use16
;disable interrupts
cli

;update the segment registers
mov   ax,cs
mov   ds,ax
mov   es,ax

;enable A20
.5:
in    al,0x64
test  al,2
jnz   .5
mov   al,0xD1
out   0x64,al
.6:
in    al,0x64
and       ax,2
jnz     .6
mov   al,0xDF
out   0x60,al

;check if vesa is supported
mov   ax,0x4F00
mov   di,VBEInfoBlock - 0x10000
int 0x10

;check if function is supported
cmp   al,0x4F
je    ctrl_else0_1
mov   si,vbe_chosen_func_not_supported
mov   ax,0xB000
mov   es,ax
mov   di,0x8000
mov   al,1
ctrl_while1_1:
or    al,al
je    ctrl_wend1_1
lodsb
or    al,al
je    ctrl_else2_1
stosb
mov   al,7
stosb
ctrl_else2_1:
ctrl_endif2_1:
jmp   ctrl_while1_1
ctrl_wend1_1:
jmp   no_vesa
ctrl_else0_1:
ctrl_endif0_1:

;select ah, check if function call was successful

;case 1: wasn't successful
case1_0:
cmp   ah,1
jne   case1_1
mov   si,vbe_func_call_error
mov   ax,0xB000
mov   es,ax
mov   di,0x8000
mov   al,1
ctrl_while2_1:
or    al,al
je    ctrl_wend2_1
lodsb
or    al,al
je    ctrl_else3_1
stosb
mov   al,7
stosb
ctrl_else3_1:
ctrl_endif3_1:
jmp   ctrl_while2_1
ctrl_wend2_1:
jmp   no_vesa
jmp   endselect1

;case 2: function isn't supported by hardware
case1_1:
cmp   ah,2
jne   case1_2
mov   si,vbe_chosen_func_not_supported_by_hw
mov   ax,0xB000
mov   es,ax
mov   di,0x8000
mov   al,1
ctrl_while2_2:
or    al,al
je    ctrl_wend2_2
lodsb
or    al,al
je    ctrl_else3_2
stosb
mov   al,7
stosb
ctrl_else3_2:
ctrl_endif3_2:
jmp   ctrl_while2_2
ctrl_wend2_2:
jmp   no_vesa
jmp   endselect1

;case 3: function mustn't be called now
case1_2:
cmp   ah,3
jne   case1_3
mov   si,vbe_func_mustnt_be_called
mov   ax,0xB000
mov   es,ax
mov   di,0x8000
mov   al,1
ctrl_while2_3:
or    al,al
je    ctrl_wend2_3
lodsb
or    al,al
je    ctrl_else3_3
stosb
mov   al,7
stosb
ctrl_else3_3:
ctrl_endif3_3:
jmp   ctrl_while2_3
ctrl_wend2_3:
jmp   no_vesa
case1_3:
endselect1:

;check if the new signature is "VESA", if not vesa isn't supported
mov   si,VbeSignature - 0x10000
lodsd
cmp   eax,0x41534556
je    ctrl_else0_2
mov   di,answer
stosd
mov   si,func_not_supported
mov   ax,0xB000
mov   es,ax
mov   di,0x8000
mov   al,1
ctrl_while1_2:
or    al,al
je    ctrl_wend1_2
lodsb
or    al,al
je    ctrl_else2_2
stosb
mov   al,7
stosb
ctrl_else2_2:
ctrl_endif2_2:
jmp   ctrl_while1_2
ctrl_wend1_2:
jmp   no_vesa
ctrl_else0_2:
ctrl_endif0_2:

;get informations about mode 0x115 (should be 800x600x32)
mov   ax,0x4F01
mov   di,VbeModeInfoBlock - 0x10000
mov   cx,0x115
int 0x10

