Note in Table 3.4 that the Athlon includes either 512KB of L2 cache viaseparate chips, running at either one-half, two-fifths, or one-third the corespeed, or 256KB of on-die L2 running at full-core speed, depending on whichversion you have. The external L2 cache in those processors containsan additional 15.5 (256KB), 31 (512KB), 62 million (1MB), or 124 million (2MB)transistors in separate chips! The transistor count figures do not include the external (off-die) 256KB,512KB, 1MB, or 2MB L2 cache built in to the Pentium Pro, Pentium II/III, Xeon,or AMD Athlon CPU packages. The Celeron IIIuses the same die as the Pentium IIIE, however half of the on-die cache isdisabled, leaving 128KB functional. The Celeron, Pentium II PE, and Pentium IIIE processors include full-corespeed L2 cache integrated directly within the processor die. The PentiumII/III processors include 512KB of 1/2 core speed L2 cache on the processorcard. Note in Table 3.3 that the Pentium Pro processor includes 256KB, 512KB, or1MB of full-core speed L2 cache in a separate die within the chip. Table 3.4 lists the Intel-compatible processorsfrom AMD, Cyrix, NexGen, IDT, and Rise. Table 3.3 lists the primary specifications for the Intel family of processorsused in IBM and compatible PCs. This type of design was then quickly adopted by the secondgeneration Pentium III, as well as the AMD K6-3, Athlon, and Duron processors.In fact virtually all future processors from Intel and AMD have adopted or willadopt on-die L2 cache as it is the only cost-effective way to include the L2 andbring the speed up. A breakthroughoccurred in the second-generation Celeron, where Intel built both the L1 and L2caches directly on the processor die, where they both ran at the full-core speedof the chip. Intel built its own high-speed cache memory chipsfor the Xeon processors, but it also made them very expensive. The problem originally forcing the L2 cache to run at less thanthe processor core speed was simple: The cache chips available on the marketsimply couldn't keep up. The Pentium Pro, Pentium II/III Xeon, newer Pentium III,Celeron, K6-3, Athlon (Model 4), and Duron processors include full-core speed L2as shown in Table 3.2. The original Pentium II, III, Celeron, and Athlon (Model 1 and 2) processorsuse 512KB of either one-half, two-fifths, or one-third speed L2 cache as Table3.1 shows: Table 3.1 L2 Cache Speeds Since thespeed of commercially available cache memory chips could not keep pace with themain processor, most of the L2 cache in these processors ran at one-half speed(Pentium II/III and AMD Athlon), while some ran the cache even slower, attwo-fifths or even one-third the processor speed (AMD Athlon). Originallythis built-in L2 cache was implemented as physically separate chips containedwithin the processor package but not a part of the processor die. Starting with the Pentium Pro and Pentium II, Intel began including L2 cachememory chips directly within the same package as the main processor. During this time the cache on the motherboard was called the second levelor L2 cache, which ran at the slower motherboard speed. This meant that the L1 cachealways ran at the full speed of the chip, especially important when the later486 chips began to run at speeds higher than the motherboards they were pluggedinto. Starting with the 486 series, processors began including what was called L1(Level 1) cache directly on the processor die. Prior to the 486 processor, the cacheon the motherboard was the only cache used in the system. Starting with 16MHzsystems, high-speed cache memory appeared on the motherboard because the mainmemory at the time could not run at 16MHz. Systems below 16MHz usually had no cache memory at all. Speed iscounted in megahertz (MHz), which means millions of cycles per second—andfaster is better! The width of a processor is a little more complicated todiscuss because there are three main specifications in a processor that areexpressed in width. The speed of a processor is a fairly simple concept. Processors can be identified by two main parameters: how wide they are andhow fast they are. The next section includes a table that lists thespecifications of virtually all PC processors. Many confusing specifications often are quoted in discussions of processors.The following sections discuss some of these specifications, including the databus, address bus, and speed.
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