2 SRAM vs DRAM SRAM DRAM Less memory cells per unit area Less Access timeUses FlipflopsRefreshing Circuitry is not requiredCostlyUsed for cache memoryMore memory cells per unit areaMore Access timeUses CapacitorsRefreshing Circuitry is requiredLess CostlyUsed 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 ROMApplication of ROMs :MicroprogrammingLibrary subroutinesSystem programsFunction tablesAdv: 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 ROMROM is created with data actually wired into chip during fabrication.This presents two problems:Data insertion includes a large fixed costThere 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 erasedErasure is performed by shining an intense ultraviolet light through a window designed into the chipThis can be performed repeatedlyEach erasure can take upto 20 minutesEPROM can be altered multiple timesEPROM is more expensive than PROMAdv: multiple update capability.
8 EEPROMIt can be written into at any time without erasing prior contents.Write operation takes longer time than read.Adv:nonvolatilityflexibility of being updatableUses ordinary bus control, address, and data lines.Disadv:More expensiveLess dense (fewer bits per chip)
9 Flash EEPROM Named so since it can be reprogrammed fast. It uses electrical erasing technologyEntire memory can be erased in few secondsIt 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 HierarchyTo implement memory systems, the following relationships hold:• Faster access time, greater cost per bit• Greater capacity, smaller cost per bit• Greater capacity, slower access timeDilemma: 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
12 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 processorThus, smaller, more expensive, faster memories are supplemented by larger, cheaper, slower memories.
13 Virtual MemoryIt allows the execution of processes that are not completely in memoryIt 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 MemoryVirtual 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