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scalariscvchisel

ListLookUp in CHISEL

I am new to Scala and I have been trying to understand the RISC-V architecture. I came across this piece of code, which I have been staring at for hours, unable to understand.

val csignals =                                                                                                   
  ListLookup(io.dat.inst,                                                                                       
                         List(N, BR_N  , OP1_X  ,  OP2_X  , ALU_X   , WB_X   , REN_0, MEN_0, M_X  , MT_X,  CSR.N),
           Array(       /* val  |  BR  |  op1   |   op2     |  ALU    |  wb  | rf   | mem  | mem  | mask |  csr  */
                        /* inst | type |   sel  |    sel    |   fcn   |  sel | wen  |  en  |  wr  | type |  cmd  */
              LW      -> List(Y, BR_N  , OP1_RS1, OP2_IMI , ALU_ADD ,  WB_MEM, REN_1, MEN_1, M_XRD, MT_W,  CSR.N),
              LB      -> List(Y, BR_N  , OP1_RS1, OP2_IMI , ALU_ADD ,  WB_MEM, REN_1, MEN_1, M_XRD, MT_B,  CSR.N),
              LBU     -> List(Y, BR_N  , OP1_RS1, OP2_IMI , ALU_ADD ,  WB_MEM, REN_1, MEN_1, M_XRD, MT_BU, CSR.N),...
  1. What ListLookUp exactly does here ?
  2. Is the value of 'csignals' Boolean or something else ?
Solution

  • The ListLookUp is roughly equivalent to a switch() statement in software programming, or a casez statement in Verilog RTL.

    The first argument (io.dat.inst) is the signal to match against.

    The second argument is the default value if no match occurs.

    The third argument is an Array of "key"->"value" tuples. The io.dat.inst is matched against the keys to find a match and the csignals is set to the value part if a match is found.

    So csignals is of a type List().