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> OS Construction > More Protected/Unreal Mode... |
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edfed 15 Feb 2008, 01:06
Code: use32 ; briefly in protected mode mov edx,8 mov ds,dx ; update hidden descriptor data for DS mov es,dx ; update hidden descriptor data for ES mov fs,dx ; update hidden descriptor data for FS mov gs,dx ; update hidden descriptor data for GS mov ss,dx ; update hidden descriptor data for SS and al,$FE mov cr0,eax jmp short $+2 use16 |
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15 Feb 2008, 01:06 |
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revolution 15 Feb 2008, 01:11
Code: mov cs,dx ; update hidden descriptor data for CS   |
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15 Feb 2008, 01:11 |
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bitRAKE 15 Feb 2008, 01:20
Currently, only need one segment descriptor updated, would need to preserve the other registers if modified. Should correct the limits on DS before returning - just to be safe.
_________________ ¯\(°_o)/¯ “languages are not safe - uses can be” Bjarne Stroustrup |
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15 Feb 2008, 01:20 |
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Goplat 15 Feb 2008, 02:49
Quote: jmp short $+2 No, CS is still 16-bit. This jmp does not actually do anything; you need a far jump to load CS from the GDT. (But if all you're doing is changing the data segments, you don't need a jump at all.) |
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15 Feb 2008, 02:49 |
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bitRAKE 15 Feb 2008, 03:09
Do you know why every BIOS I've looked at uses the short jumps when CS isn't updated? I could assume it is in error, because nothing I've seems to indicate a delay or anything is needed and it does seem to work without it.
Thank you, I've remove the "use32". In protected mode, but not a 32-bit segment. _________________ ¯\(°_o)/¯ “languages are not safe - uses can be” Bjarne Stroustrup |
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15 Feb 2008, 03:09 |
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revolution 15 Feb 2008, 03:43
bitRAKE: I guess my little hint was too obscure. Goplat is correct about the CS thing (although I'm not sure about not needing the jmp), protected mode does not automatically mean 32bit, you can also have 16bit CS descriptors in PM. That is why I suggested to load CS, so that your code inside the use32 block will work as you expected.
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15 Feb 2008, 03:43 |
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bitRAKE 15 Feb 2008, 03:45
It doesn't need to be 32bit code - 16bit code is fine.
This code is tested and working (on my machine): Code: use16 push eax push edx push ds push bp push sp push ss xor eax,eax xor edx,edx pop ax pop dx shl eax,4 ; paragraph to bytes add eax,edx ; add offset to get physical address sub eax,8*2 ; size of GDT pushd $00CF9200 pushd $0000FFFF push 0 ; spacer push eax ; physical address of GDT push 8*2-1 mov bp,sp mov eax,cr0 or al,1 cli lgdt [bp] mov cr0,eax jmp short $+2 mov edx,8 mov ds,dx ; update hidden descriptor data for DS and al,$FE mov cr0,eax jmp short $+2 sti ;###################### DS is zero and availible for 32-bit offsets ; display address of GDT mov eax,[bp+2] call MHEX ; do something with DS:00000000 @@: in al,$60 dec al jne @B ;###################### DS is zero and availible for 32-bit offsets ; clear upper byte of descriptor limit mov bp,sp mov byte [bp+14],0 mov eax,cr0 or al,1 cli lgdt [bp] mov cr0,eax jmp short $+2 mov edx,8 mov ds,dx ; update hidden descriptor data for DS and al,$FE mov cr0,eax jmp short $+2 sti add sp,8*2 ; remove GDT from stack pop bp pop ds pop edx pop eax retf ; output 32-bit hex to screen MHEX: ; change this to write to different parts of screen push eax push edx push cx mov edx,$B8000 mov cx,8 .0: rol eax,4 push eax and al,$F cmp al,10 sbb al,$69 das mov [edx],al pop eax add edx,2 loop .0 pop cx pop edx pop eax retn _________________ ¯\(°_o)/¯ “languages are not safe - uses can be” Bjarne Stroustrup |
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15 Feb 2008, 03:45 |
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revolution 15 Feb 2008, 04:03
bitRAKE wrote: It doesn't need to be 32bit code - 16bit code is fine. |
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15 Feb 2008, 04:03 |
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bitRAKE 15 Feb 2008, 05:42
I'm sorry for the confusion. The only goal is to access the full 32-bit address range without breaking anything else the BIOS is doing before or after this code runs.
_________________ ¯\(°_o)/¯ “languages are not safe - uses can be” Bjarne Stroustrup |
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15 Feb 2008, 05:42 |
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sinsi 15 Feb 2008, 06:10
If you set FS/GS as your 4gig address range you should be OK - the BIOS tends not to use those registers, and the only way to access >64k segments is using either a dword address or a dword register >0000ffff (unusual to see in 16-bit code).
One thing to be wary of is the A20 line - if it's off, you can only access even megabytes. Also, himem.sys and the BIOS INT 15h "move extended memory" usually cheat by setting unreal mode (dunno if they alter FS/GS though). |
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15 Feb 2008, 06:10 |
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