add: higher half kernel, NOT WORKING
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41786ea523
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@ -1,29 +1,41 @@
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ENTRY(_start)
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ENTRY (_start)
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SECTIONS
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{
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. = 2M;
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. = 0x00100000;
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/* The kernel will live at 3GB + 1MB in the virtual address space, */
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/* which will be mapped to 1MB in the physical address space. */
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/* Note that we page-align the sections. */
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.text BLOCK(4K) : ALIGN(4K)
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_kernel_start = .;
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.multiboot.data : {
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*(.multiboot.data)
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}
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.multiboot.text : {
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*(.multiboot.text)
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}
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. += 0xC0000000;
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/* Add a symbol that indicates the start address of the kernel. */
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.text ALIGN (4K) : AT (ADDR (.text) - 0xC0000000)
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{
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*(.multiboot)
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*(.text)
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}
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.rodata BLOCK(4K) : ALIGN(4K)
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.rodata ALIGN (4K) : AT (ADDR (.rodata) - 0xC0000000)
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{
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*(.rodata)
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}
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.data BLOCK(4K) : ALIGN(4K)
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.data ALIGN (4K) : AT (ADDR (.data) - 0xC0000000)
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{
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*(.data)
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}
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.bss BLOCK(4K) : ALIGN(4K)
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.bss ALIGN (4K) : AT (ADDR (.bss) - 0xC0000000)
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{
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*(COMMON)
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*(.bss)
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end_kernel = .;
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*(.bootstrap_stack)
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}
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}
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/* Add a symbol that indicates the end address of the kernel. */
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_kernel_end = .;
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}
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@ -8,7 +8,7 @@
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#define VGA_WIDTH 80
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#define VGA_HEIGHT 25
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#define TERM_BUF ((uint16_t *)0xB8000)
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#define TERM_BUF ((uint16_t *)0xC03FF000)
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#define TERM_COUNT 10
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struct screen {
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190
src/boot.s
190
src/boot.s
@ -1,110 +1,120 @@
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/* Declare constants for the 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 constants for the 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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/*
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Declare a multiboot header that marks the program as a kernel. These are magic
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values that are documented in the multiboot standard. The bootloader will
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search for this signature in the first 8 KiB of the kernel file, aligned at a
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32-bit boundary. The signature is in its own section so the header can be
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forced to be within the first 8 KiB of the kernel file.
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*/
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.section .multiboot
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# Declare a multiboot header that marks the program as a kernel.
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.section .multiboot.data, "aw"
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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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/*
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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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# Allocate the initial stack.
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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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/*
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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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# Preallocate pages used for paging. Don't hard-code addresses and assume they
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# are available, as the bootloader might have loaded its multiboot structures or
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# modules there. This lets the bootloader know it must avoid the addresses.
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.section .bss, "aw", @nobits
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.align 4096
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boot_page_directory:
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.skip 4096
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boot_page_table1:
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.skip 4096
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# Further page tables may be required if the kernel grows beyond 3 MiB.
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# The kernel entry point.
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.section .multiboot.text, "a"
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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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# Physical address of boot_page_table1.
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# TODO: I recall seeing some assembly that used a macro to do the
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# conversions to and from physical. Maybe this should be done in this
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# code as well?
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movl $(boot_page_table1 - 0xC0000000), %edi
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# First address to map is address 0.
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# TODO: Start at the first kernel page instead. Alternatively map the first
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# 1 MiB as it can be generally useful, and there's no need to
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# specially map the VGA buffer.
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movl $0, %esi
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# Map 1023 pages. The 1024th will be the VGA text buffer.
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movl $1023, %ecx
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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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1:
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# Only map the kernel.
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cmpl $_kernel_start, %esi
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jl 2f
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cmpl $(_kernel_end - 0xC0000000), %esi
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jge 3f
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# Map physical address as "present, writable". Note that this maps
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# .text and .rodata as writable. Mind security and map them as non-writable.
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movl %esi, %edx
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orl $0x003, %edx
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movl %edx, (%edi)
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2:
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# Size of page is 4096 bytes.
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addl $4096, %esi
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# Size of entries in boot_page_table1 is 4 bytes.
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addl $4, %edi
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# Loop to the next entry if we haven't finished.
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loop 1b
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3:
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# Map VGA video memory to 0xC03FF000 as "present, writable".
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movl $(0x000B8000 | 0x003), boot_page_table1 - 0xC0000000 + 1023 * 4
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# The page table is used at both page directory entry 0 (virtually from 0x0
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# to 0x3FFFFF) (thus identity mapping the kernel) and page directory entry
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# 768 (virtually from 0xC0000000 to 0xC03FFFFF) (thus mapping it in the
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# higher half). The kernel is identity mapped because enabling paging does
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# not change the next instruction, which continues to be physical. The CPU
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# would instead page fault if there was no identity mapping.
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# Map the page table to both virtual addresses 0x00000000 and 0xC0000000.
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movl $(boot_page_table1 - 0xC0000000 + 0x003), boot_page_directory - 0xC0000000 + 0
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movl $(boot_page_table1 - 0xC0000000 + 0x003), boot_page_directory - 0xC0000000 + 768 * 4
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# Set cr3 to the address of the boot_page_directory.
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movl $(boot_page_directory - 0xC0000000), %ecx
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movl %ecx, %cr3
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# Enable paging and the write-protect bit.
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movl %cr0, %ecx
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orl $0x80010000, %ecx
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movl %ecx, %cr0
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# Jump to higher half with an absolute jump.
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lea 4f, %ecx
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jmp *%ecx
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.section .text
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4:
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# At this point, paging is fully set up and enabled.
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# Unmap the identity mapping as it is now unnecessary.
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movl $0, boot_page_directory + 0
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# Reload crc3 to force a TLB flush so the changes to take effect.
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movl %cr3, %ecx
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movl %ecx, %cr3
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# Set up the stack.
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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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/*
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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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# Enter the high-level kernel.
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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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# Infinite loop if the system has nothing more to do.
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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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jmp 1b
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