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Index > OS Construction > 16bit opcodes

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Bluebat



Joined: 18 Mar 2006
Posts: 2
Bluebat
I have a problem with fasm. I write in my kernel mov eax,0x10001 (after going to pmode ang "use32"), then cli hlt and run it in bochs. In bochs log, there is EAX=00000001 (it is cut-off to 16bit). Then I decompiled it, and I see mov ax,0x0001 Razz
Why is that not what I written? Tried on small files (only this mov eax,0x10001 and still doesn't work)
Does someone know, what to do to get 32bit opcodes? Please, help[/code]
Post 18 Mar 2006, 13:27
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Tomasz Grysztar



Joined: 16 Jun 2003
Posts: 7801
Location: Kraków, Poland
Tomasz Grysztar
Make sure that you disassemble with the same bitness as you assembled it (32 in this case). Many disassemblers (like HIEW or BIEW, for quick example) disassemble flat binary files as "use16" by default.
Post 18 Mar 2006, 13:38
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Bluebat



Joined: 18 Mar 2006
Posts: 2
Bluebat
I disasembled it with ndisasm (nasm package) and BOCHS (cli hlt just before it). And also BOCHS executes it as mov ax,0x0001 even if in protected mode.
Post 18 Mar 2006, 17:01
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Tomasz Grysztar



Joined: 16 Jun 2003
Posts: 7801
Location: Kraków, Poland
Tomasz Grysztar
So either it is the 16-bit protected mode, or you have assembled it to 16-bit (without "use32").
Post 18 Mar 2006, 17:13
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Madis731



Joined: 25 Sep 2003
Posts: 2140
Location: Estonia
Madis731
mov eax,0x10001 should assemble to 6 bytes in 16-bit mode and 5 bytes in 32-bit mode. Check it!
Post 20 Mar 2006, 08:41
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