anchor.diff:  
      Reg#(Maybe#(Word)) csr_dcsr <- mkReg(tagged Invalid);
   
  -   // ----------------
  -   // These CSRs are technically not present in the user-mode ISA.
  -   // We have them here because even bare-metal riscv-gcc ELF binaries access them.
  -
  -   Reg#(Word)  csr_mstatus    <- mkRegU;
  -   Reg#(Word)  csr_mtvec      <- mkRegU;
   
      // ----------------------------------------------------------------
      // Non-architectural state, for this model
  
positive.code:          tagged DMem_Resp_Ok .u: begin
              case (decoded.funct3)
                 f3_LB:  begin
                            Int#(8)   s8    = unpack(truncate (u));
                            Word_S    s     = signExtend (s8);
                            Word      value = pack(s);
                            fa_finish_with_Rd(decoded.rd, value);
                         end
  
negative.code: 
     // CSRs
     Reg#(Bit #(64))   csr_cycle   <- mkConfigReg(0);
     Reg#(Bit #(64))   csr_instret <- mkConfigReg(0);
  
     // ----------------
     // These CSRs are technically not present in the user-mode ISA.
  
     Reg#(Word)  csr_mepc       <- mkRegU;
     Reg#(Word)  csr_mcause     <- mkRegU;
  