannot function without a stack.
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*/
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mov $stack_top, %esp
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/*
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This is a good place to initialize crucial processor state before the
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high-level kernel is entered. It's best to minimize the early
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environment where crucial features are offline. Note that the
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processor is not fully initialized yet: Features such as floating
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point instructions and instruction set extensions are not initialized
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yet. The GDT should be loaded here. Paging should be enabled here.
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C++ features such as global constructors and exceptions will require
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runtime support to work as well.
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*/
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mov $__init_array_start, %eax
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.again:
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cmp $__init_array_end, %eax
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je .next
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push %eax
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call *(%eax)
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pop %eax
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add $0x4, %eax
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jmp .again
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.next:
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/*
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Enter the high-level kernel. The ABI requires the stack is 16-byte
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aligned at the time of the call instruction (which afterwards pushes
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the return pointer of size 4 bytes). The stack was originally 16-byte
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aligned above and we've pushed a multiple of 16 bytes to the
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stack since (pushed 0 bytes so far), so the alignment has thus been
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preserved and the call is well defined.
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*/
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call kernel_main
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/*
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If the system has nothing more to do, put the computer into an
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infinite loop. To do that:
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1) Disable interrupts with cli (clear interrupt enable in eflags).
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They are already disabled by the bootloader, so this is not needed.
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Mind that you might later enable interrupts and return from
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kernel_main (which is sort of nonsensical to do).
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2) Wait for the next interrupt to arrive with hlt (halt instruction).
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Since they are disabled, this will lock up the computer.
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3) Jump to the hlt instruction if it ever wakes up due to a
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non-maskable interrupt occurring or due to system management mode.
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*/
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cli
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1: hlt
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jmp 1b
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/*
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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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*/
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.size _start, . - _start
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