Computer ArchitectureComputer Architecture

Part III-A: Memory

A Quote on MemoryA Quote on Memory

“With 1 MB RAM, we had a memory capacity which will NEVER be fully utilized”

- Bill Gates

MemoryMemory

Computer pioneers correctly predicted that programmers would want unlimited memory

Solution : memory hierarchy Takes advantage of of locality and cost /

performance of memory technologies Using the principle of locality plus the

guideline that smaller is faster resulted on a memory hierarchy based on memories of different speeds and sizes

Memory Measure: Access TimeMemory Measure: Access Time

Time taken to read data from a given memory location, measured from the start of a read cycle.

Components Time to get to the location of the data Time for the data to become available

from the location in memory

Memory: Not created equally!Memory: Not created equally!

The Memory SubsystemThe Memory Subsystem

CPU

Cache

L1/Primary

L2/Secondary

MainMemory

Memory OrganizationMemory Organization

Address 0

Address 1

Address 2

Address N-2

Address N-1

N words - Number of wordsis generally 2n

Bit 1 Bit 2

Word Length = n bits

About Naming Conventions:

For example,a memory system with 4096locations, each with a diff.Address and each storing 12 bitsis called a 4096 word 12-bitmemory or 4K 12-bit memory.

Getting Data from Main MemoryGetting Data from Main Memory

Memory

CPU Registers

CPU Internal Bus

MAR MDR

AddressBus

DataBus

1

2

3

4

5

Types of Memory : ROMTypes of Memory : ROM

Read-Only Memory Contains instructions for starting up the

computer Contains constants that specify the

system’s configuration

Types of ROMTypes of ROM

PROM (Programmable ROM) EPROM (Erasable PROM)

Programs are erasable via ultraviolet light EEPROM - Electrically EPROM

Programs are erasable by exposing it to an electrical charge

Types of Memory: RAMTypes of Memory: RAM

Random Access Memory Main memory Classification - Physical

Characteristics Static vs. Dynamic Volatile vs. Nonvolatile Destructive vs. Nondestructive Removable vs. Permanent

Static vs. DynamicStatic vs. Dynamic

STATIC Memory contents are

refreshed less often

Faster than DRAM but more expensive and requires more power

More stable

Access time: 10 ns

DYNAMIC Memory contents are constantly

refreshed otherwise the contents will be lost

Cheaper to build, but slower than SRAMs

Less stable than SRAM

Access time: 60 ns.

Volatile vs. NonvolatileVolatile vs. Nonvolatile

A memory device is volatile if it requires a continuous source of power to hold its value, otherwise, it is non-volatile

CD-Rs, hard disks, floppies, etc. - nonvolatile

RAMs - volatile

Destructive Read vs. Destructive Read vs. Non-Destructive ReadNon-Destructive Read

Destructive Read

When the system reads a word in memory, it destroys the value.

Characteristics of all DRAMs

In practice, the circuitry rewrites original value back to the cell via a two-phase operation: read cycle and restore cycle

Non-Destructive Read

Circuitry does not destroy the value of the memory cells

Removable vs. PermanentRemovable vs. Permanent

Removable

Active elements can be removed from system hardware

Examples: floppies, tape cartridges, hot swappable disks

Permanent

Components are not physically removed

Example: RAM, hard disks, etc.

PackagingPackaging

DIP Style DRAM package Popular when it was common for memory

to be installed directly on the computer's system board

”Through-hole" components, which means they install in holes extending into the surface of the printed circuit board

Can be soldered in place or placed in sockets

PackagingPackaging

SIMM (Single Inline Memory Module)

DIMM (Dual Inline Memory Module)

Memory BanksMemory Banks

A Word about Virtual MemoryA Word about Virtual Memory

An imaginary memory area supported by some OS in conjunction with the hardware

Purpose: To enlarge the address space, which is the set of memory addresses a program can utilize

A program using all of Virtual Memory (VM) will not fit in Main Memory (MM), but the system is able to execute it by copying the required portions into MM

PagesPages

To facilitate copying portions of VM into MM, the OS divides VM into pages

Pages: A fixed number of memory addresses

Each page is stored on the disk until its needed

DRAM types: FPMDRAM types: FPM

Fast Page Mode RAM Traditional RAM for PCs Hard to find and more expensive Access Times are 60 to 70 ns. Allows faster access to data on the same

page

DRAM types: EDODRAM types: EDO

Extended Data Output Faster by 10%-15% than FPM Copies an entire block of memory to its

internal cache; while the processor is accessing this cache, memory can collect a new block to send

EDO can access data faster if the cache controller supports PIPELINE BURST

About Pipeline BurstAbout Pipeline Burst

Purpose: Minimizes wait states so that memory can be accessed as fast as possible by the microprocessor

How? A burst mode that pre-fetches memory

contents before they are requested Pipelining so that one memory value can

be accessed in the cache at the same time another memory value is accessed in DRAM

DRAM Types: BEDODRAM Types: BEDO

Burst EDO After an address has been specified,

several bytes are then read within one clock cycle each

Transfer of information to the CPU much faster than EDO

Downside: Unable to cope well with system buses higher than 66 MHz

DRAM Types: SDRAMDRAM Types: SDRAM

Synchronous DRAM Runs on much higher clock speeds than

conventional memory Supports bus rates up to 133 MHz Done by having two memory banks, one

bank is used to get ready for access, while the second one is being accessed

PC 100/133: set of guidelines by Intel for synchronous DIMMS

DRAM types: RDRAMDRAM types: RDRAM

Rambus DRAM Initially developed by Rambus, Inc. Intel signed a contract for its endorsement

(up to 2002) in 1997 Data transfer occurs on both edges of the

clock Packaged in RIMMS and installed in pairs

in many units Requires all memory slots in the

motherboard to be populated (CRIMMS) Royalties paid by manufacturers

DDR-SDRAMDDR-SDRAM

Also known as SDRAM II Packaged in DIMMS Cheaper than RDRAM Also utilizes both edges of the clock Not backward compatible with SDRAM No royalties