When running legacy applications, a fast Pentium may outperform the first P6 processors
Tom R. Halfhill
Trick question: When is a Pentium
faster
than a P6? Surprise answer: When it's running 16-bit software, including DOS and Windows 3.1.
Intel's latest benchmark testing reveals that a 133-MHz Pentium consistently outruns a 150-MHz P6 when executing the 16-bit code found in today's most popular software. Even a 100-MHz Pentium runs neck and neck with a 150-MHz P6.
Theoretically, the sixth-generation P6 chip should blow the fifth-generation Pentium out of the water. The P6 has three-way superscalar superpipelines, speculative execution, out-of-order execution, additional registers, 2.2 million more transistors, more headroom for higher clock speeds, a closely coupled secondary cache,
and a higher price tag (see "Intel's P6," April BYTE). But some of those fancy features actually
slow down
the P6 when running 16-bit code.
The problem, says Intel, is with today's installed base of software, not with the chip. The P6 is optimized for 32 bits. When Intel engineers began designing the P6 about four years ago, they figured everyone would be running 32-bit software by now. After all, Intel's first 32-bit x86 processor (the 386) dates back to 1985. But the industry hasn't moved quite as fast as Intel and others expected: Most PCs today run 16-bit Windows. When Intel ran the SysMark application-level benchmark programs on a P6, old-generation software embarrassed Intel's next-generation chip.
It's certainly not unusual for a new processor to deliver less-than-optimum performance unless old software is recompiled to take advantage of the new design. That's especially true of RISC processors. While the P6 is still a CISC chip, it adopts several RISC-like techniques. However, it
's definitely unusual for a new CPU to run old software
slower
than existing CPUs that share the same basic architecture. (For more information on why this is, see the sidebar "Why Legacy Code Snags the P6.")
The P6 lives up to expectations with 32-bit code.
Intel's benchmarks show
that it easily outperforms the fastest Pentiums when running 32-bit applications on a 32-bit OS, such as Windows 95 or Windows NT. Interestingly, however, the P6 does much better with NT than it does with Windows 95. Intel says that there are vestiges of 16-bit code in the Windows GDI (Graphical Device Interface), while NT is thoroughly 32-bit.
The P6's poor showing with 16-bit software is probably not as serious as it seems. High prices will initially limit the P6 to servers and workstation-class desktop systems, whose performance-minded users will almost certainly be running 32-bit OSes and applications. If the P6 follows an adoption curve similar to the Pentium's, it will not appear
in mainstream PCs until 1997. By then, 80 percent of new PCs will ship with a 32-bit OS, according to International Data (Framingham, MA). And Windows 95 should accelerate the migration to 32 bits.
Intel says the P6 will get a performance boost when the company moves from its current 0.6- to 0.35-micron process. That raw performance boost should let the P6 outperform the Pentium in running legacy 16-bit software. Until then, anyone who is contemplating the purchase of a P6 should be forewarned: If you're running 16-bit software, the Pentium delivers more bang for fewer bucks.
illustration_link (26 Kbytes)
The P6 outruns the fastest Pentiums when measured by low-level 32-bit benchmarks, as seen in these SPECmark estimates from Intel (see graph on left). Although Intel initially planned to introduce the P6 at 133MHz, it's now likely the chip will debut this fall at speeds of 133, 150, and 166 MHz.
Although the P6 outruns the fastest Pentiums in low-level 32-bit benchmarks, a 150-MHz P6 is outraced by a 133-MHz Pentium and matched by a 100-MHz Pentium when running 16-bit programs under Windows 3.1, as seen in these preliminary benchmarks from Intel (see graph on right). With 32-bit Windows 95 or Windows NT, however, the P6 meets expectations. As of this writing, Intel has not permitted BYTE to run our own benchmarks on the P6.
P6 Weakness Revealed
