139 lines
2.6 KiB
ArmAsm
139 lines
2.6 KiB
ArmAsm
[bits 16]
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main:
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jmp far 0x000:0x7c05
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main2:
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mov ax, 0x0000
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mov ds, ax
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mov ss, ax
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mov ax, 0x7b00
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mov sp, ax
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mov ax, 0x0500
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mov es, ax
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clc
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int 0x12
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mov [lower_mem], ax
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; memory scan
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mov di, 0x0
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call do_e820
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jc hang
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cli
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; a20
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in al, 0x92
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or al, 2
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out 0x92, al
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; basic flat GDT
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xor eax, eax
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mov ax, ds
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shl eax, 4
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add eax, gdt_base
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mov [gdtr+2], eax
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mov eax, gdt_end
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sub eax, gdt_base
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mov [gdtr], ax
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lgdt [gdtr]
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; protected mode enable flag
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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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; set 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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; jump to protected mode entry
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extern kmain
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jmp far 0x08:(kmain)
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hang:
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jmp hang
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do_e820:
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xor ebx, ebx ; ebx must be 0 to start
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xor bp, bp ; keep an entry count in bp
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mov edx, 0x0534D4150 ; Place "SMAP" into edx
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mov eax, 0xe820
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mov [es:di + 20], dword 1 ; force a valid ACPI 3.X entry
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mov ecx, 24 ; ask for 24 bytes
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int 0x15
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jc short .failed ; carry set on first call means "unsupported function"
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mov edx, 0x0534D4150 ; Some BIOSes apparently trash this register?
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cmp eax, edx ; on success, eax must have been reset to "SMAP"
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jne short .failed
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test ebx, ebx ; ebx = 0 implies list is only 1 entry long (worthless)
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je short .failed
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jmp short .jmpin
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.e820lp:
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mov eax, 0xe820 ; eax, ecx get trashed on every int 0x15 call
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mov [es:di + 20], dword 1 ; force a valid ACPI 3.X entry
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mov ecx, 24 ; ask for 24 bytes again
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int 0x15
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jc short .e820f ; carry set means "end of list already reached"
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mov edx, 0x0534D4150 ; repair potentially trashed register
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.jmpin:
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jcxz .skipent ; skip any 0 length entries
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cmp cl, 20 ; got a 24 byte ACPI 3.X response?
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jbe short .notext
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test byte [es:di + 20], 1 ; if so: is the "ignore this data" bit clear?
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je short .skipent
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.notext:
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mov ecx, [es:di + 8] ; get lower uint32_t of memory region length
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or ecx, [es:di + 12] ; "or" it with upper uint32_t to test for zero
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jz .skipent ; if length uint64_t is 0, skip entry
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inc bp ; got a good entry: ++count, move to next storage spot
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add di, 24
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.skipent:
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test ebx, ebx ; if ebx resets to 0, list is complete
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jne short .e820lp
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.e820f:
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mov [mmap_ent], bp ; store the entry count
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clc ; there is "jc" on end of list to this point, so the carry must be cleared
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ret
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.failed:
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stc ; "function unsupported" error exit
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ret
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align 8
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; GDT pointer
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gdtr
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dw 0
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dd 0
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; GDT (null, code, data)
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gdt_base
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; null
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dq 0
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; code
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dw 0xFFFF
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dw 0
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db 0
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db 0x9a
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db 0xcf
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db 0
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; data
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dw 0xffff
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dw 0
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db 0
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db 0x92
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db 0xcf
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db 0
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gdt_end
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; memory map entry count
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global mmap_ent
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mmap_ent db 0, 0
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global lower_mem
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lower_mem db 0, 0
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