From 529c1cc2496e24c6282c3fa9e0a45272b7f500b6 Mon Sep 17 00:00:00 2001
From: Julian Blake Kongslie
Date: Sun, 2 May 2021 16:58:46 -0700
Subject: Add a bunch of microcoded instructions.

This is a minimum viable PDP-8 that can print "Hello, world!" and halt.
---
 hdl/core.sv | 93 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 90 insertions(+), 3 deletions(-)

(limited to 'hdl/core.sv')

diff --git a/hdl/core.sv b/hdl/core.sv
index 5bee974..02ff231 100644
--- a/hdl/core.sv
+++ b/hdl/core.sv
@@ -115,6 +115,7 @@ always_ff @(posedge clk) begin
                 if (`lag(mem_read_valid)) begin
                     state = FETCH;
                     {opcode, operand} = `lag(mem_read_data);
+                    $display("%d decode %x: %b %b", $time, pc-1, opcode, operand);
                     {i, z, wip} = operand;
                     if (z)
                         address = {page, wip};
@@ -130,6 +131,89 @@ always_ff @(posedge clk) begin
                                 pc = address;
                             end
                         end
+                        'b111: begin
+                            casez (operand)
+                                'b0????????: begin
+                                    automatic bit cla, cll, cma, cml, rar, ral, bsw, iac;
+                                    {cla, cll, cma, cml, rar, ral, bsw, iac} = operand[7:0];
+                                    if (cla) acc = 0;
+                                    if (cll) link = 0;
+                                    if (cma) acc = ~acc;
+                                    if (cml) link = ~link;
+                                    if (iac) {link, acc} += 1;
+                                    if (rar && !ral) begin
+                                        {link, acc} = {acc[0], link, acc[11:1]};
+                                        if (bsw) {link, acc} = {acc[0], link, acc[11:1]};
+                                    end
+                                    if (ral && !rar) begin
+                                        {link, acc} = {acc, link};
+                                        if (bsw) {link, acc} = {acc, link};
+                                    end
+                                    if (bsw && !(rar || ral)) acc = {acc[5:0], acc[11:6]};
+                                end
+                                'b1????0??0: begin
+                                    automatic bit cla, sma, sza, snl, osr, hlt;
+                                    automatic bit skip;
+                                    {cla, sma, sza, snl, osr, hlt} = {operand[7:4], operand[2:1]};
+                                    skip = 0;
+                                    if (sma && acc[11]) skip = 1;
+                                    if (sza && acc == 0) skip = 1;
+                                    if (snl && link != 0) skip = 1;
+                                    if (skip) pc++;
+                                    if (cla) acc = 0;
+                                    if (osr) begin
+                                        $display("unsupported front panel switch test");
+                                        $finish;
+                                    end
+                                    if (hlt) state = HALT;
+                                end
+                                'b1????1??0: begin
+                                    automatic bit cla, spa, sna, szl, osr, hlt;
+                                    automatic bit skip;
+                                    {cla, spa, sna, szl, osr, hlt} = {operand[7:4], operand[2:1]};
+                                    skip = 1;
+                                    if (spa && acc[11]) skip = 0;
+                                    if (sna && acc == 0) skip = 0;
+                                    if (szl && link != 0) skip = 0;
+                                    if (skip && (spa || sna || szl)) pc++;
+                                    if (cla) acc = 0;
+                                    if (osr) begin
+                                        $display("unsupported front panel switch test");
+                                        $finish;
+                                    end
+                                    if (hlt) state = HALT;
+                                end
+                                default: begin
+                                    $display("%d decoded unknown opcode %x: %b %b", $time, pc-1, opcode, operand);
+                                    $finish;
+                                end
+                            endcase
+                        end
+                        'b110: begin
+                            case (operand[8:3])
+                                'b000100: begin
+                                    case (operand[2:0])
+                                        'b001: if (!tx_valid) pc++;
+                                        'b110: begin
+                                            tx_valid = 1;
+                                            tx_data = {1'b0, acc[6:0]};
+                                        end
+                                        default: begin
+                                            $display("unsupported device op %b", operand[2:0]);
+                                            $finish;
+                                        end
+                                    endcase
+                                end
+                                default: begin
+                                    $display("unsupported device %b", operand[8:3]);
+                                    $finish;
+                                end
+                            endcase
+                        end
+                        default: begin
+                            $display("%d decoded unknown opcode %x: %b %b", $time, pc-1, opcode, operand);
+                            $finish;
+                        end
                     endcase
                 end
             end
@@ -170,8 +254,8 @@ always_ff @(posedge clk) begin
                 mem_write = 1;
                 mem_write_data = address[DATA_BITS-1:0];
                 case (opcode)
-                    'b000, 'b001, 'b010: state = `lag(mem_ready) ? PREINC : AGEN;
-                    'b011, 'b100, 'b101: state = `lag(mem_ready) ? PREINC : EXEC;
+                    'b000, 'b001, 'b010: state = `lag(mem_ready) ? AGEN : PREINC;
+                    'b011, 'b100, 'b101: state = `lag(mem_ready) ? EXEC : PREINC;
                 endcase
             end
 
@@ -231,7 +315,10 @@ always_ff @(posedge clk) begin
 
             MEMWAIT: state = `lag(mem_ready) ? FETCH : MEMWAIT;
 
-            HALT: $finish;
+            HALT: begin
+                $display("%d halt state reached", $time);
+                $finish;
+            end
         endcase
     end
 end
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