memory orgnization

					Computer Memory Organization

Elephants don’t forget… but do computers?

Memory Design
GOAL: to best supply programmers with (almost) infinitely extensible memory that is as fast as possible to use The general design is a hierarchy
The most expensive, fastest memory is in close proximity to the processor Memory contents are duplicated and expanded in banks lower in the hierarchy Successive memory banks increase in storage capacity and access latency, but decrease in cost and operational speed

Performance Considerations
Performance measurements between any two levels of hierarchy
Hit rate = ratio of times a desired element is in that level of memory to the number of total accesses to memory = (1-miss rate) Hit time = time to access the upper level of memory and deliver the item to the requesting device Miss penalty = time to bring the item into upper level and deliver the item

Memory and Design
Average memory access time is equal to (hit time + miss rate * miss penalty) Computer designers favor a block size with the lowest average access time rather than lowest miss rate Memory hierarchies also complicate the design of the CPU
Variable access time prevents memory from being directly on the datapath

The Memory Hierarchy
Cache
Smallest and closest to processor Many different organization strategies May be multiple levels of cache

Main memory
The motherboard RAM you know and love

Virtual memory
Switch out memory to disk Essential for shared, multiprocess systems

Caches
Where can you put a block in a cache?
direct mapped: a function maps a block to a unique location in the cache set associative: a function maps a block to a unique group of locations in the cache, but block can go anywhere in the cache fully associative: put the block anywhere

How do you find things in a cache?
For direct map, might invert map function Otherwise, must check each block (in parallel)

More Caches
What block is replaced on a miss?
Random selection promotes uniform allocation, easy to implement Least recently used takes advantage of temporal locality (recently used blocks are more likely to be used again)

What happens on a write?
Write through - also writes to lower-level block Write back - doesn’t write lower-level block until the corresponding cache block is replaced
dirty bit on a cache block indicates update is necessary

Main Memory
We try to make main memory contain the entire state of the currently running system process; otherwise, we swap
Penalty for swapping with virtual memory very high (magnetic, mechanical storage)

We don’t worry about sync between main memory and virtual memory because we do not replicate main memory in virtual memory (very costly)

Improving Main Memory
Make memory bus wider (more at one time)
Also must widen cache Must multiplex before entering the datapath (convert many signals into one) May be wasteful (especially on writes)

Interleave memory banks (parallelize reading)
Successive words rotate through memory banks (word 1 = bank 1, word 2 = bank 2, …, word 5 = bank 1, word 6 = bank 2, …) Must still multiplex into the datapath Allows parallel writes (if one bank not overloaded)


				
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posted:8/25/2009
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Shah Muhammad  Butt Shah Muhammad Butt IT professional
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