Use BIOS funcions from long mode ---

supernicky · **Joined:** Sun Apr 03, 2016 5:59 am **Posts:** 1

Hello,

in my OS (64bit) I would change the Displayresolution with the bios int 10h.

To switch I use the following code: viewtopic.php?f=1&t=23125

Code:

use64                  ;
bios_call:   
   cli                                                           ;Interrupts off
      ;save all GPR's
      push rax
      push rbx
      push rcx
      push rdx
      push rsi
      push rdi
      push r8
      push r9
      push r10
      push r11
      push r12
      push r13
      push r14
      push r15
      
      mov rbx, _old_esp                             ;save Stackpointer
      mov [rbx], rsp
      
      mov rsp, 6800h                                 ;new Stackpointer
      
      sidt [System.oldidt]      ;save IDT
      
      push 40h            ;Realmode Desc 64k laden
      push @f
      retfq
         
use16                  ;16 Bit P-Mode
@@:      
      
      ;PE und PM off
      mov eax, cr0
      and eax, 7FFFFFFEh
      mov cr0, eax
                                                            ;ESP to RM Addr
      mov esp, 6700h
      
                                                            ;ds, es, ss = NULL
      xor eax, eax
      mov ds, ax
      mov es, ax
      mov fs, ax
      mov gs, ax
      mov ss, ax
      
      ;Far Jump 
      jmp 0x0:@f
      
@@:      

      ;Realmode IDT
      lidt [System.nullidt]
;---------------------------------------------------------------------------------------------------------------------
                           ;BIOS Calls here
;---------------------------------------------------------------------------------------------------------------------
      ;PE und PM on
      mov eax, cr0
      or eax, 80000001h
      mov cr0, eax      
      
      jmp @f
@@:
      
      ; Enable long mode by setting the EFER.LME flag in MSR 0xC0000080 
      mov ecx, 0C0000080h
      rdmsr
      or eax, 100h
      wrmsr
      
      

      db 66h, 0eah
      dd _p64_mode3
      dw 28h

use64      
_p64_mode3:
      jmp @f
@@:
      
      lidt [System.oldidt]      
      
      ;restore Stackpointer      
      mov rbx, _old_esp
      mov rsp, [rbx]
                            ;restore all GPR's      
      pop r15
      pop r14
      pop r13
      pop r12
      pop r11
      pop r10
      pop r9
      pop r8
      pop rdi
      pop rsi
      pop rdx
      pop rcx
      pop rbx
      pop rax
   sti                                       ;Interrupt on
ret                                                     ;Exit
align 16
_old_esp dq 0
align 4
_modus_old dw 0
_oldtr dw 0

The Code works fine without Bios Calls, but when i am insert a Bios Call (e.g.

Code:

mov ax, 12h
int 10h

), the processor shutdown.

what could be the Problem?

Another Question about the GDT:

Code:

System:
.GDT:
      dd 0,0
._CODE32:               ;0x8
      dw 0xFFFF
      dw 0                       
      db 0                          
      db 10011010b
      db 11001111b         ;DB und G Bit gesetzt
      db 0
._DATA32:               ;0x10
      dw 0xFFFF
      dw 0               
      db 0                          
      db 10010010b
      db 11001111b         ;DB und G Bit gesetzt
      db 0   
._CODE64:            ;0x28
      dw 0xFFFF
      dw 0                       
      db 0                          
      db 10011010b
      db 10101111b         ;DB und G Bit gesetzt
      db 0      
      
._TSS64:   dw 104d         ;0x30
      dw 0000h
      db 1h
      db 10001001b
      dw 00000000b
._CODE16:            ;0x40
      dw 0FFFFh        ; compatibility mode code descriptor
      dw 0
      db 0
      db 10011010b
      db 0h
      db 0
._DATA16:               ;0x30
      dw 0FFFFh        ; compatibility mode data descriptor
      dw 0
      db 0
      db 10010010b
      db 0h
      db 0
;      dd 0, 0

In P-Mode VBox reads as follows:

Quote:

