3
\$\begingroup\$

Wikipedia states "An ALU is a combinational logic circuit, meaning that its outputs will change asynchronously in response to input changes."

But, the definition of a combinational logic circuit is: "combinational logic ... where the output is a pure function of the present input only."

If the output of a combinational logic circuit is purely determined by present input, is Wikipedia stating that it's asynchronous because of the time it takes for an electrical signal to travel through the circuit? If so, why is it stating it as with "this means" as if combinational logic implies a delay (as this would be the case with any circuit)?

\$\endgroup\$
3
  • 1
    \$\begingroup\$ typically the ALU sits between two synchronous components, e.g. between two pipeline registers. what happens is that, at a clock edge, the content of the input register latches and changes, and the signal travels through the ALU and stabalises by the next clock edge, at which point the result is latched by the output register. the ALU in between the registers is completely combinational \$\endgroup\$
    – Einheri
    Dec 5 '17 at 23:43
  • \$\begingroup\$ "asynchronous" here means "not in relation to some clock signal" \$\endgroup\$
    – user253751
    Dec 6 '17 at 1:04
  • \$\begingroup\$ If there are no FlipFlops or Latches inside the ALU, then input changes will cause output changes. \$\endgroup\$ Dec 6 '17 at 5:19
3
\$\begingroup\$

Synchronous logic refers specifically to that with registers whose values are changed at (synchronous) to a clock edge. The registers act as a memory, so the output is not necessarily reflective of the current state of the input.

Asynchronous means there is no clock, and so the output, as stated, depends only on the inputs. There is a delay, which is due to the finite time it takes for the signal to propagate through the logic path, but that is not why it is referred to as asynchronous logic.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.