Subversion Repositories Kolibri OS

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;;                                                              ;;

;; Copyright (C) KolibriOS team 2004-2007. All rights reserved. ;;

;; Copyright (C) MenuetOS 2000-2004 Ville Mikael Turjanmaa      ;;

;; Distributed under terms of the GNU General Public License    ;;

;;                                                              ;;

;;  BOOTCODE.INC                                                ;;

;;                                                              ;;

;;  KolibriOS 16-bit loader,                                    ;;

;;                        based on bootcode for MenuetOS        ;;

;;                                                              ;;

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

$Revision: 713 $

;==========================================================================

;

;                           16 BIT FUNCTIONS

;

;==========================================================================

putchar:

; in: al=character

        mov     ah, 0Eh

        mov     bh, 0

        int     10h

        ret

print:

; in: si->string

        mov     al, 186

        call    putchar

        mov     al, ' '

        call    putchar

printplain:

; in: si->string

        pusha

        lodsb

@@:

        call    putchar

        lodsb

        cmp     al, 0

        jnz     @b

        popa

        ret

getkey:

; get number in range [bl,bh] (bl,bh in ['0'..'9'])

; in: bx=range

; out: ax=digit (1..9, 10 for 0)

        mov     ah, 0

        int     16h

        cmp     al, bl

        jb      getkey

        cmp     al, bh

        ja      getkey

        push    ax

        call    putchar

        pop     ax

        and     ax, 0Fh

        jnz     @f

        mov     al, 10

@@:

        ret

setcursor:

; in: dl=column, dh=row

        mov     ah, 2

        mov     bh, 0

        int     10h

        ret

macro _setcursor row,column

{

        mov     dx, row*256 + column

        call    setcursor

}

boot_read_floppy:

        push    si

        xor     si, si

        mov     ah, 2   ; read

@@:

        push    ax

        int     0x13

        pop     ax

        jnc     @f

        inc     si

        cmp     si, 10

        jb      @b

        mov     si, badsect

sayerr_plain:

        call    printplain

        jmp     $

@@:

        pop     si

        ret

;=========================================================================

;

;                           16 BIT CODE

;

;=========================================================================

include 'bootvesa.inc'                 ;Include source for boot vesa

start_of_code:

        cld

; \begin{diamond}[02.12.2005]

; if bootloader sets ax = 'KL', then ds:si points to loader block

        cmp     ax, 'KL'

        jnz     @f

        mov     word [cs:cfgmanager.loader_block], si

        mov     word [cs:cfgmanager.loader_block+2], ds

@@:

; \end{diamond}[02.12.2005]

; if bootloader sets cx = 'HA' and dx = 'RD', then bx contains identifier of source hard disk

; (see comment to bx_from_load)

        cmp     cx, 'HA'

        jnz     no_hd_load

        cmp     dx,'RD'

        jnz     no_hd_load

        mov     word [cs:bx_from_load], bx              ; {SPraid}[13.03.2007]

no_hd_load:

; set up stack

        mov     ax, 3000h

        mov     ss, ax

        mov     sp, 0EC00h

; set up segment registers

        push    cs

        pop     ds

        push    cs

        pop     es

; set videomode

        mov     ax, 3

        int     0x10

if lang eq ru

 ; Load & set russian VGA font (RU.INC)

        mov     bp, RU_FNT1             ; RU_FNT1 - First part

        mov     bx, 1000h               ; 768 bytes

        mov     cx, 30h                 ; 48 symbols

        mov     dx, 80h                 ; 128 - position of first symbol

        mov     ax, 1100h

        int     10h

        mov     bp, RU_FNT2             ; RU_FNT2 -Second part

        mov     bx, 1000h               ; 512 bytes

        mov     cx, 20h                 ; 32 symbols

        mov     dx, 0E0h                ; 224 - position of first symbol

        mov     ax, 1100h

        int     10h

 ; End set VGA russian font

else if lang eq et

        mov     bp, ET_FNT              ; ET_FNT1

        mov     bx, 1000h               ;

        mov     cx, 255                 ; 256 symbols

        xor     dx, dx                  ; 0 - position of first symbol

        mov     ax, 1100h

        int     10h

end if

; draw frames

        push    0xb800

        pop     es

        xor     di, di

        mov     ah, 1*16+15

; draw top

        mov     si, d80x25_top

        mov     cx, d80x25_top_num * 80

@@:

