Decoding instructions - Classic way

• For each bit combination may lead to a different instruction

• Or better put "Map each control signal onto a separate bit of the instruction word"
  • Computers are complicated and words may become too long
  • Some combinations may cause damage (3 sate output clashes)
  • Most combinations will be useless

• To solve this use could use a smaller number of bits for the control signal, and have that within a larger number of bits needed for the control

• This is all rarely changed, due to such large changes required

Decoding instructions - Memory way

• Control signals are stored in ROM, (acts as a look up table)

• Input the instruction code, this is an address

• Outputs the control signal

• VERY MUCH OVERKILL because there can be loads of input combinations

Decoding - Programmable method

Note: connections are passed from each vertical layer, input left to the output right

• Generalised to create whatever type of outputs from the inputs

• Connections between each layer are different depending out desired outputs

• Used in most programable devices

CPLDs - Complex Programmable Logic Devices

FPGAs - Field-Programmable Gate Arrays (most commonly used)

In practice

Note: AND gates replaced with a decoder

Note: this has two fields, the first two bits and the second two bits

• Most instructions are broken down into several fields (groups of bits that control different tasks)

Bringing it all together

Harvard

• ROMs contain the lookup tables, for sources and destinations and next instruction

Von Neumann

• Remember there is a fetch cycle and a execute cycle