...since it supposedly improves the FPU performance. 
Get better x87 performance out of your Pentium 4! (INTEL)
Posted By Johan on Ace's Hardware...
Monday, May 14, 2001 - 8:41:20 AM
"A few days ago, I was testing with the Pentium 4 1.5 GHz and the AOPEN AX4T motherboard. You might remember from our last "Pentium 4 in depth" article that the Pentium's 4 x87 FPU performance was rather mediocre.
When I tested with Test 1 of the Flops benchmark, I found out that the x87 FPU performance of my test system was much lower than the results that I got on Intel's review system. The P4 system I build scored only 464 in first test, while Intel's almost identical system scored 541, or no less than 17% higher! As you know, the Flops benchmark runs totally in the L1-cache and the results depend only on the clockspeed and FP architecture of the tested CPU.
Than I stumbled upon a "FPU OPCode compatibility" BIOS option. When I disabled this option, the flops result was back at 541. Sandra Kuo of AOPEN send me this interesting comment:
FPU OP Code is a BIOS emulation for solving backward compatibility issues under old MS-DOS mode, and some performance is lost if the feature is enabled. [...]
Based on Intel recommendation, the default setting of " Compatibile FPU OPCODE" is enabled.
So it is possible that many Pentium 4 systems out there offer worse x87 performance than they should. Very few people run old DOS programs today, so if this BIOS option is available, disable it! If you disable the bios option "FPU OpCode Compatible", you get up to 17% x87 FPU performance for free!"
Conrad Song has found more information about the FPU opcode compatibility issue:
I've tracked down the issue... it can be found in Section 8.1.8 of the IA-32 Intel Architecture Software Developer?s Manual Volume 1: Basic Architecture. Also, a reference to it exists in The IA-32 Intel Architecture Software Developer?s Manual, Volume 3: System Programming Guide under Appendix B, register 0x1A0 (IA32_MISC_ENABLE).
8.1.8.1. FOPCODE COMPATIBILITY MODE Beginning with the Pentium 4 processors, the IA-32 architecture provides program control over the storing of the last instruction opcode (sometimes referred to as the fopcode). When the fopcode compatibility mode is disabled (default), the value stored in the x87 FPU opcode register is undefined (reserved). The benefit of disabling the fopcode compatibility mode is better processor performance."
A thread on this is posted in CPU/Overclocking by lifeguard1999.
Get better x87 performance out of your Pentium 4! (INTEL)
Posted By Johan on Ace's Hardware...
Monday, May 14, 2001 - 8:41:20 AM
"A few days ago, I was testing with the Pentium 4 1.5 GHz and the AOPEN AX4T motherboard. You might remember from our last "Pentium 4 in depth" article that the Pentium's 4 x87 FPU performance was rather mediocre.
When I tested with Test 1 of the Flops benchmark, I found out that the x87 FPU performance of my test system was much lower than the results that I got on Intel's review system. The P4 system I build scored only 464 in first test, while Intel's almost identical system scored 541, or no less than 17% higher! As you know, the Flops benchmark runs totally in the L1-cache and the results depend only on the clockspeed and FP architecture of the tested CPU.
Than I stumbled upon a "FPU OPCode compatibility" BIOS option. When I disabled this option, the flops result was back at 541. Sandra Kuo of AOPEN send me this interesting comment:
FPU OP Code is a BIOS emulation for solving backward compatibility issues under old MS-DOS mode, and some performance is lost if the feature is enabled. [...]
Based on Intel recommendation, the default setting of " Compatibile FPU OPCODE" is enabled.
So it is possible that many Pentium 4 systems out there offer worse x87 performance than they should. Very few people run old DOS programs today, so if this BIOS option is available, disable it! If you disable the bios option "FPU OpCode Compatible", you get up to 17% x87 FPU performance for free!"
Conrad Song has found more information about the FPU opcode compatibility issue:
I've tracked down the issue... it can be found in Section 8.1.8 of the IA-32 Intel Architecture Software Developer?s Manual Volume 1: Basic Architecture. Also, a reference to it exists in The IA-32 Intel Architecture Software Developer?s Manual, Volume 3: System Programming Guide under Appendix B, register 0x1A0 (IA32_MISC_ENABLE).
8.1.8.1. FOPCODE COMPATIBILITY MODE Beginning with the Pentium 4 processors, the IA-32 architecture provides program control over the storing of the last instruction opcode (sometimes referred to as the fopcode). When the fopcode compatibility mode is disabled (default), the value stored in the x87 FPU opcode register is undefined (reserved). The benefit of disabling the fopcode compatibility mode is better processor performance."
A thread on this is posted in CPU/Overclocking by lifeguard1999.
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