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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
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