(^136) PC Hardware: A Beginner’s Guide
Having more RAM in the PC does not improve the overall speed of the processor, but it
does improve how much data the processor can access without the need to go to the slower
hard disk drive. You may have heard that adding RAM to a slow PC will speed it up. Yes,
butonlybecausetheprocessorwasabletoperformfasterinput/output(I/O)operations.
Memory Speeds
On older, pre-Pentium PCs, RAM speeds were in the range of 80 to 120ns (the higher
number is the lower speed). Pentium and equivalent PCs have RAM speeds of 60ns or
lower (faster). For the best results, RAM speeds should be matched to the speed of the
motherboard’s bus. Typically, a motherboard’s documentation contains information on
the RAM speed it requires and supports.
NOTE: When it comes to memory speeds, higher means slower, so 120ns is slower than 60ns.
Memory Latency and Burst Mode
Memory is arranged in rows and columns much like millions of cubbyholes, each of
which stores a single byte of data. When the processor asks for data, it specifies the row
and column of the location it wishes to start fetching or storing data. First the row is
found, then the column, and finally, the required number of data cells is transferred. The
amount of delay in the process required to locate the row, the column, and then the starting
cell is calledmemory latency.
To minimize the effect of memory latency on the efficiency of the PC, memory accesses
are done in sets (bursts) of multiple data segments, using what is calledburst mode access.
Becauseofthelatency,ittakeslongertoreadthefirstsetofdatathanitdoesthenextone,two,
or three cells (four is a fairly common number of data segments in a burst operation). Burst
mode access reads the four segments, the size of which is determined by the data bus, in
series. This avoids repeating the latency for each segment. Burst mode operations are mea-
sured in the number of clock cycles required for each segment. For example, an 8-2-2-2 burst
notation indicates that the first segment requires eight clock cycles to complete because of
memory latency, but each of the remaining three segments requires only two cycles. The
benefit of burst mode access is in the numbers. In the example, a total of 14 clock cycles were
required to complete the access. Without burst mode operations, each access would require
the full 8 clock cycles, which results in a total of 32 clock cycles for all four segments.
Burst mode access works with L2 cache, which is sized to receive and buffer as many
of the burst segments as possible. For example, on a PC with a 32-bit data bus, the L2
cache of 256 bits would receive and buffer as many as two burst sets (or eight segments)
from memory.
RAM Types
ThetwobasicRAMtypesusedinaPCareDRAM(DynamicRAM)andSRAM(StaticRAM).
DRAM and SRAM are quite different beyond the similarity that they both store data and are
random access memory. Table 7-3 lists some of the more commonly used types of RAM.