Copyright © 2007 – Curt Hill Primary Memory and Electronic Storage Implementations

Primary Memory What we refer to as memory may in itself be composed of several types of memory The early machines actually used drums (similar to disks) as memory Most of what we consider here is not moving Copyright © 2007 – Curt Hill

Some characteristics to consider Location Capacity Unit of transfer Access Method Performance Physical type Physical characteristics Organization Copyright © 2007 – Curt Hill

Location Relative CPU Internal (main memory) –Same chip –Same board External (secondary) –Different board Copyright © 2007 – Curt Hill

Other Characteristics Capacity –Word size –Number of words Unit of transfer –Word –Block Access Method –Sequential access –Direct access –Random access –Associative access Copyright © 2007 – Curt Hill

Other Characteristics Performance –Access time –Cycle time –Transfer rate Physical type –Semiconductor –Bubble memory –Magnetic core –Moving magnetic –Moving optical Copyright © 2007 – Curt Hill

Other Characteristics Physical characteristics –Volatile or non-volatile –Erasable or non-erasable Organization –Physical organization of bits into words –Fixed or removable Copyright © 2007 – Curt Hill

The hierarchy reviewed CPU registers –Very expensive, very fast, temporary (volatile), not transportable –The number of registers is only of concern to programmers Cache memory: slower than registers faster than most other things, typically pico seconds –Expensive, fast, volatile, not transportable Copyright © 2007 – Curt Hill

The hierarchy reviewed Main memory: typically 10s of nanoseconds and tens of dollars per Megabyte –Same as above Fixed disks: Milliseconds (as few as 2) $ $10 a Megabyte –Much slower and cheaper than memory, permanent, not transportable Copyright © 2007 – Curt Hill

The hierarchy reviewed Floppy disk: tenths of a second.1 to 1 dollars/M –Transportable, Very slow compared even to hard disks, –Two separate costs: drive (35-150) media (1) Removable hard drives (disk packs) –Similar to hard drives but more expensive (the engineering and manufacturing costs are higher) Copyright © 2007 – Curt Hill

The Hierarchy Reviewed Optical disks (CD ROM) –Slow, read only, but cheap high capacity Tape units many seconds, pennies or less –Cheap media, though drives are somewhat expensive (supply and demand) –Sequential access –Very slow Copyright © 2007 – Curt Hill

Electronic implementations Usually implies no moving parts Several kinds –RAM –ROM –Mag bubble Copyright © 2007 – Curt Hill

RAM Random Access Memory Dynamic RAM (DRAM) –Store charge like a capacitor –This charge does tend to dissipate so they need to be refreshed periodically –Most modern CPUs have a refresh cycle built in Static RAM lasts –Uses a flip flops to retain value while there is power Both are volatile Copyright © 2007 – Curt Hill

CMOS Complementary Metal Oxide Semiconductor - CMOS Cycle time is the time that it takes for a fetch operation to be successfully satisfied –70 ns seems to be typical now Copyright © 2007 – Curt Hill

ROM Non volatile, but not changable Read Only Memory Lots of possibilities PROM EPROM –Erasable by UV light EEPROM –Erased by processor control Copyright © 2007 – Curt Hill

Bubble memory Seems to have died No moving part, non volatile, changable, slow compared with most other technologies Magnetic domains Polarity determines value In most case battery backed RAM is faster and more cost effective Copyright © 2007 – Curt Hill

Memory types Copyright © 2007 – Curt Hill Memory type CategoryErasureWrite mechanism Volatile RAMRead/WriteElectrically by byte ElectricallyYes ROMRead onlyNoMasksNo PROMRead onlyNoMasksNo EPROMRead onlyUV light, by chip ElectricallyNo Flash memory Read mostlyElectrically by block ElectricallyNo EEPROMRead mostlyElectrically by byte ElectricallyNo

Flash Memory Also known as AND/OR memory Type of EEPROM Usually used for disks rather than primary memory –BIOS is often flash though Uses Fowler-Nordheim tunneling Individual byte programming once –Then must erase whole block Copyright © 2007 – Curt Hill

Content addressable memory AKA associative memory This is special (that is expensive) type of memory that can be accessed in two ways –By address –By contents I believe I saw an implementation of a cell that used 10 gates as opposed to the four used in a flip-flop Copyright © 2007 – Curt Hill

Accessing associative memory Give me the address and I give you the contents Give me the contents or a portion of them and I give you the address or group of addresses Associative memory is much more complex than normal static/dynamic RAM Copyright © 2007 – Curt Hill