Random Access Memory

What computers use to think with

Running Programs

• Stored on hard disk drive (or other media)

• Copied into RAM from media

• Then the program is run

• CPU can’t get to hard disk drive contents from the Address Bus fast enough

DRAM

• Uses transistor and capacitor to form memory:

Organizing Memory

• Our computer thinks in terms of bits; we think in terms of Bytes.

• 2 GB of RAM is really 2 billion by 8 bits of memory.

Early RAM

• Dual Inline Pin Package (DIPPs)

Parity

• Early memory had a bad habit of forgetting data

• Parity is adding a ninth bit to a memory row for error checking

• Some early memory errors were traced to Alpha waves/particles from substrate

• Current thinking focuses on cosmic rays and no, I did not make this up!

Parity, cont.

• Here’s how it works:

1 0 1 0 0 1 01Even Odd

One’s

Zero’s Ninth bit

Parity, cont.

• Here’s how it works:

1 0 1 0 0 1 01Even Odd

One’s

Zero’s

0

Ninth bit

Parity, cont.

• Here’s how it works:

1 0 1 0 0 1 01Even Odd

One’s

Zero’s

0 1

Ninth bit

Parity, cont.

• Here’s how it works:

1 0 1 0 0 1 01Even Odd

One’s

Zero’s

0 1

1Ninth bit

Parity, cont.

• Here’s how it works:

1 0 1 0 0 1 01Even Odd

One’s

Zero’s

0 1

1 0Ninth bit

Banking

• Memory must be as wide as External Data Bus

• 386 and 486 were 32-bit EDB, so 72-pin was wide enough

• Pentiums are 64-bits wide, so we need two 32’s (32+32=64). Should be matched pair of 72-pin memory, or one 64-bit DIMM

• Total system memory is sum of modules

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SDRAM

• Synchronous DRAM came along

• Tied to FSB speed (System Clock), so we did not have to wait quite as long

• Faster than EDO in general, overall result is 4 to 6 times faster

• Speeds: 66, 100, 133 (MHz)

• Uses Serial Presence Detect (SPD) chip on each module to set timings

168-pin DIMMsSDRAM

• Dual, or Different signals on opposite sides of module; Synchronous Dynamic RAM

• 64-bits wide – only need one module per bank with Pentium

• Notice two notches to orient module

ECC

• Error Correction Code

• Used in high-end servers

• Unlike Parity, can find and correct one-bit errors; can find two-bit errors but not able to correct

• Uses 8 bits over 64 bits of data

• This one is still around

Registered DRAM

• Uses registers as buffers for control signals

• Again, found in high-end servers

• What we use is UNBUFFERED memory in desktop systems

More Speed

• Rambus developed a method for faster RAM• Had to use two sticks at a time • They did not learn from history: collected a lot of

patents and developed expensive licenses• Intel thought it was great for earliest P4’s• Ran very hot – required a heat spreader• Has gone away except for replacement sticks• Betamaxed: Good technology, small market

share drives it to extinction.

DDR

• AMD went with Double Data Rate SDRAM• Rambus had coined bus speeds (200, 400)• DDR is 184-pins, 64-bits wide, one off-center notch• DDR uses throughput volumes:

FSB Effective FSB Designation

100 200 DDR200/PC 1600

133 266 DDR266/PC 2100

166 333 DDR333/PC 2700

200 400 DDR400/PC 3200

Next Stop

• Dual-Channel Architecture

• Get some data from one module, some from another module

• Theory is great; used for about six months

• Have to use matched pairs; total system memory is sum of modules

• Still supported today

DDR2

• Next generation of DDR – faster

• 240-pins, 64-bit wide, won’t fit in DDR slot

• Electrical improvements allow for faster speeds and less power consumption

• Dual channel still supported

• PC2-3200 to PC2-8000

DDR3

• 240-pin module – not compatible with DDR2

• Higher speeds, more efficient architecture, around 30% lower power consumption

DDR3

• Some modules include XMP (Extended Memory Profile) – allows user to overclock RAM

• Others support Triple Channel Memory (stack three modules together logically)

• Doubles the buffer from 4 to 8 bits to get bandwidth improvements

• PC3-6400, 8500, 10667, 12800

Double Sided Memory

• Memory can come either single-sided,

• Or double-sided

• Some motherboards have trouble with double-sided memory

CAS

• Column Array Strobe – One element of the fetch-from-memory process; measured in FSB clock ticks. Think of it as RAM getting up off the couch.

• Another name is Latency

• There are four or five different “tech” numbers used with RAM – not to worry about them; the gamers will

Buffering

• Buffering, or Registered, DRAM is used by servers using large amounts of memory

• Extra chip on memory module, rather like a cache

• We don’t use it in desktop systems; we use non-ECC and unbuffered DRAM

Installing DIMMs

• DIMMs go straight down into slots, held in place by tabs at each end

Do you need memory?

• Probably the quickest and cheapest speed improvement you can make – to a point.

SystemSpeed

Amount of RAM

Win 98 – 128 MB, 512 MaxWin 2000 – 256 MBWin XP – 1 Gig, 2-3G MaxVista and Win 7 – 2 Gig, 4G Max

Easier than CPU-Z!

Replacing/Upgrading Memory

• Beware the evil ESD! And that 5v on the (ATX) motherboard.

• Check what you have, what your motherboard supports, empty slots

• Get the right stuff for your application

• Try not to mix speeds or capacities

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If it doesn’t add up

• Many motherboards have “on-board video”

• System properties will show less than what you installed because 8 to 64 MB is set aside for video use. That’s where the difference lies.

• If you examine the numbers closely, you may find 386Kb “missing” as it is turned off for Upper Memory Area (BIOS).

Troubleshooting RAM

• Parity errors create a Non-Maskable Interrupt (NMI). System halts.

• Can also get other Blue Screens of Death (BSOD) with memory addresses (in Hex).

• True memory errors will show same address each time; other system errors will give different addresses (think about changing the power supply).

Memory Map

Conventional Memory

Upper Memory

High Memory 64 KB

Extended Memory

(for a while was ExpandedMemory)

0

640 KB

1 MB

2 GB

384 KB

Bank A slots

Bank B slots

RAM Pricing

• Overall, memory tends to decrease in price over time. We have seen this as DDR3 dropped from $300/module to $15/module.

• Memory gets “pushed around” more than any other component – Vista drove prices up for six months, so did Win 7.

• There are only a limited number of factories for RAM and if one breaks… up goes the prices.

Troubleshooting

• Single bit failure – NMI and system halts

• Module failure – Series of beeps at startup that continue until you turn system off

• Sometimes system just will not POST

• Keep some good memory handy

• Remember the short parts list: unplug hard disk drive, optical, pull expansion cards and try restart.