[rtl] Add support for direct mode in mtvec

The Ibex RISC-V core currently lacks support for the direct mode in the
mtvec (#1419). This commit make mtvec support both vectored and direct
mode.

To ensure compatibility with existing test programs, the initial value
of mtvec.MODE is set to vectored mode.For both vectored mode and direct
mode, mtvec.BASE must be 256-byte aligned.

Author: leesum1

rtl/ibex_cs_registers.sv

@@ -575,10 +575,10 @@ module ibex_cs_registers #(
     mtval_en     = 1'b0;
     mtval_d      = csr_wdata_int;
     mtvec_en     = csr_mtvec_init_i;
-    // mtvec.MODE set to vectored
+    // mtvec.MODE default set to vectored,but can change to direct
     // mtvec.BASE must be 256-byte aligned
     mtvec_d      = csr_mtvec_init_i ? {boot_addr_i[31:8], 6'b0, 2'b01} :
-                                      {csr_wdata_int[31:8], 6'b0, 2'b01};
+                                      {csr_wdata_int[31:8], 6'b0, csr_wdata_int[1:0]};
     dcsr_en      = 1'b0;
     dcsr_d       = dcsr_q;
     depc_d       = {csr_wdata_int[31:1], 1'b0};
@@ -634,7 +634,13 @@ module ibex_cs_registers #(
         CSR_MTVAL: mtval_en = 1'b1;
 
         // mtvec
-        CSR_MTVEC: mtvec_en = 1'b1;
+        CSR_MTVEC: begin 
+          mtvec_en = 1'b1;
+          // Change to direct MODE if software writes an unsupported value
+          if ((mtvec_d[1:0] != 2'b00) && (mtvec_d[1:0] != 2'b01)) begin
+            mtvec_d[1:0] = 2'b00;
+          end
+        end
 
         CSR_DCSR: begin
           dcsr_d = csr_wdata_int;

rtl/ibex_if_stage.sv

@@ -182,11 +182,13 @@ module ibex_if_stage import ibex_pkg::*; #(
 
   // exception PC selection mux
   always_comb begin : exc_pc_mux
-    irq_vec = exc_cause.lower_cause;
-
-    if (exc_cause.irq_int) begin
+    // mtvec is vectored mode
+    if (csr_mtvec_i[1:0] == 2'b01) begin
       // All internal interrupts go to the NMI vector
-      irq_vec = ExcCauseIrqNm.lower_cause;
+      irq_vec = exc_cause.irq_int ? ExcCauseIrqNm.lower_cause : exc_cause.lower_cause;
+    end else begin
+      // mtvec is direct mode, so irq_vec is always 0
+      irq_vec = 5'b00000;
     end
 
     unique case (exc_pc_mux_i)