From 56f5c483ee516245867a312c7e1520c3f4df16c7 Mon Sep 17 00:00:00 2001 From: Clyne Sullivan Date: Fri, 27 Sep 2024 06:45:13 -0400 Subject: [PATCH] convert boot.s to c++ --- Makefile | 30 +++++-------- boot.cpp | 72 ++++++++++++++++++++++++++++++ boot.s | 127 ----------------------------------------------------- kernel.cpp | 1 - 4 files changed, 84 insertions(+), 146 deletions(-) create mode 100644 boot.cpp delete mode 100644 boot.s diff --git a/Makefile b/Makefile index 17adbac..652f617 100644 --- a/Makefile +++ b/Makefile @@ -1,20 +1,18 @@ -ASFLAGS := --32 CXXFLAGS := -m32 -ggdb -g3 -O0 -fno-pic -ffreestanding -fno-rtti -fno-exceptions -std=c++23 LDFLAGS := -m32 -static -T link.ld -ffreestanding -nostdlib -ASFILES := boot.s -CXXFILES := gdt.cpp \ - idt.cpp \ - memory.cpp \ - multiboot.cpp \ - pic.cpp \ - pit.cpp \ - tasking.cpp \ - vgaterminal.cpp \ - kernel.cpp +CXXFILES := boot.cpp \ + gdt.cpp \ + idt.cpp \ + memory.cpp \ + multiboot.cpp \ + pic.cpp \ + pit.cpp \ + tasking.cpp \ + vgaterminal.cpp \ + kernel.cpp -OBJS := $(subst .s,.o,$(ASFILES)) \ - $(subst .cpp,.o,$(CXXFILES)) +OBJS := $(subst .cpp,.o,$(CXXFILES)) all: myos.iso @@ -27,10 +25,6 @@ myos.bin: $(OBJS) link.ld @echo " LD " $@ @g++ $(LDFLAGS) -o $@ $(OBJS) -%.o: %.s - @echo " AS " $< - @as $(ASFLAGS) -c $< -o $@ - %.o: %.cpp @echo " CXX " $< @g++ $(CXXFLAGS) -c $< -o $@ @@ -41,5 +35,5 @@ clean: run: myos.iso @echo " QEMU" - @qemu-system-i386 -cdrom $< -monitor stdio -no-reboot -s -S #-d int + @qemu-system-i386 -cdrom $< -monitor stdio -no-reboot #-s -S #-d int diff --git a/boot.cpp b/boot.cpp new file mode 100644 index 0000000..0b003fd --- /dev/null +++ b/boot.cpp @@ -0,0 +1,72 @@ +#include +#include + +extern void kernel_main(); + +struct multiboot2 +{ + static constexpr std::uint32_t MAGIC = 0xE85250D6; + static constexpr std::uint32_t FLAGS = 0; + static constexpr std::uint32_t LENGTH = 16; + static constexpr std::uint32_t CHECKSUM = -(MAGIC + FLAGS + LENGTH); + + alignas(8) + std::uint32_t magic = MAGIC; + std::uint32_t flags = FLAGS; + std::uint32_t length = LENGTH; + std::uint32_t checksum = CHECKSUM; +} __attribute__((packed)); + +struct multiboot2_tag +{ + alignas(8) + std::uint16_t id; + std::uint16_t flags; + std::uint32_t length; + std::uint32_t data[]; +} __attribute__((packed)); + +__attribute__((section(".multiboot2"))) +multiboot2 multibootHeader; + +__attribute__((section(".multiboot2"))) +multiboot2_tag multibootTagInfoRequest = { + 1, 0, sizeof(multiboot2_tag) + sizeof(std::uint32_t), + {4} +}; + +__attribute__((section(".multiboot2"))) +multiboot2_tag multibootTagEnd = { + 0, 0, sizeof(multiboot2_tag), {} +}; + +alignas(16) +std::array stack; + +extern "C" +__attribute__((naked)) +void _start() +{ + asm volatile(R"( + mov %%eax, multiboot_magic + mov %%ebx, multiboot_ptr + mov %0, %%esp + )" :: "i" (stack.data() + stack.size())); + + extern std::uint32_t __init_array_start; + extern std::uint32_t __init_array_end; + + auto it = &__init_array_start; + while (it < &__init_array_end) { + auto fn = reinterpret_cast(*it); + fn(); + ++it; + } + + kernel_main(); + + asm volatile("cli"); + for (;;) + asm volatile("hlt"); +} + diff --git a/boot.s b/boot.s deleted file mode 100644 index 7908d3e..0000000 --- a/boot.s +++ /dev/null @@ -1,127 +0,0 @@ -/* Declare constants for the multiboot header. */ -.set MAGIC, 0xE85250D6 -.set FLAGS, 0x0 -.set LENGTH, 16 -.set CHECKSUM, -(MAGIC + FLAGS + LENGTH) - -.section .multiboot2 -.align 8 -.int MAGIC -.int FLAGS -.int LENGTH -.int CHECKSUM - -/* info request */ -.align 8 -.hword 1, 0 -.int 12 -.int 4 - -/* end