Chisel Cannot reassign to read-only - scala

Question

This is a continuation from the other question i asked : Instanciation of a class doesn't work (Chisel/Scala)
Actually, I wanna print the "result" variable. I wrote this command : sbt test, and I don't understand this error :
"chisel3.internal.ChiselException: Cannot reassign to read-only combinedTausworthe.b: OpResult[UInt<32>] "
This is the Test file
import chiseltest._
import org.scalatest.freespec.AnyFreeSpec
class combined_test extends AnyFreeSpec with ChiselScalatestTester {
"combinedTauswhore" in {
test(new combinedTausworthe() ){ c =>
//Initialisation
c.clock.step(1)
println(c.result)
c.clock.step(1)
println(c.result)
}
}
}
This is the class :
import chisel3._
class combinedTausworthe extends Module{
val io = IO(new Bundle {
})
val seed1 = RegInit(322769304.U(32.W))
val seed2 = RegInit(424235419.U(32.W))
val seed3 = RegInit(212119443.U(32.W))
var result= 0.U
var b = ((seed1 << 13)(31,0) ^ seed1)
seed1 := ((seed1 & 4294967294L.U) << 12)(31,0) ^ b
b := ((seed2 << 2)(31,0) ^ seed2) >> 25
seed2 := ((seed2 & 4294967294L.U) << 4)(31,0) ^ b
b := ((seed3<<3)(31,0) ^ seed3) >> 11
seed3 := ((seed3 & 4294967294L.U) <<17)(31,0) ^ b
result := (seed1 ^ seed2 ^ seed3)
}
object Main extends App {
}

Answer 1

In Chisel3, := is the connection operator for driving a signal with some value. Think about it as creating an electrical connection. In your code, it looks like you are trying to reassign to a mutable variable rather than drive the value of that variable with a value, try =:
import chisel3._
class combinedTausworthe extends Module{
val io = IO(new Bundle {
})
val seed1 = RegInit(322769304.U(32.W))
val seed2 = RegInit(424235419.U(32.W))
val seed3 = RegInit(212119443.U(32.W))
var result= 0.U
var b = ((seed1 << 13)(31,0) ^ seed1)
seed1 := ((seed1 & 4294967294L.U) << 12)(31,0) ^ b
// Note using assignment (=), not connection (:=)
b = ((seed2 << 2)(31,0) ^ seed2) >> 25
seed2 := ((seed2 & 4294967294L.U) << 4)(31,0) ^ b
b = ((seed3<<3)(31,0) ^ seed3) >> 11
seed3 := ((seed3 & 4294967294L.U) <<17)(31,0) ^ b
// Same thing here
result = (seed1 ^ seed2 ^ seed3)
}
That being said, as Scala is a functional programming language and we stylistically prefer pure/immutable constructs, try just using values:
import chisel3._
class combinedTausworthe extends Module{
val io = IO(new Bundle {
})
val seed1 = RegInit(322769304.U(32.W))
val seed2 = RegInit(424235419.U(32.W))
val seed3 = RegInit(212119443.U(32.W))
// Note result var removed and replaced with val below
val b1 = ((seed1 << 13)(31,0) ^ seed1)
seed1 := ((seed1 & 4294967294L.U) << 12)(31,0) ^ b1
val b2 = ((seed2 << 2)(31,0) ^ seed2) >> 25
seed2 := ((seed2 & 4294967294L.U) << 4)(31,0) ^ b2
val b3 = ((seed3<<3)(31,0) ^ seed3) >> 11
seed3 := ((seed3 & 4294967294L.U) <<17)(31,0) ^ b3
val result = (seed1 ^ seed2 ^ seed3)
}