CHAPTER 7 RAM
RANDOM ACCESS MEMORY
2 types

Static RAM SRAM
o Faster than DRAM, but more expensive
o Used for cache

Dynamic RAM DRAM
o Cheap, but slower than SRAM
o Used for main memory
DRAM is installed as modules that plug directly into the motherboard
SIMMS – single in-line memory module
DIMMS – dual in-line memory module
RIMMS – rambus in-line memory module
Laptops use SO modules (small outline)
SO DIMMS
SO RIMMS
SIMMS – 32 bit data path

Installed in pairs

Outdated
DIMMS – 64 bit data path

Dominate today

Uses SD RAM (synchronous DRAM, in synch with the system bus

Currently DDR-DDR2 (double data rate) are the most popular
RIMMS – direct rambus DRAM

Designed by rambus; proprietary

More expensive but slower than DDR and DDR2 DIMMS, RIMMS are starting to become obsolete
Motherboards designed to support RIMM memory must have all memory slots filled to maintain continuity
If you use RIMM memory, every memory slot without a rimm must be filled with a C-RIMM (continuity RIMM, holds
no memory) or memory will not work right if at all.
ERROR CHECKING

Parity – an error checking method that uses an extra bit. Memory with parity would use 9 bits for each byte
instead of 8

ECC – error correcting code – a newer error checking method that can detect errors in a single bit

ECC memory is more expensive, but more reliable. Often used on servers
MEMORY SPEEDS
SIMMS & asynchronous DRAM measure speed in nanoseconds(NS) the lower the number, the better
SDRAM (ddr, ddr2) & RIMMS measure speed in MHz, because they synchronize with the system bus. The larger the
number the better
PC RATING – a measure of the bandwidth (amount of data moved) of memory. The higher the number the better
A DDR DIMM has an 8 byte width (64 bits) & a system bus speed of 266 MHz. 8 X 266= 2128 MB/sec=PC 2100
WHEN BUYING MEMORY

Match tin to tin, & gold to gold

When possible, use memory made by the same manufacturer

Read the motherboard documentation. You can only install memory that the board supports

Research the memory before you purchase


Buy from a reputable source
Avoid re-marked chips.
BUFFERS & REGISTERS
Both are devices used to hold data and amplify a signal before it is written to memory
Older memory used buffers
Newer memory uses registers
Some memory has neither
Dual channeling – a technology available for DDR & DD2 DIMMS, which allows the memory controller to
communicate with 2 DIMMS at the same time, doubling the speed of memory access
For dual channeling to work, the 2 DIMMS must have the same size, speed & features. And come from the same
manufacture.
Alternate way of measuring the speed of memory
CL or RL (CAS or RAS)
Latency
A measure of how ling it takes in clock cycles to read a column or row of data off of a memory module
CAS – Column Access Strobe
RAS – Row Access Strobe