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https://github.com/gbowne1/ClassicOS.git
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4fa82854dd
- bl stage 1: move gdt+pm setup to stage 2 (avoids mode switching) - bl stage 2: at entry query e820 table from bios, then setup gdt+pm - kernel/memmap: pad map entry to 24 bytes, as exported by bl - kernel/memmap: map copy (gbowne1) - Makefile: facilitate placing the memory map at a known location - Update bl docs
96 lines
2.0 KiB
NASM
96 lines
2.0 KiB
NASM
global _start
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extern load_kernel
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%define e820_magic 0x534d4150 ; "SMAP"
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%define e820_entry_size 24
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%define e820_max_entries 128
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; ----------------------------------------------------------------
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; Real mode
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; ----------------------------------------------------------------
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[BITS 16]
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_start:
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call read_e820
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call setup_gdt
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call switch_to_pm
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read_e820:
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xor ebx, ebx
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mov es, bx
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mov di, MEMMAP_BASE+4 ; ES=0 DI=MEMMAP_BASE+4
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xor bp, bp ; Keeping count in bp
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.e820_loop:
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mov eax, 0xe820
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mov ecx, e820_entry_size
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mov edx, e820_magic
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int 0x15
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jc .done ; Error?
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cmp eax, e820_magic ; Verify "SMAP"
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jne .done
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test ecx, ecx ; Skip 0-sized entries
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jz .skip
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add di, e820_entry_size ; Advance write addr
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inc bp ; Increment count
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cmp bp, e820_max_entries ; Stop if we're at capacity
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jae .done
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.skip:
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test ebx, ebx
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jne .e820_loop
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.done:
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mov [MEMMAP_BASE], bp ; Store count
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ret
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setup_gdt:
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lgdt [gdt_descriptor]
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ret
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switch_to_pm:
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cli
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mov eax, cr0
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or eax, 1
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mov cr0, eax
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jmp 0x08:pm_entry
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e820_count:
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dw 0
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gdt_start:
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dq 0x0000000000000000 ; Null descriptor
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dq 0x00CF9A000000FFFF ; 32-bit code segment (selector 0x08)
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dq 0x00CF92000000FFFF ; 32-bit data segment (selector 0x10)
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dq 0x00009A000000FFFF ; 16-bit code segment for real mode (selector 0x18)
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gdt_descriptor:
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dw gdt_end - gdt_start - 1
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dd gdt_start
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gdt_end:
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; ----------------------------------------------------------------
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; Protected mode
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; ----------------------------------------------------------------
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[BITS 32]
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pm_entry:
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; Set up segments
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; Data segments
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mov ax, 0x10
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mov ds, ax
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mov es, ax
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mov fs, ax
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mov gs, ax
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mov ss, ax
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; Code segment
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mov ax, 0x08
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mov cs, ax
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; Stack
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mov esp, 0x90000
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call load_kernel
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jmp eax
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