Understanding PC Memory - CompTIA A+ 220-801: 1.3

Using memory

• CPU Registers
• Temporary storage
• Caches
• On die Static RAM (SRAM) caches
• External (off-die caches)

• DRAM
• Dynamic Random Access Memory
• Paging systems / Virtual memory

Memory transfer

• Bandwidth
• "Width" of the memory bus
• Bytes transferred with each clock cycle
• Memory bandwidth (8, 16, 32, 64 bits)
• Amount of information transferred
• Width of the memory module

• The number of physical chips on the memory module is irrelevant

Memory speed

• Synchronous DRAM (SDRAM)
• Synchronized to bus
• Memory speed identified as throughput in megabytes per second

• SDRAM measured in clock speed: 100 MHz bus= PC100 
• DDR, DDR2, and DDR3 measured in throughput
• 1,600 MB/sec = PC-1600
• 3,200 MB/sec = PC-3200
• 6,400 MB/sec = PC-6400

 Memory latency

• CAS
• Column Address Strobe
• Column Address Select
• CAS Latency (CL)
• Clock cycles between a memory request sent and data received
• The lower the CL number, the fewer clock cycles and faster the data transfer
• A DDR2 667 MHz CL=4 is faster than a DDR2 667 MHz CL=5

Memory that checks itself

• Used on critical computer systems
• Parity memory
• Additional parity bit
• Won't always detect an error
• Can't correct an error
• Error Correcting Code (ECC)
• Detects errors
• Corrects on the fly
• Not all systems use ECC
• It looks the same as non- ECC memory

Multi-channel memory 

• Installed in pairs or trios for maximum throughput
• Memory combinations should match
• Exact matches are best
• Memory module slots are often colored

Single-sided vs double-sided memory 

• Does not refer to the pysical layout of the memory package
• Ranks
• The "groups" of memory on a module that can be independently accessed
• Memory controller moves between the ranks
• Also called "sides"
• May be called "rows" in older documentation

Memory ranks

•  From the Intel 875P Chipset Memory Configuration Guide
• 1 Rank is single-sided DIMM
• 2 Ranks is a double-sided DIMM
• Check the motherboard documentation to ensure the correct memory