by Peter Perlsø, published 6th of March 2003, updated 30aug2009 - A perennial work in progress...
RAM is short for Random Access Memory.
General rule for computers: Fast response (= low latency) memory is expensive, slower memory is cheaper.
The memory hierarcy briefly described:
Memory unit - hierarchy level - typical access time
Registers - Level 0 - inside CPU, less than one CPU cycle
L1 cache - Level 1 - on CPU die, 1-2 CPU cycles
L2 cache - Level 1 - on or off CPU die, 5 cycles
L3 cache - Level 1 - external to CPU, 10-20 cycles
RAM/Main memory - Level 2 - 100-1000 cycles
Hard disk - Level 3 (outboard storage) - over 1e6 cycles
Tape/Backup - Level 4 (offline storage) - duh!
Cycle times, relative to bus frequency:
| Bus frequency in MHz: | Cycle time in nanoseconds (ns): |
| 8 | 125 |
| 10 | 100 |
| 12,5 | 80 |
| 14,2 | 70 |
| 16,7 | 60 |
| 20 | 50 (FPM) |
| 66,7 | 15 |
| 83,3 | 12 |
| 100 | 10 |
| 125 | 8 (SDRAM) |
| 133 | 7,5 |
| 142 | 7 |
| 167 | 6 |
| 200 | 5 |
| 225 | 4,5 |
| 250 | 4 |
| 333 | 3 (DDR SDRAM) |
SIMM - Single Inline Memory Module technologies
D (Dynamic) RAM
4 - 40 MHz, 100-150 ns access time
Requres that a signal be sent for both row and columns for each memory access.
Typical timing/latency: 5-5-5-5
FPM (Fast Page Mode) DRAM
16-66 MHz, 60-80 ns
Allows a row to be activated and held active while multiple sequential column accesses take place for memory accesses near each other. Improves access times significantly.
Typical timing/latency: 5-3-3-3
S (Static) RAM
Very fast RAM used in CPU buffers (caches). Does not need refresh signals, but requires more transistors (5 per data bit storage) and more power to operate).
BS (Burst Static) RAM
Also used in CPU buffers
DIMM - Dual Inline Memory Module technologies
EDO (D) RAM - Enhanced Data Out
aka "hyper page mode" DRAM
33 - 75 MHz, 50-60 ns
Invented in 1994. Allows a new memory access to start before the last memory access has finished, allowing for lower latencies.
Typical timing/latency: 5-2-2-2 @ 66 MHz
BEDO (Burst EDO) DRAM
Burst capable EDO RAM made possible by adding more latches and pipelining circuitry. Had a limited success in the marketplace.
Typical timing/latency: 5-1-1-1
S (Synchronous) DRAM
60-166 MHz, 4-12 ns (originally 8-10-12)
SDRAM runs on a clock signal synchronised with the chipset clock (hence the name), unlike Asynchonous RAM where RAM and chipset run on two different clock signals. Allows a dramatic increase in bus speeds over Async RAM. Allows bursting. Uses internal interleaving to allow circuits to initiate an access for the first half of the chip while the second half is finishing an access. Abolishes the use of wait states.
Debuted in 1997 and was widely adopted by 1999.
PC66 - 532 MBps *
PC100 - 0.8 GBps
PC133 - 1.064 GBps (most common type)
PC166 - 1.33 GBps **
Typical timing/latency: 5-1-1-1
V (Video) RAM
W (Window) RAM
SGRAM
RIMM - Rambus Inline Memory Module
An awfully expensive kind of RAM that died a slow, agonizing death in the early 2000's due to its poor price-performance compared to the competing, and generally adopted SDRAM technologies. Intel originally backed the technology due to its promising future, but when it turned out that it was a dead end, and RAMBUS inc. started suing everyone that had cooperated with them, Intel (wisely) jumped ship.
RD (RAMBUS Direct) RAM - 16 bit wide RAMbus
RDRAM600 - 266 MHz x 2 - 532 Mhz - 1.064 GBps
RDRAM700 - 356 MHz x 2 - 712 MHz - 1.424 GBps
RDRAM800 "PC800" - 400 MHz x 2 - 800 MHz - 1.6 GBps
RDRAM1066 "PC1066" - 533 MHz x 2 - 1066 MHz
An open competing technology to RDRAM was SLDRAM.
DDR (Double Data Rate) (SD) RAM
Debuted in 2000, and became widely used over 2001.
PC1600 - 100 MHz x 2 - DDR100/200 - 1.6 GBps
PC2100 - 133 Mhz x 2 - DDR133/266 - 2.133 GBps
PC2700 - 166 MHz x 2 - DDR333 - 2.667 GBps
!PC3000 - DDR366 **
PC3200 - DDR400 (most common) - 3.2 GBps
!PC3500 - DDR433 *
!PC3700 - DDR466 *
!PC4000 - DDR500
!PC4200 - DDR533 **
!PC4300 - DDR538? **
!PC4400 - DDR550
!PC4500 - DDR566 **
!PC4800 - DDR600 *
!) not JEDEC standardized
DDR2-SDRAM
Debuted in 2003, and was widely adopted by late 2004.
PC2-3200 - DDR2-400 - 400 MHz - 3.2 GBps
PC2-4200 - DDR2-530 - 533 MHz (most common type) - 4.267 GBps
PC2-4300 - 538 MHz *
PC2-5300 - DDR2-667 - 667 MHz - 5.333 GBps
PC2-5400 - 675 MHz *
PC2-6400 - DDR2-800 - 800 MHz ** - 6.4 GBps
PC2-8000 - 1 GHz **
PC2-8500 - 1.066 GHz **
PC2-8888 - 1.111 GHz **
PC2-9000 - 1.120 GHz **
PC2-9200 - 1.150 GHz **
PC2-10000 - 1.25 GHz **
DDR3-SDRAM
Entered market in 2008, slowly becoming widely used in 2009.