From 763beb022c799598ef4eaa9750ad259497bd2e84 Mon Sep 17 00:00:00 2001
From: Julian Blake Kongslie
Date: Sat, 22 Oct 2022 22:53:26 -0700
Subject: Merge TT_INPUT and TT_OUTPUT into TT_BITS

---
 isa/decode.cpp | 48 ++++++++++++++++++++++++++++++------------------
 1 file changed, 30 insertions(+), 18 deletions(-)

(limited to 'isa/decode.cpp')

diff --git a/isa/decode.cpp b/isa/decode.cpp
index dde5b74..41d125d 100644
--- a/isa/decode.cpp
+++ b/isa/decode.cpp
@@ -74,54 +74,66 @@ instruction_context decode(unsigned int dfifb, unsigned int pc, unsigned int bit
         case 6: // IOT
             switch ((bits >> 3) & 077) {
                 case 004:
+                    // TELETYPE TELEPRINTER/PUNCH
                     switch (bits & 07) {
                         case 0:
-                            inst.write_ctlreg = TT_OUTPUT;
+                            // Set TTO flag
+                            inst.read_ctlreg = TT_BITS;
+                            inst.write_ctlreg = TT_BITS;
                             inst.ef = [](auto &ctx) {
-                                ctx.ctlval = 0x600;
+                                ctx.ctlval.value() |= TTO_FLAG | TTO_FLAG_OLD;
                             };
                             break;
                         case 1:
-                            inst.read_ctlreg = TT_OUTPUT;
+                            // Skip if TTO flag is set
+                            inst.read_ctlreg = TT_BITS;
                             inst.possibly_redirects = true;
                             inst.ef = [](auto &ctx) {
-                                if (ctx.ctlval.value() & 0x200)
+                                if (ctx.ctlval.value() & TTO_FLAG)
                                     ctx.next_pc = (ctx.next_pc & ~07777) | ((ctx.next_pc + 1) & 07777);
                             };
                             break;
                         case 2:
-                            inst.write_ctlreg = TT_OUTPUT;
+                            // Clear TTO flag
+                            inst.read_ctlreg = TT_BITS;
+                            inst.write_ctlreg = TT_BITS;
                             inst.ef = [](auto &ctx) {
-                                ctx.ctlval = 0;
+                                ctx.ctlval.value() &= ~TTO_FLAG & ~TTO_FLAG_OLD;
                             };
                             break;
                         case 4:
+                            // Print to TTO
                             inst.need_read_acc = true;
-                            inst.read_ctlreg = TT_OUTPUT;
-                            inst.write_ctlreg = TT_OUTPUT;
+                            inst.read_ctlreg = TT_BITS;
+                            inst.write_ctlreg = TT_BITS;
                             inst.ef = [](auto &ctx) {
                                 auto &x = ctx.ctlval.value();
-                                x &= ~0x1ff;
-                                x |= (ctx.acc.value() & 0xff) ^ 0x80; // 0x7f instead of 0xff for real PDP-8 IO
-                                x |= 0x100;
+                                auto chr = ctx.acc.value() ^ 0x80;
+                                x &= ~TTO_DATA;
+                                x |= (chr << TTO_DATA_SHIFT) & TTO_DATA;
+                                x |= TTO_TX;
                             };
                             break;
                         case 5:
+                            // Skip if TTO flag is set or TTI flag is set
+                            inst.read_ctlreg = TT_BITS;
                             inst.possibly_redirects = true;
-                            // FIXME this instruction wants to read both TT_OUTPUT and TT_INPUT
                             inst.ef = [](auto &ctx) {
-                                assert(false);
+                                if (ctx.ctlval.value() & (TTI_FLAG | TTO_FLAG))
+                                    ctx.next_pc = (ctx.next_pc & ~07777) | ((ctx.next_pc + 1) & 07777);
                             };
                             break;
                         case 6:
+                            // Print to TTO and clear TTO flag
                             inst.need_read_acc = true;
-                            inst.read_ctlreg = TT_OUTPUT;
-                            inst.write_ctlreg = TT_OUTPUT;
+                            inst.read_ctlreg = TT_BITS;
+                            inst.write_ctlreg = TT_BITS;
                             inst.ef = [](auto &ctx) {
                                 auto &x = ctx.ctlval.value();
-                                x &= ~0x7ff;
-                                x |= (ctx.acc.value() & 0xff) ^ 0x80;
-                                x |= 0x100;
+                                auto chr = ctx.acc.value() ^ 0x80;
+                                x &= ~TTO_FLAG & ~TTO_FLAG_OLD & ~TTO_DATA;
+                                x |= (chr << TTO_DATA_SHIFT) & TTO_DATA;
+                                x |= TTO_TX;
                             };
                             break;
                         default:
-- 
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