The AMD-K5 is a Pentium-compatible processor developed by AMD and available as the PR75, PR90, PR100, PR120, PR133, and PR-166. Because it is designed to be physically and functionally compatible, any motherboard that properly supports the Intel Pentium should support the AMD-K5.

However, a BIOS upgrade might be required to properly recognize the AMD-K5. The K5 has the following features:

• 16KB instruction cache, 8KB write-back data cache

• Dynamic execution—branch prediction with speculative execution

• Five-stage, RISC-like pipeline with six parallel functional units

• High-performance floating-point unit

• Pin-selectable clock multiples of 1.5x and 2x

The K5 is sold under the P-Rating system, which means that the number on the chip does not indicate true clock speed, only apparent speed when running certain applications.

Note that the actual clock speeds of several of these processors are not the same as their apparent rated speeds. For example, the PR-166 version actually runs at only 117 true MHz. Sometimes this can confuse the system BIOS, which might report the true speed rather than the P-Rating, which compares the chip against an Intel Pentium of that speed.

AMD's assertion is that because of architecture enhancements over the Pentium, they do not need to run the same clock frequency to achieve that same performance. Even with such improvements, AMD marketed the K5 as a fifth-generation processor, just like the Pentium.

The AMD-K5 operates at 3.52 volts (VRE setting). Some older motherboards default to 3.3 volts, which is below specification for the K5 and could cause erratic operation. Because of the relatively low clock speeds and compatibility issues some users experienced with the K5, AMD replaced it with the K6 family of processors.