        lodsb

        stosw

        loop    @b

; draw spaces

        mov     si, space_msg

        mov     dx, 25 - d80x25_top_num - d80x25_bottom_num

dfl1:

        push    si

        mov     cx, 80

@@:

        lodsb

        stosw

        loop    @b

        pop     si

        dec     dx

        jnz     dfl1

; draw bottom

        mov     si, d80x25_bottom

        mov     cx, d80x25_bottom_num * 80

@@:

        lodsb

        stosw

        loop    @b

        mov     byte [space_msg+80], 0    ; now space_msg is null terminated

        _setcursor d80x25_top_num,0

; TEST FOR 386+

        mov     bx, 0x4000

        pushf

        pop     ax

        mov     dx, ax

        xor     ax, bx

        push    ax

        popf

        pushf

        pop     ax

        and     ax, bx

        and     dx, bx

        cmp     ax, dx

        jnz     cpugood

        mov     si, not386

sayerr:

        call    print

        jmp     $

     cpugood:

        push    0

        popf

        sti

; set up esp

        movzx   esp, sp

        push    0

        pop     es

        and     word [es:0x9031], 0

; \begin{Mario79}

; find HDD IDE DMA PCI device

; check for PCI BIOS

        mov     ax, 0xB101

        int     0x1A

        jc      .nopci

        cmp     edx, 'PCI '

        jnz     .nopci

; find PCI class code

; class 1 = mass storage

; subclass 1 = IDE controller

; a) class 1, subclass 1, programming interface 0x80

        mov     ax, 0xB103

        mov     ecx, 1*10000h + 1*100h + 0x80

        xor     si, si  ; device index = 0

        int     0x1A

        jnc     .found

; b) class 1, subclass 1, programming interface 0x8A

        mov     ax, 0xB103

        mov     ecx, 1*10000h + 1*100h + 0x8A

        xor     si, si  ; device index = 0

        int     0x1A

        jnc     .found

; c) class 1, subclass 1, programming interface 0x85

        mov     ax, 0xB103

        mov     ecx, 1*10000h + 1*100h + 0x85

        xor     si, si

        int     0x1A

        jc      .nopci

.found:

; get memory base

        mov     ax, 0xB10A

        mov     di, 0x20        ; memory base is config register at 0x20

        int     0x1A

        jc      .nopci

        and     cx, 0xFFF0      ; clear address decode type

        mov     [es:0x9031], cx

.nopci:

; \end{Mario79}

        mov     al, 0xf6        ; Ñáðîñ êëàâèàòóðû, ðàçðåøèòü ñêàíèðîâàíèå

        out     0x60, al

        xor     cx, cx

wait_loop:       ; variant 2

; reading state of port of 8042 controller

        in      al, 64h

        and     al, 00000010b  ; ready flag

; wait until 8042 controller is ready

        loopnz  wait_loop

;;;/diamond today   5.02.2008

; set keyboard typematic rate & delay

        mov     al, 0xf3

        out     0x60, al

        xor     cx, cx

@@:

        in      al, 64h

        test    al, 2

        loopnz  @b

        mov     al, 0

        out     0x60, al

        xor     cx, cx

@@:

        in      al, 64h

        test    al, 2

        loopnz  @b

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

; --------------- APM ---------------------

        and     word [es:0x9044], 0     ; ver = 0.0 (APM not found)

        mov     ax, 0x5300

        xor     bx, bx

        int     0x15

        jc      apm_end                 ; APM not found

        test    cx, 2

        jz      apm_end                 ; APM 32-bit protected-mode interface not supported

        mov     [es:0x9044], ax         ; Save APM Version

        mov     [es:0x9046], cx         ; Save APM flags

        ; Write APM ver ----

        and     ax, 0xf0f

        add     ax, '00'

        mov     si, msg_apm

        mov     [si + 5], ah

        mov     [si + 7], al

        _setcursor 0, 3

        call    printplain

        ; ------------------

        mov     ax, 0x5304              ; Disconnect interface

        xor     bx, bx

        int     0x15

        mov     ax, 0x5303              ; Connect 32 bit mode interface

        xor     bx, bx

        int     0x15

        mov     [es:0x9040], ebx

        mov     [es:0x9050], ax

        mov     [es:0x9052], cx

        mov     [es:0x9054], dx

apm_end:

