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– “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