Hardware Reference
In-Depth Information
SDRAM performance is dramatically improved over that of FPM or EDO RAM. However, because
SDRAM is still a type of DRAM, the initial latency is the same, but burst mode cycle times are much
faster than with FPM or EDO. SDRAM timing for a burst access would be 5-1-1-1, meaning that four
memory reads would complete in only eight system bus cycles, compared to 11 cycles for EDO and
14 cycles for FPM. This makes SDRAM almost 20% faster than EDO.
Besides being capable of working in fewer cycles, SDRAM is capable of supporting up to 133MHz
(7.5ns) system bus cycling. Most PC systems sold from 1998 through 2002 included SDRAM
memory.
SDRAM is sold in DIMM form and is normally rated by clock speed (MHz) rather than cycling time
(ns), which was confusing during the initial change from FPM and EDO DRAM.
Figure 6.5
(later in
this chapter) shows the physical characteristics of DIMMs.
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 considered the proper rating
for 100MHz operation, but the PC100 specification promoted 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 processor 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.
346
.
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 essence, PC150 or PC166 memory was
PC133 memory that was tested to run at overclocked 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.