        _setcursor d80x25_top_num, 0

;CHECK current of code

        cmp     [cfgmanager.loader_block], -1

        jz      noloaderblock

        les     bx, [cfgmanager.loader_block]

        cmp     byte [es:bx], 1

        mov     si, loader_block_error

        jnz     sayerr

noloaderblock:

; DISPLAY VESA INFORMATION

         call    print_vesa_info

         call    calc_vmodes_table

         call    check_first_parm  ;check and enable cursor_pos

; \begin{diamond}[30.11.2005]

cfgmanager:

; settings:

; a) preboot_graph = graphical mode

;    preboot_gprobe = probe this mode?

; b) preboot_dma  = use DMA access?

; c) preboot_vrrm = use VRR?

; d) preboot_device = from what boot?

; determine default settings

        mov     [.bSettingsChanged], 0

;.preboot_gr_end:

; following 6 lines set variables to 1 if its current value is 0

        cmp     byte[preboot_dma], 1

        adc     byte[preboot_dma], 0

        cmp     byte[preboot_vrrm], 1

        adc     byte[preboot_vrrm], 0

        cmp     byte[preboot_device], 1

        adc     byte[preboot_device], 0

; notify user

        _setcursor 5,2

        mov     si, linef

        call    printplain

        mov     si, start_msg

        call    print

        mov     si, time_msg

        call    print

; get start time

        call    .gettime

        mov     [.starttime], eax

        mov     word [.timer], .newtimer

        mov     word [.timer+2], cs

.printcfg:

        _setcursor 9,0

        mov     si, current_cfg_msg

        call    print

        mov     si, curvideo_msg

        call    print

            call    draw_current_vmode

        mov     si, usebd_msg

        cmp     [preboot_biosdisk], 1

        call    .say_on_off

        mov     si, vrrm_msg

        cmp     [preboot_vrrm], 1

        call    .say_on_off

        mov     si, preboot_device_msg

        call    print

        mov     al, [preboot_device]

        and     eax, 7

        mov     si, [preboot_device_msgs+eax*2]

        call    printplain

.wait:

        _setcursor 25,0         ; out of screen

; set timer interrupt handler

        cli

        push    0

        pop     es

        push    dword [es:8*4]

        pop     dword [.oldtimer]

        push    dword [.timer]

        pop     dword [es:8*4]

;        mov     eax, [es:8*4]

;        mov     [.oldtimer], eax

;        mov     eax, [.timer]

;        mov     [es:8*4], eax

        sti

; wait for keypressed

        xor     ax,ax

        int     16h

        push    ax

; restore timer interrupt

;        push    0

;        pop     es

        mov     eax, [.oldtimer]

        mov     [es:8*4], eax

        mov     [.timer], eax

        _setcursor 7,0

        mov     si, space_msg

        call    printplain

        pop     ax

; switch on key

        cmp     al, 13

        jz      .continue

        or      al, 20h

        cmp     al, 'a'

        jz      .change_a

        cmp     al, 'b'

        jz      .change_b

        cmp     al, 'c'

        jz      .change_c

        cmp     al, 'd'

        jnz     .wait

        _setcursor 15,0

        mov     si, bdev

        call    print

        mov     bx, '14'

        call    getkey

        mov     [preboot_device], al

        _setcursor 13,0

.d:

        mov     [.bSettingsChanged], 1

        call    clear_vmodes_table             ;clear vmodes_table

        jmp    .printcfg

.change_a:

.loops:

        call    draw_vmodes_table

        xor     ax,ax

        int     0x16

;        call    clear_table_cursor             ;clear current position of cursor

        mov     si,word [cursor_pos]

        cmp     ah,0x48;x,0x48E0               ; up

        jne     .down

        cmp     si,modes_table

        jbe     .loops

        sub     word [cursor_pos],size_of_step

        jmp     .loops

.down:  cmp     ah,0x50;x,0x50E0               ; down

        jne     .enter

        cmp     word[es:si+10],-1

        je      .loops

        add     word [cursor_pos],size_of_step

        jmp     .loops

.enter: cmp     al,0x0D;x,0x1C0D               ; enter

        jne     .loops

        push    word [cursor_pos]

        pop     word [preboot_graph]           ;save choose

        jmp    .d

.change_b:

