Inside the Machine

Caches and Front End

Let’s take a short look at the caches for the 970 and the G4e. Table 10-1 should give you a rough idea of how the two chips compare.

Table 10-1. The Caches of the PowerPC 970 and G4e

	L1 I-cache	L1 D-cache	L2 Cache
PowerPC 970	64KB, direct-mapped	32KB, two-way assoc.	512KB, eight-way assoc.
G4e	32KB, eight-way assoc.	32KB, eight-way assoc.	256KB, eight-way assoc.

Table 10-1 shows that the 970 sports a larger instruction cache than the G4e. This is because the 970’s pipeline is roughly twice the length of the G4e’s, which means that like the Pentium 4, the 970 pays a much higher performance penalty when its pipeline stalls due to a miss in the I-cache. In short, the 970’s 64KB I-cache is intended to keep pipeline bubbles out of the chip’s ...