9/26: Memory ROM vs. RAM ROM –BIOS RAM –changeable –different kinds & uses inc. VRAM, SRAM –how it works image courtesy of How Computers Work CD

ROM? RAM? Both refer to chip-based memory hardware –(not hard drives, CD-ROMs, etc.) ROM: Read-Only Memory –doesn’t change (much if at all) –BIOS stored here. RAM: Random Access Memory –changes frequently –short-term memory

ROM: Read-Only Memory Chips hardwired with start-up instructions Nonvolatile memory –doesn’t change Non-dynamic –not dependent on electrical power to store information Necessary to start up the operating system (OS) Has a few options: disk drive boot order, etc.

RAM: Random Access Memory Synonyms: “main memory”, “primary memory”, “user memory” Volatile memory –contents can be changed Why “random access”? –sequential access vs. random access –analogy: cassette vs. CD Programs, files stored here while in use. image courtesy of How Computers Work CD

How much RAM do you need? Current recommendation: 128 MB total RAM for a new computer Windows 98 uses ~ 20 MB of RAM Windows NT uses ~ 40 MB of RAM McAfee Virus Scan & Task Manager ~ 7 MB MS Word uses ~ 7 MB Netscape Communicator uses ~ 10 MB MS Internet Explorer uses ~ 7 MB Each file you open is held in RAM

Varieties of RAM: DRAM DRAM: Dynamic RAM –dynamic: needs electricity to hold data. Must be continually replenished with electricity. –FPM: Fast Page Mode –EDO: Extended Data Out can send data while receiving other data –BEDO: Burst EDO sends “pipelined” data, speeding up EDO –SDRAM: Synchronous DRAM runs at the CPU clock speed (faster than prev. types) today’s main DRAM.

Future of DRAM –DRDRAM: Direct Rambus TM DRAM revolutionary: completely new architecture closed standard supported by Intel and Rambus: manufacturers will owe royalty payments to the companies. If Intel remains as the giant of the industry, this technology will likely win out. –SLDRAM: Synchronous-Link DRAM evolutionary: doubles the SDRAM transfer rate by using both “sides” of the clock signal (double beats) open standard supported by SLDRAM consortium: royalty-free design makes it more likely to win out.

Varieties of RAM: SRAM SRAM: Static RAM –static: does not need to be continually refreshed. –more expensive, bigger, and faster than DRAM. –used in CPU chips (L1, L2, and L3 caches) –used in “flash memory”

Varieties of RAM: VRAM Video RAM Designed for high throughput: –dual-ported: one for incoming data and one for outgoing data

DRAM: Configurations Memory modules SIMMs - 32-bit, require pairing on Pentium systems DIMMs - 64-bit, do not require pairing on Pentium systems image courtesy of gamespot.com

How RAM works RAM chips have millions of transistor/capacitor pairs. Capacitors can briefly hold an electric charge. Gates are opened and closed by the address lines. Data goes in and out on the data lines Capacitor Transistor image courtesy of How Computers Work CD

How RAM works: writing to it The address lines point out the spot. The data lines send electrical pulses to charge the capacitors. image courtesy of How Computers Work CD

How RAM works: reading from it The address lines point out the spot. The data lines sense the electrical charge put off by the capacitor. image courtesy of How Computers Work CD