Mill Computing, Inc. › Forums › The Mill › Architecture › Scratchpad design decision › Reply To: Scratchpad design decision
I’m sorry that I sound far more ignorant than I am. Apparently I’m bad a communication. Sorry.
The belt uses temporal addressing; the scratchpad uses spatial addressing. There are two addresses involved in a spill/fill: the belt address of the value to be spilled somewhere, and the “somewhere” address needed to choose what value to fill. The present Mill uses temporal for the first, and like any reference the spill must execute before its target drops off the belt.
But the addressing logic to get such a scratch entry back into the space where adds and such could address it is too expensive because it would push up the size of the crossbar. So we keep the addresses space separate.
These are all things I know, have known, or figured were the case. I have never been confused on them, but it seems I sounded that way. Again, sorry.
Perhaps you are thinking of a scheme whereby the spill op would push the value into a side stack and the fill would use a stack offset rather than a temporal reference to address it.
While I have considered a stack or second belt for the Scratchpad (as I’m sure you have), no, I was thinking of a spatially-addressed, disjoint Scratchpad addressed by entry rather than one of those. I figured something similar was already present in the hardware for another purpose (holding old-but-still-present items on the belt).
Therefore either the team never thought of it or there were complications that prevented its use. And if the later, I wondered what those were. And I now have an answer to that.
The only question left is whether this holding place for “out of the output latches but not yet fallen off” belt entries exists or not. And if so, whether its mechanism would actually be similar to what a spatially-addressed, entry-based Scratchpad would require for mapping entries to its storage.
(I thought the Spiller would be a sensible place for this storage space-saving transfer time on function call spill/fill. Hence our entire Spiller digression.)