        _setcursor 15,0

;        mov     si, ask_dma

;        call    print

;        mov     bx, '13'

;        call    getkey

;        mov     [preboot_dma], al

        mov     si, ask_bd

        call    print

        mov     bx, '12'

        call    getkey

        mov     [preboot_biosdisk], al

        _setcursor 11,0

        jmp     .d

.change_c:

        _setcursor 15,0

        mov     si, vrrmprint

        call    print

        mov     bx, '12'

        call    getkey

        mov     [preboot_vrrm], al

        _setcursor 12,0

        jmp     .d

;;;;;;;;;;;;;;;;;;;;;;;;;;;;

.say_on_off:

        pushf

        call    print

        mov     si, on_msg

        popf

        jz      @f

        mov     si, off_msg

@@:     jmp     printplain

; novesa and vervesa strings are not used at the moment of executing this code

virtual at novesa

.oldtimer dd ?

.starttime dd ?

.bSettingsChanged db ?

.timer dd ?

end virtual

.loader_block dd -1

.gettime:

        mov     ah, 0

        int     1Ah

        xchg    ax, cx

        shl     eax, 10h

        xchg    ax, dx

        ret

.newtimer:

        push    ds

        push    cs

        pop     ds

        pushf

        call    [.oldtimer]

        pushad

        call    .gettime

        sub     eax, [.starttime]

        sub     ax, 18*5

        jae     .timergo

        neg     ax

        add     ax, 18-1

        mov     bx, 18

        xor     dx, dx

        div     bx

if lang eq ru

; ¯®¤®¦¤¨â¥ 5 ᥪ㭤, 4/3/2 ᥪ㭤ë, 1 ᥪ㭤ã

        cmp     al, 5

        mov     cl, ' '

        jae     @f

        cmp     al, 1

        mov     cl, 'ã'

        jz      @f

        mov     cl, 'ë'

@@:     mov     [time_str+9], cl

else if lang eq et

        cmp     al, 1

        ja      @f

        mov     [time_str+9], ' '

        mov     [time_str+10],' '

@@:

else

; wait 5/4/3/2 seconds, 1 second

        cmp     al, 1

        mov     cl, 's'

        ja      @f

        mov     cl, ' '

@@:     mov     [time_str+9], cl

end if

        add     al, '0'

        mov     [time_str+1], al

        mov     si, time_msg

        _setcursor 7,0

        call    print

        _setcursor 25,0

        popad

        pop     ds

        iret

.timergo:

        push    0

        pop     es

        mov     eax, [.oldtimer]

        mov     [es:8*4], eax

        mov     sp, 0EC00h

.continue:

        sti

        _setcursor 6,0

        mov     si, space_msg

        call    printplain

        call    printplain

        _setcursor 6,0

        mov     si, loading_msg

        call    print

        _setcursor 15,0

        cmp     [.bSettingsChanged], 0

        jz      .load

        cmp     [.loader_block], -1

        jz      .load

        les     bx, [.loader_block]

        mov     eax, [es:bx+3]

        push    ds

        pop     es

        test    eax, eax

        jz      .load

        push    eax

        mov     si, save_quest

        call    print

.waityn:

        mov     ah, 0

        int     16h

        or      al, 20h

        cmp     al, 'n'

        jz      .loadc

        cmp     al, 'y'

        jnz     .waityn

        call    putchar

        mov     byte [space_msg+80], 186

        pop     eax

        push    cs

        push    .cont

        push    eax

        retf

.loadc:

        pop     eax

.cont:

        push    cs

        pop     ds

        mov     si, space_msg

        mov     byte [si+80], 0

        _setcursor 15,0

        call    printplain

        _setcursor 15,0

.load:

; \end{diamond}[02.12.2005]

