Memory - Upgrading and Repairing PCs - page 399

To meet the stringent timing demands of its chipsets, Intel created specifications for
SDRAM called PC66, PC100, and PC133. For example, you would think 10ns would be
consideredtheproperratingfor100MHzoperation,butthePC100specificationpromoted
by Intel called for faster 8ns memory to ensure all timing parameters could be met with
sufficient margin for error.
In May 1999, JEDEC created a specification called PC133. It achieved this 33MHz speed
increase by taking the PC100 specification and tightening up the timing and capacitance
parameters. The faster PC133 quickly caught on for any systems running a 133MHz pro-
cessor bus. The original chips used in PC133 modules were rated for exactly 7.5ns or
133MHz; later ones were rated at 7.0ns, which is technically 143MHz. These faster chips
were still used on PC133 modules, but they allowed for improvements in column address
strobe latency (abbreviated as CAS or CL), which somewhat improves overall memory
cycling time.
SDRAM normally came in 168-pin DIMMs, running at several speeds. Table 6.4 shows
the standard single data rate SDRAM module speeds and resulting throughputs.
Table 6.4 JEDEC Standard SDRAM Module (168-Pin DIMM) Speeds and Transfer Rates
See “ Memory Modules ,” p. 333 (this chapter).
Some module manufacturers sold modules they claimed were “PC150” or “PC166,” even
though those speeds did not exist as official JEDEC or Intel standards, and no chipsets
or processors officially supported those speeds. These modules actually used hand-picked
133MHz-rated chips that could run overclocked at 150MHz or 166MHz speeds. In es-
sence,PC150orPC166memorywasPC133memorythatwastestedtorunatoverclocked
speeds not supported by the original chip manufacturer. This overclockable memory was
sold at a premium to enthusiasts who wanted to overclock their motherboard chipsets,
thereby increasing the speed of the processor and memory bus.