Memory Organization

Module IV

SRAM vs DRAM

SRAM• Less memory cells per unit

area• Less Access time• Uses Flipflops• Refreshing Circuitry is not

required• Costly• Used for cache memory

DRAM• More memory cells per unit

area• More Access time• Uses Capacitors• Refreshing Circuitry is

required• Less Costly• Used for main memory

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

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

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.

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.

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.

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)

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

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

Memory Hierarchy

Memory Hierarchy

• As one goes down the hierarchy:a. Decreasing cost per bitb. Increasing capacityc. Increasing access timed. Decreasing frequency of access of the memory

by the processor• Thus, smaller, more expensive, faster

memories are supplemented by larger, cheaper, slower memories.

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.

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

Virtual Memory