;check if function is supported
cmp   al,0x4F
je    ctrl_else0_3
mov   si,vbe_chosen_func_not_supported
mov   ax,0xB000
mov   es,ax
mov   di,0x8000
mov   al,1
ctrl_while1_3:
or    al,al
je    ctrl_wend1_3
lodsb
or    al,al
je    ctrl_else2_3
stosb
mov   al,7
stosb
ctrl_else2_3:
ctrl_endif2_3:
jmp   ctrl_while1_3
ctrl_wend1_3:
jmp   no_vesa
ctrl_else0_3:
ctrl_endif0_3:

;select ah, check if function call was successful

;case 1: wasn't successful
case2_0:
cmp   ah,1
jne   case2_1
mov   si,vbe_func_call_error
mov   ax,0xB000
mov   es,ax
mov   di,0x8000
mov   al,1
ctrl_while2_4:
or    al,al
je    ctrl_wend2_4
lodsb
or    al,al
je    ctrl_else3_4
stosb
mov   al,7
stosb
ctrl_else3_4:
ctrl_endif3_4:
jmp   ctrl_while2_4
ctrl_wend2_4:
jmp   no_vesa
jmp   endselect2

;case 2: function isn't supported by hardware
case2_1:
cmp   ah,2
jne   case2_2
mov   si,vbe_chosen_func_not_supported_by_hw
mov   ax,0xB000
mov   es,ax
mov   di,0x8000
mov   al,1
ctrl_while2_5:
or    al,al
je    ctrl_wend2_5
lodsb
or    al,al
je    ctrl_else3_5
stosb
mov   al,7
stosb
ctrl_else3_5:
ctrl_endif3_5:
jmp   ctrl_while2_5
ctrl_wend2_5:
jmp   no_vesa
jmp   endselect2

;case 3: function mustn't be called now
case2_2:
cmp   ah,3
jne   case2_3
mov   si,vbe_func_mustnt_be_called
mov   ax,0xB000
mov   es,ax
mov   di,0x8000
mov   al,1
ctrl_while2_6:
or    al,al
je    ctrl_wend2_6
lodsb
or    al,al
je    ctrl_else3_6
stosb
mov   al,7
stosb
ctrl_else3_6:
ctrl_endif3_6:
jmp   ctrl_while2_6
ctrl_wend2_6:
jmp   no_vesa
case2_3:
endselect2:

;VESA and mode 800x600x32 are supported
mov   si,vesa_supported
mov   ax,0xB000
mov   es,ax
mov   di,0x8000
mov   al,1
ctrl_while0_1:
or    al,al
je    ctrl_wend0_1
lodsb
or    al,al
je    ctrl_else1_1
stosb
mov   al,7
stosb
ctrl_else1_1:
ctrl_endif1_1:
jmp   ctrl_while0_1
ctrl_wend0_1:

;Enable 800x600x32 with linear frame buffer
mov   ax,0x4F02
mov   bx,0x4115
int 0x10

;Say the PM code, that VESA is on
mov   [IsVESA - 0x10000],1

no_vesa:

;switch to PM

;fill the segment registers with 0 (else they won't select the dummy descriptor and there'll be a GPF)
xor   ax,ax
mov   ds,ax
mov   es,ax
mov   ss,ax
mov   fs,ax
mov   gs,ax
xor   esp,esp

;Trying to load the GDT
lgdt [gdt_desc]
;This point here will never be executed! The program is frozen here.

;If the program would continue, it would enter the PM here.
mov   eax,cr0
or    eax,1
mov   cr0,eax

;Flush the pipeline and use 32 bit commands
jmp   far 0x28:pmode
;The selected descriptor has the following attributes:
;Base is on 0x10000
;Size is 0xFFFEF * 4KB
;It's a readable code segment with DPL = 0
;It's for 80386+

use32

pmode:

