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Posted: **Sat Jun 23, 2007 6:11 am**

I did notice an strange thing while programming a boot sector. I expected that real mode is the same as 16-bit mode (Real mode forces processor to execute instruction like 8088/8086 processors). So for example the dump of one instruction in my program in windows looks like this:

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00 00    ADD byte ptr[eax], al

use raw write on diskette and start it like boot sector, assuming this result:

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00 00    ADD byte ptr[ax], al

but it isn't right. In real mode instruction becomes:

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00 00    ADD byte ptr[bx+si], al

just like protected mode:

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67 00 00    ADD byte ptr[bx+si], al

Is there a list of mod reg r/m in real mode?
I found some information over internet how to convert opcodes to mod reg r/m but it is for protected 32-bit mode.

Sorry about bad english

Posted: **Sat Jun 23, 2007 6:34 am**

Intel manual, Vol 2A, section 2.1.3, Table 2.1 for 16-bit and table 2.2 for 32-bit.

Alternatively, see table 17-2 and 17-3.

Regards,
John.

OSDev.org

mod reg r/m question

mod reg r/m question