abc80.v: remove (useless) support for X MMU; allow NMI disable

The X-select in the MMU never worked (the M1 phase is only for the
actual opcode fetch, not immediates) so remove it.  Allow bit 2 in
port 7 to disable NMI; this is needed for CP/M and seems like an
entirely reasonable addition to Mikrodatorn's 64K extension mod.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 11 insertions, 8 deletions

diff --git a/abc80.v b/abc80.v
index 9791e23..f27dca6 100644
--- a/abc80.v
+++ b/abc80.v
@@ -1,6 +1,6 @@
 // -----------------------------------------------------------------------
 //   
-//   Copyright 2003-2010 H. Peter Anvin - All Rights Reserved
+//   Copyright 2003-2014 H. Peter Anvin - All Rights Reserved
 //
 //   This program is free software; you can redistribute it and/or modify
 //   it under the terms of the GNU General Public License as published by
@@ -488,11 +488,10 @@ module abc80 (
    wire 	 mmu_wr_e;	// MMU write enable
    wire [15:0] 	 mmu_rd_data;	// Read data output from MMU
 
-   reg [5:0] 	 mmu_map_sel;	// XXWWRR
+   reg [3:0] 	 mmu_map_sel;	// WWRR
    wire [1:0] 	 mmu_map;
 
    assign 	 mmu_map =
-		 ~cpu_m1_n ? mmu_map_sel[5:4] :
 		 ~cpu_wr_n ? mmu_map_sel[3:2] :
 		 mmu_map_sel[1:0];
 
@@ -900,7 +899,8 @@ module abc80 (
    reg [7:0]  intio_do;	 // Data out from MMU I/O ports
 
    reg        turbo_set;
-      
+   reg        nmi_dis;
+
    assign mmu_wr_e = intio_sel & ~cpu_wr_n & (cpu_a[5:0] == 5'b00011);
 
    always @(negedge rst_n or posedge fast_clk)
@@ -911,6 +911,8 @@ module abc80 (
 	     mmu_mod_data <= 0;
 	     mmu_map_sel  <= 0;
 
+	     nmi_dis      <= 0;
+
 	     turbo_set    <= 0;
 
 	     prog_led_ctl <= 0;
@@ -938,7 +940,7 @@ module abc80 (
 		    5'b00010:
 		      mmu_mod_data <= cpu_do[7:0];
 		    5'b001xx:
-		      mmu_map_sel <= cpu_do[5:0];
+		      mmu_map_sel <= cpu_do[3:0];
 		    5'b100xx:
 		      turbo_set <= 1;
 		    5'b10100:
@@ -958,7 +960,8 @@ module abc80 (
 	     else if ( ~abc_out_n[7] )
 	       begin
 		  // For compatibility with Mikrodatorn's 64K hack
-		  mmu_map_sel <= { cpu_do[1:0], cpu_do[1:0], cpu_do[1:0] };
+		  mmu_map_sel <= { cpu_do[1:0], cpu_do[1:0] };
+		  nmi_dis <= cpu_do[2];
 	       end
 	     else if ( intio_sel & ~cpu_rd_n )
 	       begin
@@ -972,7 +975,7 @@ module abc80 (
 		    5'b00011:
 		      intio_do <= mmu_rd_data[15:8];
 		    5'b001xx:
-		      intio_do <= { 2'b0, mmu_map_sel };
+		      intio_do <= { 4'b0, mmu_map_sel };
 		    5'b100xx:
 		      intio_do <= { 6'b0, cpu_turbo };
 		    5'b10100:
@@ -1042,7 +1045,7 @@ module abc80 (
    // ------------------------------------------------------------------------
 
    assign     cpu_int_n = pioa_int_n & piob_int_n;
-   assign     cpu_nmi_n = ctr_20ms;
+   assign     cpu_nmi_n = ctr_20ms | nmi_dis;
    wire       cpu_wait_n = 1'b1; // See cpu_clk_en
    wire       cpu_clk_en;