;Fill the segment registers with the correct values
mov   ax,0x10
mov   ds,ax
mov   es,ax
mov   ss,ax
;The selected descriptor has the following attributes:
;Base is on 0x0000
;Size is 0xFFFFF * 4KB
;It's a writeable data segment with DPL = 0
;It's for 80386+

xor   ax,ax
mov   fs,ax
mov   gs,ax

mov   esp,0x1FFFFF

jmp   far 0x08:start
;The selected descriptor has the following attributes:
;Base is on 0x0000
;Size is 0xFFFFF * 4KB
;It's a readable code segment with DPL = 0
;It's for 80386+

func_not_supported db "VESA functions aren't supported., answer is "
answer db 0,0,0,0,0
vbe_chosen_func_not_supported db "The chosen VBE function isn't supported.",0
vbe_chosen_func_not_supported_by_hw db "The chosen VBE function isn't supported by hardware.",0
vesa_supported db "VESA 800x600x32 is supported.",0
vbe_func_call_error db "The function call wasn't successful.",0
vbe_func_mustnt_be_called db "The function mustn't be called.",0
vbe_invalid_video_mode db "Invalid video mode.",0

org 0x10000+$

;Here is the GDT
gdt_start:
gdt_entry_dummy: ;Index 0 [selector = 0x00]
dd 0, 0
gdt_entry_sys_code: ;Index 1 [selector = 0x08]
dw 0xFFFF
dw 0x0000
db 0x00
db 10011010b
db 11001111b
db 0x00
;Code executable/readable, DPL = 0, Base = 0x00000000, Size = 0xFFFFF * 4KB, 80386

gdt_entry_sys_data: ;Index 2 [selector = 0x10]
dw 0xFFFF
dw 0x0000
db 0x00
db 10010010b
db 11001111b
db 0x00
;Data readable/writeable, DPL = 0, Base = 0x00000000, Size = 0xFFFFF * 4KB, 80386

gdt_entry_sys_video: ;Index 3 [selector = 0x18]
dw 0xFA00
dw 0x0000
db 0x0A
db 10110010b
db 01000000b
db 0x00
;Data readable/writeable, DPL = 1, Base = 0x000A0000, Size = 0xFA00 * 1B, 80386
;This descriptor was used for VGA mode 0x13

gdt_entry_sys_text: ;Index 4 [selector = 0x20]
dw 0x8000
dw 0x8000
db 0x0B
db 10010010b
db 01000000b
db 0x00
;Data readable/writeable, DPL = 0, Base = 0x000B8000, Size = 0x8000 * 1B, 80386

gdt_entry_sys_scode: ;Index 5 [selector = 0x28]
dw 0xFFEF
dw 0x0000
db 0x01
db 10011010b
db 11001111b
db 0x00
;Code executable/readable, DPL = 0, Base = 0x00010000, Size = 0xFFFEF * 4KB, 80386

gdt_entry_sys_sdata: ;Index 6 [selector = 0x30]
dw 0x1000
dw 0x0000
db 0x01
db 10010010b
db 11000000b
db 0x10
;Data readable/writeable, DPL = 0, Base = 0x00010000, Size = 0xFFFEF * 4KB, 80386

gdt_end:

gdt_desc:
dw gdt_end - gdt_start - 1
dd gdt_start
dw 0 ;Don't know why this could be important, but somewhere you're able to see that

VBEInfoBlock:
VbeSignature db "VBE2"
VbeVersion dw 0
VbeOEMStringPtr dd 0
VbeCapabilities dd 0
VbeVideoModePtr dd 0
VbeTotalMemory dw 0
VbeOEMSoftwareRev dw 0
VbeOEMVendorNamePtr dd 0
VbeOEMProductNamePtr dd 0
VbeOEMProductRevPtr dd 0
VbeReserver  db 222 DUP(0)
VbeOEMData  db 256 DUP(0)