; ASK GRAPHICS MODE

        call    set_vmode

; GRAPHICS ACCELERATION

; force yes

        mov     [es:0x901C], byte 1

; DMA ACCESS TO HD

        mov     al, [preboot_dma]

        mov     [es:0x901F], al

; VRR_M USE

        mov     al,[preboot_vrrm]

        mov     [es:0x9030], al

        mov     [es:0x901E], byte 1

; BOOT DEVICE

        mov     al, [preboot_device]

        dec     al

        mov     [boot_dev], al

; READ DISKETTE TO MEMORY

;        cmp     [boot_dev],0

        jne     no_sys_on_floppy

        mov     si,diskload

        call    print

        xor     ax, ax            ; reset drive

        xor     dx, dx

        int     0x13

; do we boot from CD-ROM?

        mov     ah, 41h

        mov     bx, 55AAh

        xor     dx, dx

        int     0x13

        jc      .nocd

        cmp     bx, 0AA55h

        jnz     .nocd

        mov     ah, 48h

        push    ds

        push    es

        pop     ds

        mov     si, 0xa000

        mov     word [si], 30

        int     0x13

        pop     ds

        jc      .nocd

        push    ds

        lds     si, [es:si+26]

        test    byte [ds:si+10], 40h

        pop     ds

        jz      .nocd

; yes - read all floppy by 18 sectors

        mov     cx, 0x0001      ; startcyl,startsector

.a1:

        push    cx dx

        mov     al, 18

        mov     bx, 0xa000

        call    boot_read_floppy

        mov     si, movedesc

        push    es

        push    ds

        pop     es

        mov     cx, 256*18

        mov     ah, 0x87

        int     0x15

        pop     es

        pop     dx cx

        test    ah, ah

        jnz     sayerr_floppy

        add     dword [si+8*3+2], 512*18

        inc     dh

        cmp     dh, 2

        jnz     .a1

        mov     dh, 0

        inc     ch

        cmp     ch, 80

        jae     ok_sys_on_floppy

        pusha

        mov     al, ch

        shr     ch, 2

        add     al, ch

        aam

        xchg    al, ah

        add     ax, '00'

        mov     si, pros

        mov     [si], ax

        call    printplain

        popa

        jmp     .a1

.nocd:

; no - read only used sectors from floppy

; now load floppy image to memory

; at first load boot sector and first FAT table

        mov     cx, 0x0001      ; startcyl,startsector

        xor     dx, dx          ; starthead,drive

        mov     al, 1+9         ; no of sectors to read

        mov     bx, 0xB000      ; es:bx -> data area

        call    boot_read_floppy

; and copy them to extended memory

        mov     si, movedesc

        mov     [si+8*2+3], bh

        push    es

        push    ds

        pop     es

        mov     cx, 256*10

        mov     ah, 0x87

        int     0x15

        test    ah, ah

        jz      @f

sayerr_floppy:

        mov     dx, 0x3f2

        mov     al, 0

        out     dx, al

        mov     si, memmovefailed

        jmp     sayerr_plain

@@:

        add     dword [si+8*3+2], 512*10

; copy FAT to second copy

        mov     byte [si+8*2+3], 0xB2

        mov     cx, 256*9

        mov     ah, 0x87

        int     0x15

        pop     es

        test    ah, ah

        jnz     sayerr_floppy

        add     dword [si+8*3+2], 512*9

; calculate total number of sectors to read

        mov     ax, 1+9+14      ; boot+FAT+root

        mov     di, 0xB203

.calc_loop:

        test    word [es:di], 0xFFF

        jz      @f

        inc     ax

@@:

        test    word [es:di+1], 0xFFF0

        jz      @f

        inc     ax

@@:

        add     di, 3

        cmp     di, 0xB200+1440*3

        jb      .calc_loop

        push    ax

        mov     bp, 1+9         ; already read sectors

; now read rest

        mov     byte [si+8*2+3], 0xA0

        mov     di, 2-14        ; absolute sector-31

        mov     cx, 0x0002      ; cylinder=0, sector=2

        mov     dx, 0x0100      ; head=1, disk=0

.read_loop:

