Slots
Introduction
Let’s say you have some hardware which has to keep track of multiple similar ongoing activities, you may want to implement an array of “slots” to do so. This example show how to do it using Area, OHMasking.first, onMask and reader.
Implementation
This implementation avoid the use of Vec. Instead, it use Area which allow to mix signal, registers and logic definitions in each slot.
Note that the reader API is for SpinalHDL version comming after 1.9.1
package spinaldoc.examples.advanced
import spinal.core._
import spinal.lib._
import scala.language.postfixOps
case class SlotsDemo(slotsCount : Int) extends Component {
//...
//Create the hardware for each slot
//Note each slot is an Area, not a Bundle
val slots = for(i <- 0 until slotsCount) yield new Area{
//Because the slot is an Area, we can define mix signal, registers, logic definitions
//Here are the registers for each slots
val valid = RegInit(False)
val address = Reg(UInt(8 bits))
val age = Reg(UInt(16 bits)) //Will count since how many cycles the slot is valid
//Here is some hardware behaviour for each slots
//Implement the age logic
when(valid){
age := age + 1
}
//removeIt will be used as a slot interface later on
val removeIt = False
when(removeIt){
valid := False
}
}
//Logic to allocate a new slot
val insert = new Area{
val cmd = Stream(UInt(8 bits)) //interface to issue requests
val free = slots.map(!_.valid)
val freeOh = OHMasking.first(free) //Get the first free slot (on hot mask)
cmd.ready := free.orR //Only allow cmd when there is a free slot
when(cmd.fire){
//slots.onMask(freeOh)(code) will execute the code for each slot where the corresponding freeOh bit is set
slots.onMask(freeOh){slot =>
slot.valid := True
slot.address := cmd.payload
slot.age := 0
}
}
}
//Logic to remove the slots which match a given address (assuming there is not more than one match)
val remove = new Area{
val cmd = Flow(UInt(8 bits))//interface to issue requests
val oh = slots.map(s => s.valid && s.address === cmd.payload) //oh meaning "one hot"
when(cmd.fire){
slots.onMask(oh){ slot =>
slot.removeIt := True
}
}
val reader = slots.reader(oh) //Create a facility to read the slots using "oh" as index
val age = reader(_.age) //Age of the slot which is selected by "oh"
}
//...
}
object SlotsDemo extends App {
SpinalVerilog(SlotsDemo(4))
}
In practice
For instance, this kind of slot pattern is used in Tilelink coherency hub to keep track of all ongoing memory probes in flight:
As well in the DRAM / SDR / DDR memory controller to implement the handeling of multiple memory transactions at once (having multiple precharge / active / read / write running at the same time to improve performances) :
As well in the NaxRiscv (out of order CPU) load-store-unit to handle the store-queue / load-queue hardware (a bit too scary to show here in the doc XD)