VbeModeInfoBlock:
VbeModeModeAttributes dw 0
VbeModeWinAAttributes db 0
VbeModeWinBAttributes db 0
VbeModeWinGranularity dw 0
VbeModeWinSize dw 0
VbeModeWinASegment dw 0
VbeModeWinBSegment dw 0
VbeModeWinFuncPtr dd 0
VbeModeBytesPerScanLine dw 0
VbeModeXResolution dw 0
VbeModeYResolution dw 0
VbeModeXCharSize db 0
VbeModeYCharSize db 0
VbeModeNumberOfPlanes db 0
VbeModeBitsPerPixel db 0
VbeModeNumberOfBanks db 0
VbeModeMemoryModel db 0
VbeModeBankSize db 0
VbeModeNumOfImagePages db 0
VbeModeReserved_page db 0
VbeModeRedMaskSize db 0
VbeModeRedMaskPos db 0
VbeModeGreenMaskSize db 0
VbeModeGreenMaskPos db 0
VbeModeBlueMaskSize db 0
VbeModeBlueMaskPos db 0
VbeModeReservedMaskSize db 0
VbeModeReservedMaskPos db 0
VbeModeDirColorModeInfo db 0
VbeModePhysBasePtr dd 0
VbeModeOffScreenMemOffs dd 0
VbeModeOffScreenMemSize dw 0
VbeModeLinByPerScanLine dw 0
VbeModeBnkNumberOfPages db 0
VbeModeLinNumberOfPages db 0
VbeModeLinRedMaskSize db 0
VbeModeLinRedFieldPos db 0
VbeModeLinGreenMaskSize db 0
VbeModeLinGreenFieldPos db 0
VbeModeLinBlueMaskSize db 0
VbeModeLinBlueFieldPos db 0
VbeModeLinRsvdMaskSize db 0
VbeModeLinRsvdFieldPos db 0
VbeModeMaxPixelClock dd 0
VbeModeReserved  db 190 DUP(0)

IsVESA db 0

start:

;Here follows other system code    

00000450000i[WGUI ] dimension update x=720 y=400 fontheight=16 fontwidth=9 bpp=8
00000695508e[HD   ] device set to 0 which does not exist
00000695801e[HD   ] device set to 1 which does not exist
00000802818i[CLVGA] VBE set bpp (24)
00000802840i[CLVGA] VBE set xres (800)
00000802921i[CLVGA] VBE set yres (600)
00000802959i[CLVGA] VBE enabling x 800, y 600, bpp 24, 1440000 bytes visible
00000802959i[WGUI ] dimension update x=800 y=600 fontheight=0 fontwidth=0 bpp=24
00003303000p[WGUI ] >>PANIC<< POWER button turned off.
00003303000i[SYS  ] Last time is 1186562163
00003303000i[CPU0 ] real mode
00003303000i[CPU0 ] CS.d_b = 16 bit
00003303000i[CPU0 ] SS.d_b = 16 bit
00003303000i[CPU0 ] | EAX=41530000  EBX=00004115  ECX=000a0115  EDX=00000000
00003303000i[CPU0 ] | ESP=00000000  EBP=00007c00  ESI=0000029c  EDI=0000803a
00003303000i[CPU0 ] | IOPL=0 NV UP DI PL ZR NA PE NC
00003303000i[CPU0 ] | SEG selector     base    limit G D
00003303000i[CPU0 ] | SEG sltr(index|ti|rpl)     base    limit G D
00003303000i[CPU0 ] |  CS:1000( 0000| 0|  0) 00010000 0000ffff 0 0
00003303000i[CPU0 ] |  DS:0000( 0000| 0|  0) 00000000 0000ffff 0 0
00003303000i[CPU0 ] |  SS:0000( 0000| 0|  0) 00000000 0000ffff 0 0
00003303000i[CPU0 ] |  ES:0000( 0000| 0|  0) 00000000 0000ffff 0 0
00003303000i[CPU0 ] |  FS:0000( 0000| 0|  0) 00000000 0000ffff 0 0
00003303000i[CPU0 ] |  GS:0000( 0000| 0|  0) 00000000 0000ffff 0 0
00003303000i[CPU0 ] | EIP=000001bb (000001bb)
00003303000i[CPU0 ] | CR0=0x00000010 CR1=0 CR2=0x00000000
00003303000i[CPU0 ] | CR3=0x00000000 CR4=0x00000000
00003303000i[     ] restoring default signal behavior
00003303000i[CTRL ] quit_sim called with exit code 1