**RAM** (Random Access Memory)

Memory that can be accessed and written to randomly (by address location). The Northbridge/Memory Controller Hub coordinate these functions.

  * **SRAM** (Static RAM) 
    * Used for cache
    * Faster than DRAM, more expensive
    * Takes more physical space (each bit requires 6 or more transistors)
    * Does not need refreshing

  * **DRAM** (Dynamic RAM)
    * Used for main memory
    * Slower than SRAM, less expensive
    * Needs periodic refreshing (leaky capacitors)

  * **SDRAM** (Synchronous DRAM)
    * The memory bus is synchronized with the front-side bus

  * **RDRAM** (Rambus DRAM)
    * Proprietary RAM architecture
    * 16 or 32 bit bus widths
    * Not used much anymore

  * **DDR SDRAM** (Double Data Rate SDRAM)
    * Data is transmitted on both the rising and falling edge of the clock cycle
    * Doubles the memory throughput of basic SDRAM
    * 2-bit Prefetch

  * **DDR2 SDRAM**
    * Uses "double pumping" like DDR but has a faster I/O bus
    * Theoretical memory throughput quadruples basic SDRAM
    * Typically has a higher CAS latency than regular DDR (slower access time by the memory controller hub and more CPU cycles pass before getting the data)
    * Uses less power than DDR modules
    * 4-bit Prefetch

  * **Parity RAM**
    * An additional bit is added for every byte of data stored in RAM. Basic comparison tests can be done with these extra bits to determine if a sequence of bytes are bad or not.
    * Can only detect errors, not correct them.
    * CAS Latency (CL) is generally worse than non-parity RAM
    * Not really used anymore

  * **Error Correcting Code (ECC) RAM**
    * Uses codes to test and fix errors
    * Can correct memory errors on the fly
    * CAS Latency (CL) is generally worse than non-ECC RAM
    * ECC memory is more expensive
    * Typically found in workstations and servers.

  * **Registered Memory**
    * Additional memory chips are added to act as a buffer
    * The buffer regulates data traffic flow over the memory bus
    * More expensive than non-buffered RAM modules
    * Typically found in workstations and servers

  * **Dual-Channel**
    * The Northbridge/Memory controller hub uses two data channels to transmit data
    * The theoretical throughput is doubled on top of what DDR or DDR2 modules might provide
    * Matching pairs must occupy memory slots for this architecture to provide any bandwidth benefits

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**ROM** (Read-only Memory)

Memory that can be read only. The term is a bit misleading in that all but one type of ROM permits some sort of additional writing.

  * **ROM** - Can only be read and not written to except by the manufacturer of the chip.
  * **PROM** (Programmable ROM)  - Can be written to once.
  * **EPROM** (Erasable Programmable ROM) - Can be written to and erased using UV light.
  * **EEPROM** (Electrically Erasable Programmable ROM) - Can be written to and erased electrically. This type of ROM is also known as Flash memory.