4 ms·

Well for example imagine you have a 64-bit adder - and you add two numbers together - let's assume that on e of the in puts is '1' - how long does it take until

Well for example imagine you have a 64-bit adder - and you add two numbers together - let's assume that on e of the in puts is '1' - how long does it take until the output is stable? it depends on the second value an d more importantly how long it takes for all the carries to propagate to the MSB - for a naive circuit and input of 0 the output will stabilise very quickly, for an input of 0xffff_ffff_ffff_ffff it will take 64 adder delays - an async circuit can have simple additions run faster than the worst case ones (which will still work). While a synchronous circuit would have a clock that could only go as fast as the slowest case (or pipeline things so that the output appears multiple clocks later)

jacquesm 5y ago
That's a pretty naive adder implementation. A more efficient version would stabilize after two ticks.
- Taniwha 5y ago
  yup, but I'm using it as an example to make a point here (hence my use of the word 'naive') - also the whole point of doing async stuff is that you don't have to worry about 'ticks'
  - jacquesm 5y ago
    Even in async you have to worry about ticks, it's just that the ticks aren't global. Think of the extent to which a clock can propagate as a lightcone, within that domain everything is synchronized, but it need not be synchronized with things on the outside. The smaller the domains the more asynchronous a design gets. But you'll never be async all the way down, at some point you will have to worry about stabilizing your outputs and passing them on to the next stage in that stable situation. Compare synchronous serial lines with an asynchronous interface such as a centronics printer interface. The former will happily send zeros + clocks all day long absent a signal, the latter will strobe it's 'ready' output only when there actually is data, but it will still have to output that pulse, which serves as a very local clock.
Const-me 5y ago
> imagine you have a 64-bit adder Integer addition is too easy. On all modern computers, add instructions take at most 1 cycle. Even vector ones like vpaddq AVX2 which adds four 64-bit numbers to another four numbers.

jacquesm 5y ago

That's a pretty naive adder implementation. A more efficient version would stabilize after two ticks.

Taniwha 5y ago
yup, but I'm using it as an example to make a point here (hence my use of the word 'naive') - also the whole point of doing async stuff is that you don't have to worry about 'ticks'
- jacquesm 5y ago
  Even in async you have to worry about ticks, it's just that the ticks aren't global. Think of the extent to which a clock can propagate as a lightcone, within that domain everything is synchronized, but it need not be synchronized with things on the outside. The smaller the domains the more asynchronous a design gets. But you'll never be async all the way down, at some point you will have to worry about stabilizing your outputs and passing them on to the next stage in that stable situation. Compare synchronous serial lines with an asynchronous interface such as a centronics printer interface. The former will happily send zeros + clocks all day long absent a signal, the latter will strobe it's 'ready' output only when there actually is data, but it will still have to output that pulse, which serves as a very local clock.

Const-me 5y ago

> imagine you have a 64-bit adder Integer addition is too easy. On all modern computers, add instructions take at most 1 cycle. Even vector ones like vpaddq AVX2 which adds four 64-bit numbers to another four numbers.