Question

• The speed at which a processor can process instructions can be increased by making its clock speed higher, or by increasing the amount of L1 cache memory on the processor chip.

  Explain the terms clock speed and L1 cache and briefly discuss how increasing each of them increases the speed at which instructions can be processed.

  The maximum word limit for Question 3(a) is 150 words.

• b.You have the choice of buying two processors:

  • Processor 1 has a clock speed of 4.0 GHz with 32 KiB of L1 cache.
  • Processor 2 has a clock speed of 2.0 GHz with 64 KiB of L1 cache.

  In order to decide which to buy, you decide to estimate the time each processor will take to process a program with 10,000 RISC instructions. Each RISC instruction takes one clock pulse to execute once it is in the registers, and has a size of 4 bytes.

  • i.Show that a program consisting of 10,000 RISC instructions will fit into 64 KiB of L1 cache, but will not fit into 32 KiB of L1 cache.
  • ii.It takes 1 nanosecond (1 x 10^-9 s) to move an instruction from L1 cache to the registers and 9 nanoseconds to move an instruction from L2 cache to the L1 cache. Assume that for Processor 2, all of the instructions can be found in L1 cache and for Processor 1, all of the instructions that won’t fit in L1 cache can be found in L2 cache.

    How much time is needed to load all 10,000 instructions into the registers for each processor. Write your answers in seconds, using scientific notation.

  • iii.For each processor, calculate the time it takes to execute 10,000 RISC instructions. Write your answers in seconds, using scientific notation.
  • iv.Using your results from part (ii) and part (iii), for each processor, calculate the total amount of time to load 10,000 RISC instructors from cache memory to the registers, and then to execute them.

    You may assume that no data is needed for this test program. in each case you will only need to calculate the time to move 10,000 instructions into the registers and the time to execute the 10,000 instructions. You will then find the total time for each processor by adding these two values.

    Write your answers in seconds, using scientific notation.

Answer 1

Cache memory

The cache is a small amount of memory which is a part of the CPU - closer to the CPU than RAM. It is used to temporarily hold instructions and data that the CPU is likely to reuse.

The CPU control unit automatically checks a cache for instructions before requesting data from RAM. This saves fetching the instructions and data repeatedly from RAM – a relatively slow process which might otherwise keep the CPU waiting. Transfers to and from cache take less time than transfers to and from RAM.

The more cache there is, the more data can be stored closer to the CPU.

A cache is graded as Level 1 (L1), Level 2 (L2) and Level 3 (L3):

• L1 is usually part of the CPU chip itself and is both the smallest and the fastest to access. Its size is often restricted to between 8 KB and 64 KB.
• L2 and L3 caches are bigger than L1. They are extra caches built between the CPU and the RAM. Sometimes L2 is built into the CPU with L1. L2 and L3 caches take slightly longer to access than L1. The more L2 and L3 memory available, the faster a computer can run.

Not a lot of physical space is allocated for cache. There is more space for RAM, which is usually larger and less expensive.

Clock speed

The clock speed - also known as clock rate - indicates how fast the CPU can run. This is measured in megahertz (MHz) or gigahertz (gHz) and corresponds with how many instruction cycles the CPU can deal with in a second. A 2 gHz CPU performs two billion cycles a second. A faster CPU uses more energy and creates more heat.

A computer will normally have a maximum clock speed set by default, but it is possible to change this speed in the computer BIOS. Some people increase a CPU clock speed to try to make their computer run faster - this is called overclocking.

There are limits to how fast a CPU can run and its circuitry cannot always keep up with an overclocked speed. If the clock tells the CPU to execute instructions too quickly, the processing will not be completed before the next instruction is carried out. If the CPU cannot keep up with the pace of the clock, the data is corrupted. CPUs can also overheat if they are forced to work faster than they were designed to work.