; determine whether sector must be read

        cmp     di, 2

        jl      .read

        mov     bx, di

        shr     bx, 1

        jnc     .even

        test    word [es:bx+di+0xB200], 0xFFF0

        jmp     @f

.even:

        test    word [es:bx+di+0xB200], 0xFFF

@@:

        jz      .skip

.read:

        mov     bx, 0xA000

        mov     al, 1           ; 1 sector

        call    boot_read_floppy

        inc     bp

        push    es

        push    ds

        pop     es

        pusha

        mov     cx, 256

        mov     ah, 0x87

        int     0x15

        test    ah, ah

        popa

        pop     es

        jnz     sayerr_floppy

.skip:

        add     dword [si+8*3+2], 512

        inc     cx

        cmp     cl, 19

        jnz     @f

        mov     cl, 1

        inc     dh

        cmp     dh, 2

        jnz     @f

        mov     dh, 0

        inc     ch

@@:

        pop     ax

        push    ax

        pusha

; draw percentage

; total sectors: ax

; read sectors: bp

        xchg    ax, bp

        mov     cx, 100

        mul     cx

        div     bp

        aam

        xchg    al, ah

        add     ax, '00'

        mov     si, pros

        cmp     [si], ax

        jz      @f

        mov     [si], ax

        call    printplain

@@:

        popa

        inc     di

        cmp     di, 2880-31

        jnz     .read_loop

;        mov     cx, 0x0001      ; startcyl,startsector

;        xor     dx, dx          ; starthead,drive

;        push    word 80*2               ; read no of sect

;       reads:

;        pusha

;        xor     si,si

;       newread:

;        mov     bx,0xa000               ; es:bx -> data area

;        mov     ax,0x0200+18            ; read, no of sectors to read

;        int     0x13

;        test    ah, ah

;        jz      goodread

;        inc    si

;        cmp     si,10

;        jnz     newread

;        mov     si,badsect-0x10000

;sayerr_plain:

;        call    printplain

;        jmp     $

;       goodread:

;        ; move -> 1mb

;        mov     si,movedesc-0x10000

;        push    es

;        push    ds

;        pop     es

;        mov     cx,256*18

;        mov     ah,0x87

;        int     0x15

;        pop    es

;

;        test    ah,ah                  ; was the move successfull ?

;        je      goodmove

;        mov     dx,0x3f2              ; floppy motor off

;        mov     al,0

;        out     dx,al

;        mov     si,memmovefailed-0x10000

;        jmp    sayerr_plain

;      goodmove:

;

;    add    dword [movedesc-0x10000+0x18+2], 512*18

;        popa

;        inc     dh

;        cmp     dh,2

;        jnz     bb2

;        mov     dh,0

;        inc     ch

;        pusha                        ; print prosentage

;        mov     si,pros-0x10000

;    shr    ch, 2

;    mov    al, '5'

;    test    ch, 1

;    jnz    @f

;    mov    al, '0'

;@@:

;    mov    [si+1], al

;    shr    ch, 1

;    add    ch, '0'

;    mov    [si], ch

;        call    printplain

;        popa

;       bb2:

;        pop     ax

;        dec     ax

;        push    ax

;        jnz     reads

;       readdone:

;        pop     ax

ok_sys_on_floppy:

        mov     si, backspace2

        call    printplain

        mov     si, okt

        call    printplain

no_sys_on_floppy:

        xor     ax, ax          ; reset drive

        xor     dx, dx

        int     0x13

        mov     dx, 0x3f2       ; floppy motor off

        mov     al, 0

        out     dx, al

; SET GRAPHICS

        xor     ax, ax

        mov     es, ax

        mov     ax, [es:0x9008]         ; vga & 320x200

        mov     bx, ax

        cmp     ax, 0x13

        je      setgr

        cmp     ax, 0x12

        je      setgr

        mov     ax, 0x4f02              ; Vesa

setgr:

        int     0x10

        test    ah, ah

        mov     si, fatalsel

        jnz     v_mode_error

; set mode 0x12 graphics registers:

        cmp     bx, 0x12

        jne     gmok2

        mov     al, 0x05

        mov     dx, 0x03ce

        push    dx

        out     dx, al      ; select GDC mode register

        mov     al, 0x02

        inc     dx

        out     dx, al      ; set write mode 2

        mov     al, 0x02

        mov     dx, 0x03c4

        out     dx, al      ; select VGA sequencer map mask register

        mov     al, 0x0f

        inc     dx

        out     dx, al      ; set mask for all planes 0-3

        mov     al, 0x08

        pop     dx

        out     dx, al      ; select GDC bit mask register

                           ; for writes to 0x03cf

gmok2:

        push    ds

        pop     es