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65db388f00
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0d16e3e35a
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@ -1,48 +1,95 @@
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; A20 Wraparound check as gotten from osdev.org
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check_a20_wraparound:
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pushf
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push ds
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push es
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push di
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push si
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.prolog:
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push dword esi
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sub esp, 64
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mov esi, esp
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xor ax, ax ; ax = 0
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mov es, ax
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; EAX is used for the return value,
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; so it doesn't have to be preserved.
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mov [esi + (64 - 8)], ebx
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mov [esi + (64 - 12)], edi
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not ax ; ax = 0xFFFF
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mov ds, ax
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.test_a20:
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; Save the values which are going to be overwritten
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mov edi, 0x0800
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mov eax, [edi]
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mov edi, 0x00100800
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mov ebx, [edi]
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mov di, 0x0500
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mov si, 0x0510
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; Write to two locations which, if A20 wraps around,
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; will both be written to by the second statement.
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; Write 0 to 0x08000 to avoid the unlikely case that
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; those bytes are there coincidentally and A20 is on.
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mov edi, 0x0800
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mov [edi], dword 0x00000000
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mov edi, 0x00100800
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mov [edi], dword 0xffaa00dd
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mov al, byte [es:di]
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push ax
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; Check the lower address (which should NOT contain)
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; this value for the value. If it does contain it, the
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; memory space wraps around to 0 every 2^19 bytes.
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cmp [0x0800], dword 0xffaa00dd
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je .return_a20_not_enabled ; If the value at 0x0800
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; was written to instead
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; (or at the same time) of
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; writing to 0x100800,
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; the A20 wasn't enabled.
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; Reset the values at the addresses to which the test
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; values were written znd return that A20 is enabled.
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mov edi, 0x0800
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mov [edi], eax
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mov edi, 0x00100800
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mov [edi], ebx
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mov al, byte [ds:si]
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push ax
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.return_a20_enabled:
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mov byte [es:di], 0x00
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mov byte [ds:si], 0xFF
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mov eax, 1
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cmp byte [es:di], 0xFF
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pop ax
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mov byte [ds:si], al
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pop ax
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mov byte [es:di], al
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mov ax, 0
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je .exit
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mov ax, 1
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.exit:
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pop si
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pop di
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pop es
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pop ds
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popf
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; Restore the one used register which
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; doesn't contain the return value.
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mov ebx, [esi + (64 - 8)]
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add esp, 64
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pop dword esi
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ret
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.return_a20_not_enabled:
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; It's only necessary to write to one of the addresses
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; as both of them are equal either way.
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mov edi, 0x0800
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mov [edi], eax
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mov eax, 0
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; Restore the one used register which
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; doesn't contain the return value.
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mov ebx, [esi + (64 - 12)]
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mov edi, [esi + (64 - 8)]
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add esp, 64
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pop dword esi
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ret
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enable_a20_through_keyboard_controller:
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.prolog:
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push dword esi
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sub esp, 64
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mov esi, esp
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mov [esi + (64 - 8)], ebx
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mov [esi + (64 - 12)], ecx
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mov [esi + (64 - 16)], edx
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mov [esi + (64 - 20)], edi
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.epilog:
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mov ebx, [esi + (64 - 8)]
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mov ecx, [esi + (64 - 12)]
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mov edx, [esi + (64 - 16)]
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mov edi, [esi + (64 - 20)]
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add esp, 64
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pop dword esi
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ret
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@ -2,8 +2,6 @@
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GDT_MEMORY_AREA equ 0x7e00
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GDT_ENTRY_COUNT equ 3
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; Arguments (note: all arguments are 4-byte padded):
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; [Furthest from EBP]
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;
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@ -111,6 +109,114 @@ make_regular_data_segment_in_gdt:
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; Arguments (note: all arguments are 4-byte padded):
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; [Furthest from EBP]
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;
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; 4. uint32 privilege_level
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; - Only 0 to 3; 0 being kernel and 3 being userland.
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; Everything above 3 is an error.
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;
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; 3 uint32 num_pages
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; - Only 20 bits are usable. Everything above will
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; be cut off to be written to the segment descriptor.
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;
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; 2. ptr32 base_address
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;
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; 1. uint32 segment_index
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; Note:
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; - Zero is NOT usable and will throw an error.
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; the highest theoretical value for this is 8191,
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; but generally, one should stick to GDT_ENTRY_COUNT.
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;
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; [Nearest to EBP]
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; COMMENTED OUT DUE TO BEING BUGGY!
