From: jim AT curved-logic DOT com (James Shaw)
Newsgroups: comp.os.msdos.programmer,comp.lang.asm.x86,comp.os.msdos.djgpp,comp.unix.pc-clone.32bit
Subject: Re: FPU and protected mode
Date: Thu, 27 Mar 1997 12:05:42 GMT
Organization: None
Lines: 95
Message-ID: <333d6018.5876843@snews2.zippo.com>
References: <3339DFBB DOT 3A4B AT turbotek DOT co DOT kr>
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To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

Heo Sung-Gwan <heo AT turbotek DOT co DOT kr> wrote:

>Hi,
>
>I am coding FPU context switch with Pentium processor using fnsave and
>frstor FPU instructions. And I found these FPU instructions behaves
>differently between real mode and protected mode.
>
>See the following code:
>
>	int	i;
>	char	fpusave[124];
>
>	fpusave[0] = 0x7f;	fpusave[1] = 0x03;
>	fpusave[2] = 0x00;	fpusave[3] = 0x00;
>	fpusave[4] = 0xff;	fpusave[5] = 0xff;
>
>	asm  {
>		frstor	fpusave
>		fwait
>		fnsave	fpusave
>		fwait
>	}
>	for( i = 0; i < 124; i++ ) 
>		printf("%x ", fpusave[i]);		
>
>When compiled with bcc -B, it output 7f 03 00 00 ff ....
>but when compiled with bcc32 and PMC protectd library, it output 7f 03
>ff ff 7f 7f .....
>Also when compiled with djgpp in DOS and gcc in linux after inline
>assembly code is modified, it output the same as bcc32 and PMC lib: 7f
>03 ff ff 7f 7f .... 
>
>What happened to FPU in protected mode?

The data structures saved out by frstor are different in protected
mode and real mode.  In real mode the structure is 94 bytes, in pmode
it's 108 bytes.

The two values you notice that have changed are marked as reserved
(and so could contain anything).

Real Mode structure
controlword:WORD
statusword:WORD
tagword:WORD
ipoffset:WORD
csselector:WORD
operandoffset:WORD
operandselector:WORD

V86 structure
controlword:WORD
statusword:WORD
tagword:WORD
ip bits 0-15:WORD
ip bits 0-19,0,opcode bits 0-10:WORD  (opcode is lower bits, ip is
higher bits)
operand ptr bits 0-15:WORD
operand ptr bits 16-19,12 0's:DWORD (ditto)

32bit Real Mode
controlword:WORD
reserved:WORD
statusword:WORD
reserved:WORD
tagword:WORD
reserved:WORD
ip bits 0-15:WORD
reserved:WORD
0,0,0,0,ip bits 16-31,0,opcode bits 0-10:DWORD
operand ptr bits 0-15:WORD
reserved:WORD
0,0,0,0,operand ptr bits 16-31,12 0's:DWORD (hi->low bit order)

32bit Protected Mode
controlword:WORD
reserved:WORD
statusword:WORD
reserved:WORD
tagword:WORD
reserved:WORD
ip offset:DWORD
cs selector:WORD
0,0,0,0,opcode bits 0-10:WORD
data operand offset:DWORD
operand selector:WORD
reserved:WORD

All this comes from
Pentium Processor Family Developer's Manual
Volume 3: Architecture and Programming Manual
Ch6 pp17-18

Jim