0008 CodeER Bas=00000000 Lim=fffff000 DPL=0 P A G BIG AVL=0 L=0
0010 DataRW Bas=00000000 Lim=fffff000 DPL=0 P A G BIG AVL=0 L=0
0018 CodeER Bas=00000000 Lim=fffff000 DPL=3 P NA G BIG AVL=0 L=0
0020 DataRW Bas=00000000 Lim=fffff000 DPL=3 P NA G BIG AVL=0 L=0
0028 CodeER Bas=00000000 Lim=fffff000 DPL=0 P NA G AVL=0 L=1
0030 Tss32A Bas=00010000 Lim=00000068 DPL=0 P NB AVL=0 R=0
0040 CodeER Bas=00000000 Lim=0000ffff DPL=0 P NA AVL=0 L=0
0048 VERR_INVALID_SELECTOR

In L-Mode VBox reads as follows:

Quote:

0008 CodeER Bas=00000000 Lim=ffffffff DPL=0 P A G BIG AVL=0 L=0
0010 DataRW Bas=00000000 Lim=ffffffff DPL=0 P A G BIG AVL=0 L=0
0018 CodeER Bas=00000000 Lim=ffffffff DPL=3 P NA G BIG AVL=0 L=0
0020 DataRW Bas=00000000 Lim=ffffffff DPL=3 P NA G BIG AVL=0 L=0
0028 CodeER Bas=00000000 Lim=ffffffff DPL=0 P A G AVL=0 L=1
0030 Tss64B Bas=0000000000010000 Lim=00000068 DPL=0 P B AVL=0 R=0
003c ConfER Bas=f053f000 Lim=0000ef57 DPL=3 P A AVL=0 L=0
0044 CodeEO Bas=f04dc000 Lim=00000022 DPL=3 P NA AVL=0 L=0
004c Ill-3 41 f8 00 f0 fe e3 00 f0 DPL=3 P
0054 CodeEO Bas=f059f000 Lim=0000e739 DPL=3 P NA AVL=0 L=0
005c Trap64 Sel:Off=f000:f000f0a4f000e82e DPL=3 P
0068 VERR_INVALID_SELECTOR

Why has ._Code64 Index 0x28 and not 0x18?

I am grateful for every advice,
Nicky

If you need more Code let me know.

iansjack · **Posted:** Sun Apr 03, 2016 7:46 am

You can't use BIOS calls from Long Mode.

Octocontrabass · **Joined:** Mon Mar 25, 2013 7:01 pm **Posts:** 5137

supernicky wrote:

what could be the Problem?

Have you tried running this in Bochs yet? The Bochs log will provide tons of useful information about the CPU state when it crashes; that will be very helpful for figuring out what's going wrong.

If all you want to do is set the video mode, there are other methods:

Write a native driver for the video card
Set the video mode in the bootloader
Run the video card's (legacy or UEFI) PCI option ROM in an emulator

iansjack wrote:

You can't use BIOS calls from Long Mode.

Of course you can't. This is switching to real mode first, and then using BIOS calls from real mode.

SpyderTL · **Joined:** Sun Sep 19, 2010 10:05 pm **Posts:** 1074

I think you may be missing a few steps. Check out the Real Mode wiki page for a full list.

rdos · **Joined:** Wed Oct 01, 2008 1:55 pm **Posts:** 3192

It's perfectly possible to call the BIOS (in V86 mode, not real mode) from long mode provided your OS also sets up "shadow" 32-bit protected-mode interrupt handlers. When calling BIOS, you switch the IDT and processor to 32-bit handlers, and then switch to V86 mode. When you return, you switch back the IDT and processor to long mode. I have this feature in my OS, and it works just fine. Microsoft and the Linux community couldn't solve it, but that doesn't mean it can't be done.

Octocontrabass · **Joined:** Mon Mar 25, 2013 7:01 pm **Posts:** 5137

rdos wrote:

Microsoft and the Linux community couldn't solve it, but that doesn't mean it can't be done.

They did solve it. They use an interpreter, instead of switching the CPU mode.

alexfru · **Joined:** Tue Mar 04, 2014 5:27 am **Posts:** 1108

rdos wrote:

It's perfectly possible to call the BIOS (in V86 mode, not real mode) from long mode provided your OS also sets up "shadow" 32-bit protected-mode interrupt handlers. When calling BIOS, you switch the IDT and processor to 32-bit handlers, and then switch to V86 mode. When you return, you switch back the IDT and processor to long mode. I have this feature in my OS, and it works just fine. Microsoft and the Linux community couldn't solve it, but that doesn't mean it can't be done.

Windows gets the necessary drivers, no need for BIOS in most cases. For those odd cases where falling back to BIOS is better than nothing there's an emulator (I'd guess, it was there in one shape or another from the days of Windows NT).