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; make_stack_segment_in_gdt:
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; .prolog:
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; push esi
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; add esp, 64
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; mov esi, esp
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; mov [esi + (64 - 4)], eax
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; mov [esi + (64 - 8)], ebx
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; mov [esi + (64 - 12)], ecx
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; mov [esi + (64 - 16)], edx
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; mov [esi + (64 - 20)], edi
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; .get_arguments:
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; mov eax, [ebp - 4] ; segment_index
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; mov ebx, [ebp - 8] ; base_address
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; mov ecx, [ebp - 12] ; num_pages
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; mov edx, [ebp - 16] ; privilege_level
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; mov [esi + 16], eax ; segment_index
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; mov [esi + 20], ebx ; base_address
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; mov [esi + 24], ecx ; num_pages
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; mov [esi + 28], edx ; privilege_level
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; .validate_arguments:
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; cmp eax, 8191
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; ja .segment_index_too_tall
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; cmp eax, 0
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; je .segment_index_is_zero
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; cmp edx, 3
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; ja .privilege_level_too_large
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; .calculate_work_address:
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; shl eax, 3 ; Entry offset from table start
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; add eax, GDT_MEMORY_AREA ; Entry address
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; mov [esi + 12], eax
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; .write_segment_limit:
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; mov [eax], cx
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; shr ecx, 16
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; and ecx, 0x0f
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; mov [eax + 6], cl
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; .write_base_address:
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; mov [eax + 2], bx
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; shr bx, 16
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; mov [eax + 4], bl
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; mov [eax + 7], bh
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; .set_default_flags:
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; or [eax + 6], word (0b1100 << 4)
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; .set_stack_specific_access_byte:
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; mov al, 0b1001
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; ; OR the privilege level into the access byte
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; mov dl, [ebp - 16]
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; shl dl, 1
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; and dl, 0b0110
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; or al, dl
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; shl al, 4 ; Move the 4 bits of general flags to their place
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; ; Stack specific part
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; or al, 0b0110 ; DATA | STACK | READ_WRITE | NOT_ACCESSED
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; mov ebx, [esi + 12]
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; mov [ebx + 5], al ; Write to the final destination
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; .epilog:
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; mov edi, [esi + (64 - 20)]
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; mov edx, [esi + (64 - 16)]
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; mov ecx, [esi + (64 - 12)]
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; mov ebx, [esi + (64 - 8)]
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; mov eax, [esi + (64 - 4)]
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; sub esp, 64
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; pop esi
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; ret
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; .segment_index_too_tall:
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; .segment_index_is_zero:
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; .privilege_level_too_large:
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; ; todo(logs): Write debug message.
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; cli
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; hlt
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; Arguments (note: all arguments are 4-byte padded):
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; [Furthest from EBP]
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;
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@ -217,7 +323,6 @@ make_code_segment_in_gdt:
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hlt
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; [Furthest to EBP]
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; 1) Return Address
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; [Nearest to EBP]
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@ -241,17 +346,28 @@ load_flat_gdt:
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mov ebp, esp
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push dword 2 ; Segment Index
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push dword 0 ; Start Offset
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push dword 0x000fffff ; Page Count
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push dword 0xfffff ; Page Count
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push dword 0 ; Required privilege level
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call make_regular_data_segment_in_gdt
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mov esp, ebp
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pop ebp
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; COMMENTED OUT DUE TO BEING BUGGY
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; push ebp
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; mov ebp, esp
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; push dword 3 ; Segment Index
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; push dword LOWER_STACK_POINTER ; Start Offset
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; push dword 0xfffff ; Page Count
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; push dword 0 ; Required privilege level
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; call make_stack_segment_in_gdt
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; mov esp, ebp
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; pop ebp
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push ebp
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mov ebp, esp
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push dword 1 ; Segment Index
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push dword 0x0000 ; Start Offset
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push dword 0x000fffff ; Page Count
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push dword 0xfffff ; Page Count
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push dword 0 ; Required privilege level
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call make_code_segment_in_gdt
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mov esp, ebp
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@ -259,39 +375,42 @@ load_flat_gdt:
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mov bx, 0
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mov ds, bx
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mov es, bx
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mov fs, bx
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mov gs, bx
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mov ss, bx
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lgdt [.gdtr]
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mov ebx, cr0
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or ebx, 1
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mov cr0, ebx
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jmp dword (1 << 3):.epilog
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bits 32
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.epilog:
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mov bx, (2 << 3)
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mov ds, bx
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mov es, bx
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mov fs, bx
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mov gs, bx
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; mov bx, (3 << 3)
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mov ss, bx
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mov ebx, cr0
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or ebx, 1
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mov cr0, ebx
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xor bx, bx
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mov ds, bx
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jmp dword 0x08:.epilog
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bits 32
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.epilog:
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mov edi, [esi + (64 - 20)]
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mov edx, [esi + (64 - 16)]
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mov ecx, [esi + (64 - 12)]
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mov ebx, [esi + (64 - 8)]
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mov eax, [esi + (64 - 4)]
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jmp dword stage2_true_entry
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add esp, 64
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pop esi
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jmp stage2_true_entry
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; jmp after_gdt_loaded
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align 16
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.gdtr:
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.gdtr.length:
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dw 23
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dw 15
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.gdtr.pointer:
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dd GDT_MEMORY_AREA
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