exit on error: JMP Invalid Descriptor Type 0 \ VOGONS

exit on error: JMP Invalid Descriptor Type 0

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First post, by ThatGuy2244@live.com

Posted on 2011-05-16, 18:10

ThatGuy2244@live.com Offline

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I am trying to make a DOS program that is 32-bit, but when I try to enter 32-bit mode I get an error from DOSbox ("exit on error: JMP Invalid Descriptor Type 0"). I have tried different things to try to fix or isolate this problem and it seems to have nothing to do with the descriptors I set up, but only has a problem with my JMP instruction to a 32-bit descriptor. My code for entering the 32-bit environment is:

1cli
2        lgdt [GDT]
3	mov eax, cr0
4	or eax, 1
5	mov cr0, eax
6        sti
7	jmp 0x08:PMode
8	
9	GDT:
10	dw GDTEnd - GDTStart - 1
11	dd GDTStart
12	
13	GDTStart:
14	dd 0x00000000 ;Descriptor 0 Null
15	dd 0x00000000
16	dw 0xffff ;Descriptor 1 Code
17	dw 0x0000
18	db 0x00
19	db 10011010b
20	db 11001111b
21	db 0x00
22	dw 0xffff ;Descriptor 2 Data
23	dw 0x0000
24	db 0x00
25	db 10010010b
26	db 11001111b
27	db 0x00
28	GDTEnd:

My descriptors are set up to be a flat memory model for data and code.

Reply 1 of 6, by wd

Posted on 2011-05-16, 18:17

wd Offline

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Can you attach the compiled executable as well?

Reply 2 of 6, by ThatGuy2244@live.com

Posted on 2011-05-16, 19:08

ThatGuy2244@live.com Offline

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Sure, it's attached also I attached part of my source code and after the switch to 32-bit mode I just made the program idle the computer so just shut down DOSbox to exit the program. Also this program is meant to run as a .com file and was assembled with nasm.

-- I reran the program and the error is actually: "Exit to error: JMP Invalid Descriptor Type 10" It used to be Type 0, but it seems to be Type 10 now.

Reply 3 of 6, by wd

Posted on 2011-05-16, 20:27

wd Offline

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The GDT is wrong, the setup is limit(word) and base(dword) but your executable has
the offset of the label at that adress only. If you use the dosbox debugger to step through
your code you can use GDT right after the LGDT instruction to check the contents.

Reply 4 of 6, by ThatGuy2244@live.com

Posted on 2011-05-16, 21:43

ThatGuy2244@live.com Offline

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How do I use the DOSbox debugger--do I download it or is it built in, if so how do I enable it?

Reply 5 of 6, by wd

Posted on 2011-05-16, 21:48

wd Offline

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If you can compile dosbox from sources you can enable it through a configure
switch
or in config.h (you'll need the curses library for that). The dev forum may have
a pre-built executable.

Reply 6 of 6, by ThatGuy2244@live.com

Posted on 2011-05-17, 14:48

ThatGuy2244@live.com Offline

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Joined: 2011-05-16, 17:58

Ok, I have DOSbox debugger v.7.2 and I'll take a look at how to set up a GDT again, thanks.

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