tag */ -.align 8 -.hword 0, 0 -.int 8 - -/* -The multiboot standard does not define the value of the stack pointer register -(esp) and it is up to the kernel to provide a stack. This allocates room for a -small stack by creating a symbol at the bottom of it, then allocating 16384 -bytes for it, and finally creating a symbol at the top. The stack grows -downwards on x86. The stack is in its own section so it can be marked nobits, -which means the kernel file is smaller because it does not contain an -uninitialized stack. The stack on x86 must be 16-byte aligned according to the -System V ABI standard and de-facto extensions. The compiler will assume the -stack is properly aligned and failure to align the stack will result in -undefined behavior. -*/ -.section .bss -.align 16 -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 -.type _start, @function -_start: - /* - The bootloader has loaded us into 32-bit protected mode on a x86 - machine. Interrupts are disabled. Paging is disabled. The processor - 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 - and any code it provides as part of itself. There's no printf - function, unless the kernel provides its own header and a - printf implementation. There are no security restrictions, no - safeguards, no debugging mechanisms, only what the kernel provides - itself. It has absolute and complete power over the - machine. - */ - mov %eax, multiboot_magic - mov %ebx, multiboot_ptr - - /* - 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 - in assembly as languages such as C cannot function without a stack. - */ - mov $stack_top, %esp - - /* - This is a good place to initialize crucial processor state before the - high-level kernel is entered. It's best to minimize the early - environment where crucial features are offline. Note that the - processor is not fully initialized yet: Features such as floating - point instructions and instruction set extensions are not initialized - yet. The GDT should be loaded here. Paging should be enabled here. - C++ features such as global constructors and exceptions will require - runtime support to work as well. - */ - mov $__init_array_start, %eax -.again: - cmp $__init_array_end, %eax - je .next - push %eax - call *(%eax) - pop %eax - add $0x4, %eax - jmp .again - -.next: - - /* - Enter the high-level kernel. The ABI requires the stack is 16-byte - aligned at the time of the call instruction (which afterwards pushes - 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 - stack since (pushed 0 bytes so far), so the alignment has thus been - preserved and the call is well defined. - */ - call kernel_main - - /* - If the system has nothing more to do, put the computer into an - infinite loop. To do that: - 1) Disable interrupts with cli (clear interrupt enable in eflags). - They are already disabled by the bootloader, so this is not needed. - Mind that you might later enable interrupts and return from - kernel_main (which is sort of nonsensical to do). - 2) Wait for the next interrupt to arrive with hlt (halt instruction). - Since they are disabled, this will lock up the computer. - 3) Jump to the hlt instruction if it ever wakes up due to a - non-maskable interrupt occurring or due to system management mode. - */ - cli -1: hlt - jmp 1b - -/* -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 - diff --git a/kernel.cpp b/kernel.cpp index 41c3a61..dab93c6 100644 --- a/kernel.cpp +++ b/kernel.cpp @@ -10,7 +10,6 @@ static VGATerminal vga; TextOutput& term = vga; -extern "C" void kernel_main(void) { term.write("Clyne's kernel, v2024\n\n");