Memory Hierarchy & Semiconductor Memory Types

Hello everyone !! ^_^

Today, i want to continues the tutorial with the Memory Hierarchy and the Semiconductor Memory Types.

When you want to go to purchase memory,(whatever RAM,internal/external hard disk,etc) 

I think your guys may consider about :

1) the capacity(how much for the amount you need?),

2) the cost(how much i need to pay for it?)

3) the speed/access time(how fast for the transfer speed,access speed?)

Is I am right?haha...

Memory Hierarchy Diagram----->

From up to down hierarchy:

•decreasing the cost per bit

•increasing the capacity

•increasing access time

•decreasing frequency of access of memory by proccessor

Sometimes, i am thinking...

Is that possible to build a computer which only use static RAM,

As that might be more faster and it would need no cache,although it would take a very large amount of cost.

Principle of Locality of Reference:

-During the course of the execution of a program, memory references by the processor tend to cluster

-Over a short period of time, processor is working with fixed clusters.

For example: loop,subroutines.As once a loop/subroutine is entered, there are repeated references to small set of instance.

We can consider to use two-level memory for example: 

--Level 2 memory contain all program instructions and data

--Place the current clusters in level 1

--From time to time, change the clusters to new –old clusters

swapping back to main memory

As the diagram below,

↓Semi-Conductor Memory↓

RAM( Random Access Memory)

-A form of computer data storage

-Read and Write only  in a predetermined order

-Volatile memory

-2 types: Static(SRAM) & Dynamic(DRAM)

DRAM(Dynamic RAM)

-Store each bit of data in a separate capacitor within an integrated circuit.

-Structural simplicity.

-Loses its data quickly when power is removed

-Cheaper

-Slower

-Need refresh circuits

-Main memory

DRAM Operation

# During the bit is read or written,the address line will be active.

-transistor switch closed

# Write

- High for 1, low for 0

- Signal apply to address line(change to capacitor)

# Read

-Select the address line, then transistor will turn on

-Charge from capacitor fed by bit line to sense amplifier

-Capacitor charge must be restored.

SRAM (Static RAM)

♂ Uses bis-table latching circuitry to store each bit.
♂ Periodically refreshed.
♂ More expensive.
♂ Less dense.
♂ Used in cache.
♂ Faster.
♂ Digital.
♂ Larger per bits.
♂ Complex construction.

SRAM Operation

~Standby
~Reading
~Writing

ROM(Read Only Memory)
∷It's permanent storage(non-volatile) and random access but can not be written to it.

∷Data stored in ROM cannot be modified (overwrite).

Types of ROM:


①PROM(Programmable ROM)
- Only program ONCE by user.
-Non-volatile.
-Programmed by burning the fuse using high current pulse.
-Flexible and convenient compared to ROM.

②EPROM(Erasable Programmable ROM)
∞ Re-programmable
∞ Erased by UV light
∞ More expensive than PROM
∞ Must be erased before to write it

③EEPROM
(Electrically Erasable PROM)
-No requirement of physically removed from the circuit for re-programming.
-Use special voltage level to erase data.
-Take much longer to write and read.
-More expensive than EPROM.
-Less dense.

④Flash Memory
- Special type of EEPROM
- Erase whole memory electrically, per block or per chip erasable.
- Suitable for used as solid state disk such as MemoryStick, SD(Scan Disk), MD(Micro Disk).







Ok. that's all for today. Hope your guys will learn something from my tutorial post. =)
I will continues to post the Cache at next tutorial post. See ya ! =D

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