Optimize csr address handling

rtl/serv_decode.v

@@ -147,28 +147,27 @@ module serv_decode
    /*
     Bits 26, 22, 21 and 20 are enough to uniquely identify the eight supported CSR regs
     mtvec, mscratch, mepc and mtval are stored externally (normally in the RF) and are
-    treated differently from mstatus, mie, mcause and mip which are stored in serv_csr.
+    treated differently from mstatus, mie and mcause which are stored in serv_csr.
     
-    The former get a 2-bit address (as found in serv_params.vh) while the latter get a
+    The former get a 2-bit address as seen below while the latter get a
     one-hot enable signal each.
     
-    Hex|2 222|Reg
+    Hex|2 222|Reg     |csr
-    adr|6 210|name
+    adr|6 210|name    |addr
-    ---|-----|-------
+    ---|-----|--------|----
-    300|0_000|mstatus
+    300|0_000|mstatus | xx
-    304|0_100|mie
+    304|0_100|mie     | xx
-    305|0_101|mtvec
+    305|0_101|mtvec   | 01
-    340|1_000|mscratch
+    340|1_000|mscratch| 00
-    341|1_001|mepc
+    341|1_001|mepc    | 10
-    342|1_010|mcause
+    342|1_010|mcause  | xx
-    343|1_011|mtval
+    343|1_011|mtval   | 11
-    344|1_100|mip
     
     */
 
    //true  for mtvec,mscratch,mepc and mtval
-   //false for mstatus, mie, mcause, mip
+   //false for mstatus, mie, mcause
-   wire csr_valid = op20 | (op26 & !op22 & !op21);
+   wire csr_valid = op20 | (op26 & !op21);
 
    assign o_rd_csr_en = csr_op;
 
@@ -180,11 +179,7 @@ module serv_decode
    assign o_csr_source = funct3[1:0];
    assign o_csr_d_sel = funct3[2];
    assign o_csr_imm_en = opcode[4] & opcode[2] & funct3[2];
-
+   assign o_csr_addr = {op26 & op20, !op26 | op21};
-   assign o_csr_addr = (op26 & !op20) ? CSR_MSCRATCH :
-		       (op26 & !op21) ? CSR_MEPC :
-		       (op26)         ? CSR_MTVAL :
-		       CSR_MTVEC;
 
    assign o_alu_cmp_eq = funct3[2:1] == 2'b00;
 

rtl/serv_rf_if.v

@@ -83,13 +83,31 @@ module serv_rf_if
    //0 : RS1
    //1 : RS2 / CSR
 
-
    assign o_rreg0 = {1'b0, i_rs1_raddr};
-   assign o_rreg1 =
+
-		 i_trap   ? {4'b1000, CSR_MTVEC} :
+   /*
-		 i_mret   ? {4'b1000, CSR_MEPC} :
+    The address of the second read port (o_rreg1) can get assigned from four
-		 i_csr_en ? {4'b1000, i_csr_addr} :
+    different sources
-		 {1'b0,i_rs2_raddr};
+
+    Normal operations : i_rs2_raddr
+    CSR access        : i_csr_addr
+    trap              : MTVEC
+    mret              : MEPC
+
+    Address 0-31 in the RF are assigned to the GPRs. After that follows the four
+    CSRs on addresses 32-35
+
+    32 MSCRATCH
+    33 MTVEC
+    34 MEPC
+    35 MTVAL
+
+    The expression below is an optimized version of this logic
+    */
+   wire sel_rs2 = !(i_trap | i_mret | i_csr_en);
+   assign o_rreg1 = {~sel_rs2,
+		     i_rs2_raddr[4:2] & {3{sel_rs2}},
+		     {1'b0,i_trap} | {i_mret,1'b0} | ({2{i_csr_en}} & i_csr_addr) | ({2{sel_rs2}} & i_rs2_raddr[1:0])};
 
    assign o_rs1 = i_rdata0;
    assign o_rs2 = i_rdata1;