1. Module IV
Memory Organization

2. SRAM vs DRAM SRAM DRAM Less memory cells per unit area
Less Access time
Uses Flipflops
Refreshing Circuitry is not required
Costly
Used for cache memory
More memory cells per unit area
More Access time
Uses Capacitors
Refreshing Circuitry is required
Less Costly
Used for main memory

3. ROM It contains a permanent pattern of data that cannot be changed.
It is non-volatile (no power is required)
It is possible to only read from ROM
Application of ROMs :
Microprogramming
Library subroutines
System programs
Function tables
Adv: Data or program is permanent in main memory

4. Types of ROM ROM - Read Only Memory.
PROM - Programmable Read Only Memory.
EPROM - Erasable Programmable Read Only Memory.
EEPROM - Electrically Erasable Programmable Read Only Memory.
Flash EEPROM

5. ROM
ROM is created with data actually wired into chip during fabrication.
This presents two problems:
Data insertion includes a large fixed cost
There is no room or error. If one bit is wrong, the whole batch of ROMs must be thrown out.

6. PROM It is non-volatile and wrote only once.
The writing process is performed electrically and may be performed later than during fabrication.
Special equipment is required for the writing or “programming” process.
It provide flexibility and convenience.

7. EPROM It is read and written electrically.
Before a write operation, all storage cells is erased
Erasure is performed by shining an intense ultraviolet light through a window designed into the chip
This can be performed repeatedly
Each erasure can take upto 20 minutes
EPROM can be altered multiple times
EPROM is more expensive than PROM
Adv: multiple update capability.

8. EEPROM
It can be written into at any time without erasing prior contents.
Write operation takes longer time than read.
Adv:
nonvolatility
flexibility of being updatable
Uses ordinary bus control, address, and data lines.
Disadv:
More expensive
Less dense (fewer bits per chip)

9. Flash EEPROM Named so since it can be reprogrammed fast.
It uses electrical erasing technology
Entire memory can be erased in few seconds
It is possible to erase blocks of memory rather than entire chip.
It does not provide byte-level erasure.
Adv: It uses only one transistor per bit and hence high density

10. Memory Hierarchy
To implement memory systems, the following relationships hold:
• Faster access time, greater cost per bit
• Greater capacity, smaller cost per bit
• Greater capacity, slower access time
Dilemma: Designer would prefer large-capacity memory but to improve performance he needs to use faster low capacity memories.
Way out : Do not rely on a single memory type but to employ a memory hierarchy

11. Memory Hierarchy

12. Memory Hierarchy As one goes down the hierarchy:
a. Decreasing cost per bit
b. Increasing capacity
c. Increasing access time
d. Decreasing frequency of access of the memory by the processor
Thus, smaller, more expensive, faster memories are supplemented by larger, cheaper, slower memories.

13. Virtual Memory
It allows the execution of processes that are not completely in memory
It abstracts main memory into an extremely large storage, separating logical memory from physical memory.
It frees programmers from the concerns of memory limitations.
It allows processes to share files easily.
It is not easy to implement and decrease performance if it is used carelessly.

14. Virtual Memory
Virtual memory involves the separation of logical memory as perceived by users from physical memory.
This separation allows an extremely large virtual memory for programmers when only a smaller physical memory is available

15. Virtual Memory