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@ -5,14 +5,13 @@
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#include <arch/multiboot.h>
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/* see arch/x86/config/kernel.ld */
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.extern _kernel_end
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.extern _image_end
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.extern _stack_end
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.extern _image_start_phys
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.extern _kernel_end_phys
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.extern _image_end_phys
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.extern _boot
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.section .multiboot
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.section .multiboot.data, "aw"
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.global mb2_load_start
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mb2_load_start:
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.align MB2_HEADER_ALIGN
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@ -37,11 +36,11 @@ address_tag_start: /* struct mb2_header_tag_address */
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/* header_addr */
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.long header_start
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/* load_addr */
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.long mb2_load_start
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.long _image_start_phys
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/* load_end_addr */
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.long _kernel_end
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.long _kernel_end_phys
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/* bss_end_addr */
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.long _image_end
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.long _image_end_phys
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address_tag_end:
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.align MB2_TAG_ALIGN
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@ -85,11 +84,145 @@ end_tag_start: /* struct mb2_header_tag */
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end_tag_end:
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header_end:
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.size header_start, . - header_start
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.text
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/*
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* Actual boot sequence comes here.
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*
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* This is based on the example code from the OSDev.org wiki:
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* <https://wiki.osdev.org/Higher_Half_x86_Bare_Bones>
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*
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* We basically just turn on paging and map the lower 4 MB (including the VGA
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* console framebuffer) to 0xc0000000 where the kernel image itself is linked
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* to, and then jump to that region and set up the stack. Form there, _boot()
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* takes care of everything.
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*
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* We need to pay special attention to not touch eax and ebx during the entire
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* routine, because those store the multiboot2 magic number and tag table
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* pointer respectively which need to be passed on to _boot(). We can't push
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* them on the stack, because the stack doesn't exist yet.
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*/
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.extern _boot /* main boot routine -- see ./boot.c */
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.extern pd0 /* initial page directory -- see ../mm/page.c */
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.extern pt0 /* first page table -- see ../mm/page.c */
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.section .multiboot.text, "a"
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#define PAGE_SIZE 4096
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/*
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* referencing symbols from C requires subtracting the relocation offset
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* first because the C code is linked against virtual address space
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*/
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#define phys_addr(c_symbol) (c_symbol - CFG_KERNEL_RELOCATE)
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asmfn_begin(_start)
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mov $_stack_end, %esp
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/*
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* 1023 of the 1024 pages in the page table are mapped to the low memory
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* starting at 1 MiB, the address where the kernel image is loaded
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* ($_image_start_phys). We currently assume the kernel is < 4 MiB
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* and therefore can be mapped within a single page table.
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* As the kernel gets more and more bloated, this might not be the case
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* in the future anymore, so we should ideally add support for multiple
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* page tables soon.
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*/
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mov $phys_addr(pt0), %edi
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xor %esi, %esi
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1: cmp $_image_start_phys, %esi
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jl 2f /* skip the pages that are below the kernel image */
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/* TODO: grub stores the multiboot tags right after the kernel image,
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so we might need to map more than just what we do here */
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cmp $_image_end_phys, %esi
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jge 3f /* exit the loop when we have mapped the entire kernel image */
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mov %esi, %edx
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or $0x003, %edx /* set present and rw flags, see below */
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mov %edx, (%edi)
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2: add $PAGE_SIZE, %esi /* advance to next physical page address */
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add $4, %edi /* advance to next pointer in the page table */
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loop 1b
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/*
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* Conveniently, the VGA character framebuffer fits exactly into one
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* page. The physical address range
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* 0x000b8000 - 0x000b8fff
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* gets mapped to the virtual address range
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* 0xc03ff000 - 0xc03fffff
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* which is the last page of our 4 MiB page table (index 1023).
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* We also set the Present and RW bits by OR'ing with 0x003.
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*/
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3: movl $(0x000b8000 | 0x003), phys_addr(pt0) + 1023 * 4
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/*
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* We are mapping the lowest 4 MiB of physical memory both to itself and
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* to the relocated region. Thus, the physical address range:
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* 0x00000000 - 0x00400000
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* becomes available at two virtual address ranges:
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* 0x00000000 - 0x00400000 (identity mapping)
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* 0xc0000000 - 0xc0400000 (relocated mapping)
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*
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* The identity mapping is necessary because when we turn on paging in
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* the next lines, the program counter still refers to physical memory
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* and would thus immediately cause a page fault.
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*
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* The address of pt0 is OR'd with 0x003 (we actually have to add here
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* because of dumb compiler stuff but that doesn't matter because it's
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* the same result) to set the Present and RW flags in the page table
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* entry (the lower 12 bits in page table addresses are always zero
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* because they are page aligned so they are used as flags for the MMU,
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* see ../include/arch/page.h).
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*
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* The offset added to pd0 is the page number multiplied with the size
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* of a single entry (a pointer size):
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* (0x00000000 / PAGE_SIZE) / 1024 entries in a page table = 0
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* (0xc0000000 / PAGE_SIZE) / 1024 entries in a page table = 786
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*/
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movl $(phys_addr(pt0) + 0x003), phys_addr(pd0) + 0 * 4 /* 0x00000000 */
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movl $(phys_addr(pt0) + 0x003), phys_addr(pd0) + 768 * 4 /* 0xc0000000 */
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/* put the (physical) address of pd0 into cr3 so it will be used */
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mov $phys_addr(pd0), %ecx
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mov %ecx, %cr3
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/* set the paging and write-protect bit in cr0 */
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mov %cr0, %ecx
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or $0x80010000, %ecx
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mov %ecx, %cr0
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/*
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* Alright, we are on virtual addresses!
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* Now, we are going to do an absolute jump to the mapped kernel code
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* somewhere at 0xc01*****.
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*/
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lea 4f, %ecx
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jmp *%ecx
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/*
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* this is just because you can't span functions across multiple sections,
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* the actual code flow makes a jump from low (.multiboot.text section)
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* to high memory (.text section).
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*/
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asmfn_end(_start)
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.text
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/*
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* Now that we've completely transitioned to high memory, we can remove
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* the identity mapping because we don't need it anymore.
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*/
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4: movl $0, pd0 + 0 * 4
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/* bonk the TLB by reloading cr3 to apply the updated page table */
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mov %cr3, %ecx
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mov %ecx, %cr3
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/* set up the initial stack frame */
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mov $stack_top, %ebp
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mov %ebp, %esp
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/* reset EFLAGS */
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pushl $0
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@ -104,10 +237,15 @@ asmfn_begin(_start)
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call _boot
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/* this should never(TM) be reached */
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halt_loop:
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hlt
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jmp halt_loop
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asmfn_end(_start)
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cli
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5: hlt
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jmp 5b
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/* TODO: There are probably nicer ways of reserving stack memory. */
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.section .bootstrap_stack, "aw", @nobits
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stack_bottom:
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.skip 16384 /* 16 KiB for the stack should be plenty for now */
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stack_top:
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/*
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* This file is part of GayBSD.
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