;=----------------------------------------------------------------------------=;
;                            GNU GPL OS/K                                      ;
;                                                                              ;
;   Desc:   Basic Memory Realmode Driver                                       ;
;                   (x86_64 architecture only)                                 ;
;                                                                              ;
;                                                                              ;
;   Copyright © 2018-2019 The OS/K Team                                        ;
;                                                                              ;
;   This file is part of OS/K.                                                 ;
;                                                                              ;
;   OS/K is free software: you can redistribute it and/or modify               ;
;   it under the terms of the GNU General Public License as published by       ;
;   the Free Software Foundation, either version 3 of the License, or          ;
;   (at your option) any later version.                                        ;
;                                                                              ;
;   OS/K is distributed in the hope that it will be useful,                    ;
;   but WITHOUT ANY WARRANTY; without even the implied warranty of             ;
;   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the              ;
;   GNU General Public License for more details.                               ;
;                                                                              ;
;   You should have received a copy of the GNU General Public License          ;
;   along with OS/K.  If not, see <https://www.gnu.org/licenses/>.             ;
;=----------------------------------------------------------------------------=;

[BITS 16]

;; GDT WITH DOC
GDT64:
        NULL_SELECTOR:    ;; null selector within 64 bits
                        dw  GDT_LENGTH      ; limit of GDT
                        dw  GDT64           ; linear address of GDT
                        dd  0x0             ;

        CODE_SELECTOR:    ;; 32-bit code selector (ring 0)
                        dw  0x0000FFFF      ; Segment Limit
                        db  0x0, 0x0, 0x0   ; Base Address
                        db  10011010b       ;    |7|6|5|4|3|2|1|0|
                                            ;     | | | | | | | `----- 1 when segment used.
                                            ;     | | | | | | `------ 1 when writable.
                                            ;     | | | | | `------- 1 if "conformant". Don't know what is it... spectral ? xD
                                            ;     | | | | `-------- 1 always
                                            ;     | | | `--------- 1 for segment descriptor, 0 for system descriptor
                                            ;     | | `---------- DPL !!! 0 for ring 0
                                            ;     | `----------- DPL (2/2)
                                            ;     `------------ 1 if in physical memory, 0 if page fault
                        db  11001111b       ;    |7|6|5|4|3|2|1|0|
                                            ;     | | | | | | | `----- Limit 16
                                            ;     | | | | | | `------ Limit 17
                                            ;     | | | | | `------- Limit 18
                                            ;     | | | | `-------- Limit 19
                                            ;     | | | `--------- available for use
                                            ;     | | `---------- 0 always
                                            ;     | `----------- size of data. 1 for 32bits
                                            ;     `------------ 0 if limit is in Bytes, 1 if it's in pages (4ko)
                        db  0x0             ; Base Address

        DATA_SELECTOR:    ;; flat data selector (ring 0)
                        dw  0x0000FFFF      ; Segment Limit
                        db  0x0, 0x0, 0x0   ; Base Address
                        db  10010010b       ;    |7|6|5|4|3|2|1|0|
                                            ;     | | | | | | | `----- 1 when segment used.
                                            ;     | | | | | | `------ 1 when writable.
                                            ;     | | | | | `------- expansion direction. 1 for a LIFO
                                            ;     | | | | `-------- 1 always
                                            ;     | | | `--------- 1 for segment descriptor, 0 for system descriptor
                                            ;     | | `---------- DPL !!! 0 for ring 0
                                            ;     | `----------- DPL (2/2)
                                            ;     `------------ 1 if in physical memory, 0 if page fault
                        db  10001111b       ;    |7|6|5|4|3|2|1|0|
                                            ;     | | | | | | | `----- Limit 16
                                            ;     | | | | | | `------ Limit 17
                                            ;     | | | | | `------- Limit 18
                                            ;     | | | | `-------- Limit 19
                                            ;     | | | `--------- available for use
                                            ;     | | `---------- 0 always
                                            ;     | `----------- size of data. 1 for 32bits
                                            ;     `------------ 0 if limit is in Bytes, 1 if it's in pages (4ko)
                        db  0x0             ; Base Address

        LONG_SELECTOR:    ;; 64-bit code selector (ring 0)
                        dw  0x0000FFFF      ;  Segment Limit
                        db  0x0, 0x0, 0x0   ; Base Address
                        db  10011010b       ;    |7|6|5|4|3|2|1|0|
                                            ;     | | | | | | | `----- 1 when segment used.
                                            ;     | | | | | | `------ 1 when writable.
                                            ;     | | | | | `------- 1 if "conformant". Don't know what is it... spectral ? xD
                                            ;     | | | | `-------- 1 always
                                            ;     | | | `--------- 1 for segment descriptor, 0 for system descriptor
                                            ;     | | `---------- DPL !!! 0 for ring 0
                                            ;     | `----------- DPL (2/2)
                                            ;     `------------ 1 if in physical memory, 0 if page fault
                        db  10101111b       ;    |7|6|5|4|3|2|1|0|
                                            ;     | | | | | | | `----- Limit 16
                                            ;     | | | | | | `------ Limit 17
                                            ;     | | | | | `------- Limit 18
                                            ;     | | | | `-------- Limit 19
                                            ;     | | | `--------- available for use
                                            ;     | | `---------- 0 always
                                            ;     | `----------- size of data. 1 for 32bits
                                            ;     `------------ 0 if limit is in Bytes, 1 if it's in pages (4ko)
                        db  0x0             ; Base Address
    GDT_LENGTH:

;; TEXT

set_a20:
    push ax
    in al, 0x92
    or al, 2
    out 0x92, al
    pop ax
    ret