144 lines
4.6 KiB
PHP
144 lines
4.6 KiB
PHP
; Copyright (c) 2024 Elaina Claus
|
|
;
|
|
; Permission is hereby granted, free of charge, to any person obtaining a copy
|
|
; of this software and associated documentation files (the "Software"), to deal
|
|
; in the Software without restriction, including without limitation the rights
|
|
; to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
|
|
; copies of the Software, and to permit persons to whom the Software is
|
|
; furnished to do so, subject to the following conditions:
|
|
;
|
|
; The above copyright notice and this permission notice shall be included in all
|
|
; copies or substantial portions of the Software.
|
|
;
|
|
; THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
|
|
; IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
|
|
; FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
|
|
; AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
|
|
; LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
|
|
; OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
|
|
; SOFTWARE.
|
|
|
|
%ifndef __INC_A20ENABLE
|
|
;
|
|
;INT 0x15 Function 2400 - Disable A20
|
|
;Returns:
|
|
;
|
|
; CF = clear if success
|
|
; AH = 0
|
|
; CF = set on error
|
|
; AH = status (01=keyboard controller is in secure mode, 0x86=function not supported)
|
|
;
|
|
;INT 0x15 Function 2401 - Enable A20
|
|
;Returns:
|
|
;
|
|
; CF = clear if success
|
|
; AH = 0
|
|
; CF = set on error
|
|
; AH = status (01=keyboard controller is in secure mode, 0x86=function not supported)
|
|
;
|
|
;INT 0x15 Function 2402 - A20 Status
|
|
; Returns:
|
|
;
|
|
; CF = clear if success
|
|
; AH = status (01: keyboard controller is in secure mode; 0x86: function not supported)
|
|
; AL = current state (00: disabled, 01: enabled)
|
|
; CX = set to 0xffff is keyboard controller is no ready in 0xc000 read attempts
|
|
; CF = set on error
|
|
;
|
|
;INT 0x15 Function 2403 - Query A20 support
|
|
;Returns:
|
|
;
|
|
;CF = clear if success
|
|
;AH = status (01: keyboard controller is in secure mode; 0x86: function not supported)
|
|
;BX = status.
|
|
;
|
|
;BX contains a bit pattern:
|
|
;
|
|
; Bit 0 - supported on keyboard controller
|
|
; Bit 1 - if supported on bit 1 of I/O port 0x92
|
|
; Bits 2:14 - Reserved
|
|
; Bit 15 - 1 if additional data is available.
|
|
;
|
|
; I/O Port 0x92 infomation:
|
|
;
|
|
; Bit 0 - Setting to 1 causes a fast reset
|
|
; Bit 1 - 0: disable A20; 1: enable A20
|
|
; Bit 2 - Manufacturer defined
|
|
; Bit 3 - power on password bytes (CMOS bytes 0x38-0x3f or 0x36-0x3f). 0: accessible, 1: inaccessible
|
|
; Bits 4-5 - Manufacturer defined
|
|
; Bits 6-7 - 00: HDD activity LED off; any other value is "on"
|
|
ALIGN 4, db 0x90
|
|
EnableA20:
|
|
__CDECL16_ENTRY
|
|
push ds
|
|
push es
|
|
.a20_check:
|
|
cli
|
|
|
|
xor ax, ax
|
|
mov es, ax
|
|
|
|
not ax ; ax = 0xFFFF
|
|
mov ds, ax
|
|
|
|
mov di, 0x0500 ; scratch location 1
|
|
mov si, 0x0510 ; scratch location 2
|
|
|
|
mov al, byte [es:di]
|
|
push ax ; save whatever is at 0x0000:0500, physical location 0x0500
|
|
|
|
mov al, byte [ds:si]
|
|
push ax ; save whatever is at 0xFFFF:0510 [clarification: 0x100500 physical location (0x100500 - 1MB = 0x0500)]
|
|
|
|
mov byte [es:di], 0x00 ; zero non-wraped location and write 0xFF to it after (ab)using wrapping
|
|
mov byte [ds:si], 0xFF ; if the non-wrapped location is 0xFF, then we wraped and A20 is disabled
|
|
|
|
cmp byte [es:di], 0xFF
|
|
|
|
pop ax
|
|
mov byte [ds:si], al ; restore original contents of scratch location 2
|
|
|
|
pop ax
|
|
mov byte [es:di], al ; restore original contents of scratch location 1
|
|
|
|
mov ax, 0 ; return 0 if es:di == ds:si (memory wraps)
|
|
je EnableA20.end_check
|
|
mov ax, 1 ; return 1 if es:di != ds:si (A20 is enabled)
|
|
.end_check:
|
|
sti
|
|
cmp ax, 1
|
|
je EnableA20.endp ; A20 is already enabled
|
|
|
|
mov ax, 0x2403
|
|
int 0x15
|
|
jc EnableA20.do_fallback_a20 ; carry = error...not supported?
|
|
cmp ah, 0
|
|
ja EnableA20.do_fallback_a20 ; non-zero return = error as well
|
|
|
|
mov al, bl
|
|
and al, 0000_0010b
|
|
cmp al, 0000_0010b
|
|
je EnableA20.do_fast_a20 ; if fast a20 is supported use it
|
|
|
|
jmp EnableA20.do_bios_a20 ; else fall back to enabling via BIOS
|
|
|
|
.do_fallback_a20:
|
|
ERROR STAGE2_A20_FAILED
|
|
.do_bios_a20:
|
|
mov ax, 0x2401
|
|
int 0x15
|
|
jmp EnableA20.a20_check
|
|
.do_fast_a20:
|
|
in al, 0x92 ; read from FAST A20 port
|
|
or al, 2 ; bit 0 is a fast reset, bit 1 is fast A20
|
|
and al, 0xFE ; make sure bit 0 is 0
|
|
out 0x92, al ; enable A20
|
|
jmp EnableA20.a20_check
|
|
.endp:
|
|
pop es
|
|
pop ds
|
|
__CDECL16_EXIT
|
|
ret
|
|
|
|
%endif
|
|
%define __INC_A20ENABLE |