Add menvcfg CSR, and comment explaining why we don't have mseccfg CSR

Readme.md

@@ -29,7 +29,7 @@ Hazard3 is still under development.
 
 # Links to Specifications
 
-These are links to the ratified versions of the extensions. In general, later drafts or releases of these specifications describing the _same extension versions_ may be used for reference, and often have minor improvements to wording.
+These are links to the ratified versions of the extensions.
 
 | Extension  | Specification |
 |----------- |---------------|

hdl/hazard3_csr.v

@@ -555,8 +555,17 @@ always @ (*) begin
 		decode_match = match_mrw;
 	end
 
-	// MEDELEG, MIDELEG should not exist for M-only implementations. Will raise
+	// MEDELEG, MIDELEG should not exist for no-S-mode implementations. Will
-	// illegal instruction exception if accessed.
+	// raise illegal instruction exception if accessed.
+
+	// MENVCFG is seemingly mandatory as of M-mode v1.12, if U-mode is
+	// implemented. All of its fields are tied to 0 in our implementation.
+	MENVCFGH: if (U_MODE) begin
+		decode_match = match_mrw;
+	end
+	MENVCFG: if (U_MODE) begin
+		decode_match = match_mrw;
+	end
 
     // ------------------------------------------------------------------------
 	// Trap-handling CSRs
@@ -845,6 +854,9 @@ always @ (*) begin
 		rdata = pmp_cfg_rdata;
 	end
 
+	// MSECCFG is strictly optional, and we don't implement any of its
+	// features (ePMP etc) so we don't decode it.
+
     // ------------------------------------------------------------------------
     // U-mode CSRs
 
@@ -1272,3 +1284,7 @@ end
 `endif
 
 endmodule
+
+`ifndef YOSYS
+`default_nettype wire
+`endif

hdl/hazard3_csr_addr.vh

@@ -54,6 +54,9 @@ localparam PMPADDR13      = 12'h3bd;
 localparam PMPADDR14      = 12'h3be;
 localparam PMPADDR15      = 12'h3bf;
 
+localparam MSECCFG        = 12'h747;
+localparam MSECCFGH       = 12'h757;
+
 // Performance counters (RW)
 localparam MCYCLE         = 12'hb00; // Raw cycles since start of day
 localparam MINSTRET       = 12'hb02; // Instruction retire count since start of day
@@ -150,6 +153,10 @@ localparam MHPMEVENT29    = 12'h33d; // WARL (we tie to 0)
 localparam MHPMEVENT30    = 12'h33e; // WARL (we tie to 0)
 localparam MHPMEVENT31    = 12'h33f; // WARL (we tie to 0)
 
+// Other standard M-mode CSRs:
+localparam MENVCFG        = 12'h30a;
+localparam MENVCFGH       = 12'h31a;
+
 // Custom M-mode CSRs:
 localparam MEIE0          = 12'hbe0; // External interrupt enable register 0
 localparam MEIE1          = 12'hbe1; // External interrupt enable register 1