Revision Mid 2, Cache Prof. Sin-Min Lee Department of Computer Science

Implementing with a D AND a T flip-flop Using this FSM with three states, an operating only on inputs and transitions from one state to another, we will be using both D and T flip-flops.

Implementing with a D AND a T flip-flop Since we have no state “11”, our Q(t+1) is “don't care” = “XX” for both of these transitions. Consider the first column of the Q(t+1) values to be “D” and the second to be “T” and then we derive two corresponding charts. DT

Implementing with a D AND a T flip-flop Then we need to derive the corresponding equations.

Implementing with a D AND a T flip-flop We assume that Q(t) is actually a pair of Q D Q T. Now, with these equations, we can graph the results.

Implementing with a D AND a T flip-flop

Memory Hierarchy  Can only do useful work at the top  rule: 90% of time is spent of 10% of program  Take advantage of locality  temporal locality keep recently accessed memory locations in cache  spatial locality keep memory locations nearby accessed memory locations in cache

The connection between the CPU and cache is very fast; the connection between the CPU and memory is slower

The Root of the Problem: Economics  Fast memory is possible, but to run at full speed, it needs to be located on the same chip as the CPU Very expensive Limits the size of the memory  Do we choose: A small amount of fast memory? A large amount of slow memory?

Memory Hierarchy Design (1)  Since 1987, microprocessors performance improved 55% per year and 35% until 1987  This picture shows the CPU performance against memory access time improvements over the years Clearly there is a processor-memory performance gap that computer architects must take care of

Memory Hierarchy Design (1)  Since 1987, microprocessors performance improved 55% per year and 35% until 1987  This picture shows the CPU performance against memory access time improvements over the years Clearly there is a processor-memory performance gap that computer architects must take care of

The Root of the Problem: Economics  Fast memory is possible, but to run at full speed, it needs to be located on the same chip as the CPU Very expensive Limits the size of the memory  Do we choose: A small amount of fast memory? A large amount of slow memory?

Memory Hierarchy Design (1)  Since 1987, microprocessors performance improved 55% per year and 35% until 1987  This picture shows the CPU performance against memory access time improvements over the years Clearly there is a processor-memory performance gap that computer architects must take care of

Memory Hierarchy Design (1)  Since 1987, microprocessors performance improved 55% per year and 35% until 1987  This picture shows the CPU performance against memory access time improvements over the years Clearly there is a processor-memory performance gap that computer architects must take care of

The Cache Hit Ratio  How often is a word found in the cache?  Suppose a word is accessed k times in a short interval 1 reference to main memory (k-1) references to the cache  The cache hit ratio h is then

Reasons why we use cache Cache memory is made of STATIC RAM – a transistor based RAM that has very low access times (fast) STATIC RAM is however, very bulky and very expensive Main Memory is made of DYNAMIC RAM – a capacitor based RAM that has very high access times because it has to be constantly refreshed (slow) DYNAMIC RAM is much smaller and cheaper

Performance (Speed)  Access time Time between presenting the address and getting the valid data (memory or other storage)  Memory cycle time Some time may be required for the memory to “recover” before next access cycle time = access + recovery  Transfer rate rate at which data can be moved for random access memory = 1 / cycle time (cycle time) -1

Comparison of Placement Algorithms