The Chip That Ate My Batteries

No sooner do battery developers give us a better power-to-weight ratio than CPU makers chew up the extra power to give us a constant 1-hour notebook-battery life. Why not give us a simpler, less powerful processor that gobbles less power?

In "Keeping It Simple" (October 1997 Core), Tom R. Halfhill says the C6 is simpler. It is neither superscalar nor superpipelined, which means fewer transistors. No branch prediction means even fewer transistors.

But for all that, the C6 has more transistors than the Pentium P55C. And, although it has more transistors, its die size is 37 percent smaller. One would th ink this is because caches are more regular and more dense. But 32KB of cache accounts for maybe 800,000 transistors -- not enough to account for the greater number of transistors, nor the reduction in chip size.

This article makes it look simple, but it begs many questions. And, by the time I got to the third column, it was already clear that I still do not have my wish for a suitable notebook chip.

K. C. Toh
Petaling Jaya, Malaysia

The reason why the C6 has more transistors than a P55C-series Pentium (5.4 million versus 4.5 million) is indeed the larger caches. The C6 has 32KB each of instruction cache and data cache. It's possible to estimate how many transistors that 64KB accounts for: 64KB = 524,288 bits. Static RAM (SRAM) cells typically have six transistors per bit when used this way. That totals 3.1 million transistors, without considering the control logic associated with the caches.

The large proportion of transistors in the caches (more than 50 percent of the chip's total) indeed does account for the C6's tiny die size. As you correctly point out, memory occupies less space than logic because it's more dense, but you've underestimated the amount of memory.

Despite the C6's simpler design, the new Tillamook-class Pentiums still consume less power at the same clock frequency. That's because the Tillamooks are manufactured on a 0.25-micron process, while the C6 is still at 0.35 micron. The Tillamook can run at a lower voltage.

You make a good point about simple designs rarely remaining simple. The C6, for instance, will add branch prediction, a better FPU, and better MMX capability this year, as well as an integrated L2 cache. However, its die size and price will remain small, because Centaur is aiming the C6 at the low-end PC market.

Note the new address for Centaur's Web site: http://www.winchip.com/. -- Tom R. Halfhill, senior editor