Initial commit

boot/boot.S

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+/* Declare constants for the multiboot header. */
+.set ALIGN,    1<<0             /* align loaded modules on page boundaries */
+.set MEMINFO,  1<<1             /* provide memory map */
+.set FLAGS,    ALIGN | MEMINFO  /* this is the Multiboot 'flag' field */
+.set MAGIC,    0x1BADB002       /* 'magic number' lets bootloader find the header */
+.set CHECKSUM, -(MAGIC + FLAGS) /* checksum of above, to prove we are multiboot */
+
+/*
+Declare a multiboot header that marks the program as a kernel. These are magic
+values that are documented in the multiboot standard. The bootloader will
+search for this signature in the first 8 KiB of the kernel file, aligned at a
+32-bit boundary. The signature is in its own section so the header can be
+forced to be within the first 8 KiB of the kernel file.
+*/
+.section .multiboot
+.align 4
+.long MAGIC
+.long FLAGS
+.long CHECKSUM
+
+#.include "gdt.S"
+.section .data
+gdt:
+.quad 0x0000000000000000
+.quad 0x00CF9A000000FFFF
+.quad 0x00CF92000000FFFF
+.quad 0x00CFFA000000FFFF
+.quad 0x00CFF2000000FFFF
+
+gdtr:
+    .word 40
+    .long gdt # For base storage
+/*
+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 <stdio.h> 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.
+	*/
+
+	/*
+	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
+
+	// store multiboot on stack
+	pushl %ebx
+
+	/*
+	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.
+	*/
+	cli
+    lgdt (gdtr)
+    ljmp $0x8, $continue_boot
+
+continue_boot:
+    sti
+	/*
+	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.
+	*/
+	// top of stack is multiboot object
+	call kmain
+
+	/*
+	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

boot/gdt.S

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+.section .data
+gdt_start:  # don't remove the labels, they're needed to compute sizes and jumps
+    # the GDT starts with a null 8-byte
+    .long 0x0 # 4 byte
+    .long 0x0 # 4 byte
+
+# GDT for code segment. base = 0x00000000, length = 0xfffff
+# for flags, refer to os-dev.pdf document, page 36
+gdt_code: 
+    .word 0xffff # segment length, bits 0-15
+    .word 0x0    # segment base, bits 0-15
+    .byte 0x0    # segment base, bits 16-23
+    .byte 0b10011010 # flags (8 bits)
+    .byte 0b11001111 # flags (4 bits) + segment length, bits 16-19
+    .byte 0x0    # segment base, bits 24-31
+
+# GDT for data segment. base and length identical to code segment
+# some flags changed, again, refer to os-dev.pdf
+gdt_data: 
+    .word 0xffff
+    .word 0x0
+    .byte 0x0
+    .byte 0b10010010
+    .byte 0b11001111
+    .byte 0x0
+
+gdt_end: 
+
+# GDT descriptor
+gdt_descriptor:
+.long gdt_end-gdt_start-1
+.long gdt_start
+//MISMATCH: "    dw gdt_end - gdt_start - 1 "
+//MISMATCH: "    dd gdt_start "
+
+# define some constants for later use
+code_seg: .long gdt_code - gdt_start
+data_seg: .long gdt_data - gdt_start
+