RAM (random access memory) RAM is a semiconductor that stores charges with transistors and capacitors. DRAM- Dynamic RAM Most popular type of electronic memory in the PC world. Must be refreshed constantly or it loses its contents SRAM- Static RAM- very expensive does not have to be refreshed

RAM Random Access Memory Originally RAM had a 640 K limit The 8088 could use only use 256 K per row Needed improved MCC before more RAM could be added

MCC and Parity MCC- Memory Controller Chip Parity “Fetches” memory for the CPU from RAM Parity Extra chip For error detection Usually seen as a “ninth” or odd number chip MCC must be designed to handle parity

Evolution of RAM packages DIPPS- first generation of DRAM chips 2 rows on either side easy to install wrong or break Installed a chip at a time Needed to create a row So why have to install all chips?

30 pin SIPPS Page 160- With SIPPS RAM was on a small board that installed on motherboard Memory was more than one bit wide, so you could have more memory and fewer rows But pins were still easy to break

30 Pin SIMMS No visible pins hanging off, so you couldn’t break it Always 8 bits wide, though depths could vary Can’t tell depth by looking at it Whether you need parity depends on motherboard You can disable parity in CMOS

Access in Nanoseconds (ns) The lower the better. 200 ns on the 8088, now less than 50 ns

Banking Accessing more than one row of DRAM at a time Only possible with the 286 and later Width of RAM must equal external data bus RAM was always 8 bits wide More rows were added to equal data bus 8 bits times 4 rows equals 32 bits

Banking continued All SIMMs in the same bank must be identical You can have different total types but all of the members of one bank must be identical All SIMMs in bank should be same speed Totally “populated” or totally “unpopulated”

On to the 72 pin SIMMs Modern Computers needed too many 30 pin SIMMs to make a bank to match the modern 64 bit data bus New memory needed, to eliminate space The 72 pin SIMM is 32 bits wide, not 8 bits like previous RAM like 30 pin SIMMs Only 2 SIMMs needed for bank in Pentium

168 Pin DIMM DIMM: Dual-inline memory module (has DRAM chips on both sides) 64 bits wide, not 32 Each side of each pin has separate function SO-DIMMS used in laptops and have only 72 pins so are much shorter A bank is formed when X * Width of SIMM chip=Width of external data bus X=sticks in one bank Can you mix DIMMs and SIMMs?

Types of RAM EDO- Extended data out- doesn’t need to be refreshed as often. Can be on either 72 pin SIMM or 168 pin DIMM. Don’t mix with FPM RAM SDRAM- Synchronized DRAM tied to system clock, 5 times faster than DRAM. Is available only on DIMMs ECC RAM- Errors detected and fixed

ROM- Read-only memory PROM- Programmable Read Only Memory- can be programmed only once and are then read only- cannot be erased or changed EPROM- Erasable Programmable Read Only Memory EEPROM- Electrically Erasable Read-Only Memory