Cache

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 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.

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.

Each CPU core has its own L1 cache, but may share L2 and L3 caches.

Processor type

There are two main types of CPU: CISC and RISC.

Most desktop or laptop computers use CISC (complex instruction set computing) architecture made by Intel or AMD. Smartphones and tablets use RISC (reduced instruction set computing) ARM architecture.

The key differences between the two CPUs are:

  • Instructions - RISC has fewer instructions than CISC. CISC generally have hundreds of instructions whereas RISC usually has a lot less. To do complex tasks, RISC CPUs must combine simple operations from their reduced instruction set. The instruction cycle in CISC is more complex than RISC, so RISC can be more efficient at performing simpler tasks.
  • Physical size - in order to handle all those instructions CISC CPUs are larger and require more silicon to make.
  • Speed - RISC CPUs run at a lower clock speed than CISC CPUs. They can perform simpler tasks more quickly than CISC, but more intensive tasks will be better on a CISC CPU. However, smartphones and tablets are generally not used to do intensive tasks like playing the most hi-spec advanced games.
  • Energy consumption - because CISC CPUs are larger, they use more electricity. RISC CPUs are designed to use less power and they can go into 'sleep mode' when not actively processing a program.
  • Design - smartphones and tablets combine their processing architecture into a system on a chip (SOC). As RISC CPUs are much smaller than CISC, more functions (including the memory and other hardware) can be combined with the CPU in one chip. CISC CPUs are usually built into a system which has a heat sink and fan to cool them down. Smartphones or tablets do not have space for heat sinks or fans.
  • Cost - RISC CPUs use less power and are cheaper to make.

CPU expert Steve Furber explains why the ARM processor was created and why it is now used in all smartphones and tablets