RAM (random access memory)

1. 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

2. 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

3. 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

4. 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?

5. 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

6. 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

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

8. 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

9. 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”

10. 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

11. 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?

12. 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

13. 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