[codex]

Fundamentally, having more registers increases the speed of light delays in accessing the register. If it did not, we would just operate on main memory itself. However, two few registers and you lose the ability to perform complex computations efficiently. So I believe it is, indeed, a compromise between speed and a need to maintain scratch state. I would be surprised if Intel and AMD didn't constantly run simulations of common computations in an effort to find the optimal size of all on-chip structures.

[joosters]

That's definitely another factor but I suspect it isn't the limiting factor. Sure, design a chip with a million registers and you'll end up constructing them like RAM. But with orders-of-magnitude fewer registers, 16 or 32 or whatever, the size of the register banks on the CPU can't be that significant to incur speed-of-light style delays, surely?

[sliverstorm]

WITH 16x fewer registers, that equates to about 1 chip's worth of registers (remember a stick of DRAM often has 8-16 individual chips on it). While this is already clearly a huge problem, consider additionally that DRAM is made with trench capacitors, unlike SRAM. DRAM is dramatically slower and more dense than SRAM. So we either sacrifice speed, or bloat our one-chip's-worth of area by a few factors, say x4-8.

Then there's practicalities like sense amp design. Large register arrays are not read in a digital fashion, and current L2 and L3 sizes already press their sense amps to their limits. DRAM also uses sense amps, but the amps are again slower and larger.

http://en.wikipedia.org/wiki/Sense_amplifier

[masklinn]

Probably not, but there are definitely delay effects at play or L2 and L3 cache would be unnecessary, you could just have humongous L1s.