What happens in your system when you go all the way from long mode to v86, do smth in there and then come back? Do you reprogram interrupt handlers? Disable some or all interrupts for many CPU cycles? Change page tables? Invalidate caches (TLB and what not)? Acquire and hold global locks for a long time? IOW, you can do it, but at what cost?

rdos · **Joined:** Wed Oct 01, 2008 1:55 pm **Posts:** 3192

Octocontrabass wrote:

rdos wrote:

Microsoft and the Linux community couldn't solve it, but that doesn't mean it can't be done.

They did solve it. They use an interpreter, instead of switching the CPU mode.

That's not a solution.

Octocontrabass · **Joined:** Mon Mar 25, 2013 7:01 pm **Posts:** 5137

rdos wrote:

That's not a solution.

Why not?

rdos · **Joined:** Wed Oct 01, 2008 1:55 pm **Posts:** 3192

alexfru wrote:

What happens in your system when you go all the way from long mode to v86, do smth in there and then come back? Do you reprogram interrupt handlers?

No. You only need to switch the IDT register. The only limitation is that all the IRQ-handlers must exist in both 32-bit protected mode form and long mode form. When a new IRQ-handler is added, it must be added to both IDT-tables.

alexfru wrote:

Disable some or all interrupts for many CPU cycles? Change page tables? Invalidate caches (TLB and what not)? Acquire and hold global locks for a long time? IOW, you can do it, but at what cost?

No, the only costly operation is to switch mode, which do invalidate the TLB as you need to turn off paging and turn it on again in 32-bit mode (or long mode when you go the other way). There is also a need for a code-stub that is located at a physical address that is mapped to the same linear address where the mode-switching code needs to reside. There is no need for global locks. The switch code itself is linked to the scheduler, and the scheduler knows if a thread is for 64-bit or 32-bit, and would automatically switch between modes when needed. In the current design, 32-bit processes will run in protected mode and 64-bit processes will run in long mode, which is more efficient since 32-bit processes can still use callgates to transfer from usermode to kernel, something that is not supported in compatibility mode within long mode.

gerryg400 · **Posted:** Tue Apr 05, 2016 3:41 am

rdos originally wrote:

It's perfectly possible to call the BIOS (in V86 mode, not real mode) from long mode ....

rdos later wrote:

No, the only costly operation is to switch mode, which do invalidate the TLB as you need to turn off paging and turn it on again in 32-bit mode (or long mode when you go the other way). There is also a need for a code-stub that is located at a physical address that is mapped to the same linear address where the mode-switching code needs to reside. There is no need for global locks. The switch code itself is linked to the scheduler, and the scheduler knows if a thread is for 64-bit or 32-bit, and would automatically switch between modes when needed. In the current design, 32-bit processes will run in protected mode and 64-bit processes will run in long mode, which is more efficient since 32-bit processes can still use callgates to transfer from usermode to kernel, something that is not supported in compatibility mode within long mode.

Rdos, it sounds like you are not actually calling the BIOS from long mode at all. Am I missing something?

rdos · **Joined:** Wed Oct 01, 2008 1:55 pm **Posts:** 3192

gerryg400 wrote:

rdos originally wrote:

It's perfectly possible to call the BIOS (in V86 mode, not real mode) from long mode ....

rdos later wrote:

No, the only costly operation is to switch mode, which do invalidate the TLB as you need to turn off paging and turn it on again in 32-bit mode (or long mode when you go the other way). There is also a need for a code-stub that is located at a physical address that is mapped to the same linear address where the mode-switching code needs to reside. There is no need for global locks. The switch code itself is linked to the scheduler, and the scheduler knows if a thread is for 64-bit or 32-bit, and would automatically switch between modes when needed. In the current design, 32-bit processes will run in protected mode and 64-bit processes will run in long mode, which is more efficient since 32-bit processes can still use callgates to transfer from usermode to kernel, something that is not supported in compatibility mode within long mode.

Rdos, it sounds like you are not actually calling the BIOS from long mode at all. Am I missing something?

The task that calls the BIOS is 32-bit, so you are partly right. However, the feature to be able to run both long mode applications and call the BIOS without any emulation is present. I think that is the main issue, and the switch takes place even if it is the scheduler that does it.

gerryg400 · **Posted:** Tue Apr 05, 2016 4:10 am

Rdos, I get it now. Actually I think I recall reading a thread as you were developing this some time ago.

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Use BIOS funcions from long mode ---

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