Chapter 11. The L2 cache

The level 2 cache is normally much bigger (and unified), such as 256, 512 or 1024 KB. The purpose of the L2 cache is to constantly read in slightly larger quantities of data from RAM, so that these are available to the L1 cache.

In the earlier processor generations, the L2 cache was placed outside the chip: either on the motherboard (as in the original Pentium processors), or on a special module together with the CPU (as in the first Pentium II’s).

Fig.  71. An old Pentium II module. The CPU is mounted on a rectangular printed circuit board, together with the L2 cache, which is two chips here. The whole module is installed in a socket on the motherboard. But this design is no longer used.

As process technology has developed, it has become possible to make room for the L2 cache inside the actual processor chip. Thus the L2 cache has been integrated and that makes it function much better in relation to the L1 cache and the processor core.

The L2 cache is not as fast as the L1 cache, but it is still much faster than normal RAM.

CPU	L2 cache
Pentium, K5, K6	External, on the motherboard
Pentium Pro	Internal, in the CPU
Pentium II, Athlon	External, in a module close to the CPU
Celeron (1st generation)	None
Celeron (later gen.), Pentium III, Athlon XP, Duron, Pentium 4	Internal, in the CPU

Fig.  72. It has only been during the last few CPU generations that the level 2 cache has found its place, integrated into the actual CPU.

Traditionally the L2 cache is connected to the front side bus. Through it, it connects to the chipset’s north bridge and RAM: