Transcript RAM Random Access Memory

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
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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-
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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
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
 EDO-
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