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Rehan Azmat
Lecture 2Memory Organization and Hierarchy
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The main memory is used to store the
program and data which are currentlymanipulated by the CPU.
The secondary memory provides the long-
program.
Before the data and program in the secondarymemory can be manipulated by the CPU, they
must first be loaded into the main memory.
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Location
Capacity Unit of transfer
Access method
er ormance Physical type
Physical characteristics
Organisation
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CPU
Internal External
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Word size◦
The natural unit of organisation Word Length
◦ 8,16 or 32 bits
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Internal◦ Usually governed by data bus width
External◦ Usually a block which is much larger than a word
◦ Smallest location which can be uniquely addressed
◦ Word internally
◦ Cluster on disks
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Sequential◦ Start at the beginning and read through in order
◦ Access time depends on location of data andprevious location
◦ e.g. tape
Direct◦ Individual blocks have unique address
◦ Access is by jumping to vicinity plus sequentialsearch
◦ Access time depends on location and previouslocation
◦ e.g. disk
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Random◦ Individual addresses identify locations exactly
◦ Access time is independent of location or previous access
◦ e.g. RAM
◦ Data is located by a comparison with contents of a portionof the store
◦ Word is accessed based on its contents rather then address
◦ Access time is independent of location or previous access
◦ e.g. cache
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Access time◦ Time between presenting the address and getting
the valid data
Memory Cycle time◦ Time ma be re uired for the memor to “recover”
before next access◦ Cycle time is access + recovery
Transfer Rate◦
Rate at which data can be moved into or out frommemory unit
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Semiconductor◦ RAM
Magnetic◦ Disk & Tape
◦ CD & DVD
Others◦ Bubble
◦ Hologram
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Memory bandwidth denotes the amount of data that can beaccessed from a memory per second:
There are two basic techniques to increase the bandwidth of agiven memory:
Reduce the memory cycle time
• Expensive
• Memory size limitation Divide the memory into several banks, each of which has its
own control unit.
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What do we need?
◦ A memory to store very large programs and to work at a speed
comparable to that of the CPU. The reality is:
◦ the larger a memory, the slower it will be;
◦ the faster the memory, the greater the cost/bit.
A solution:◦ To build a composite memory system which combines a small
and fast memory with a large and slow memory, and behaves(most of the time) like a large fast memory.
The two level principle above can be extended into a
hierarchy of many levels. The effectiveness of such a memory hierarchy is based on
property of programs called the locality of reference.
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During the course of the execution of aprogram, memory references tend to cluster◦ e.g. Instruction: Loops
◦ e.g. Data: arrays
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