From 430ed65253573017d16fdc5f7a0aafc25b83e24d Mon Sep 17 00:00:00 2001
From: Julian Blake Kongslie
Date: Wed, 2 Nov 2022 14:04:08 -0700
Subject: Avoid rereading the FLAGS register; use FLAGS_SAVED for GTF.

---
 isa/decode.cpp | 37 +++++++++++++++----------------------
 1 file changed, 15 insertions(+), 22 deletions(-)

(limited to 'isa')

diff --git a/isa/decode.cpp b/isa/decode.cpp
index 77bbe8b..6ea3bb5 100644
--- a/isa/decode.cpp
+++ b/isa/decode.cpp
@@ -7,9 +7,9 @@
 instruction_context decode(std::uint_fast32_t flags, unsigned int pc, unsigned int bits, bool interrupt)
 {
     //bool gt = (flags >> 10) & 1;
-    //bool ir = (flags >> 9) & 1;
+    bool ir = (flags >> 9) & 1;
     //bool ii = (flags >> 8) & 1;
-    //bool ie = (flags >> 7) & 1;
+    bool ie = (flags >> 7) & 1;
     //bool u = (flags >> 6) & 1;
     bool ifb = (flags >> 3) & 7;
     bool df = flags & 7;
@@ -86,41 +86,37 @@ instruction_context decode(std::uint_fast32_t flags, unsigned int pc, unsigned i
                     switch (bits & 07) {
                         case 0:
                             // SKON skip if interrupts enabled
-                            inst.read_ctlreg = FLAGS;
                             inst.possibly_redirects = true;
-                            inst.ef = [](auto &ctx) {
-                                if (ctx.ctlval.value() & FLAG_INT_ENABLE)
+                            inst.ef = [ie](auto &ctx) {
+                                if (ie)
                                     ctx.next_pc = (ctx.next_pc & 07777) | ((ctx.next_pc + 1) & 07777);
                             };
                             break;
                         case 1:
                             // ION set int_enable_delay
-                            inst.read_ctlreg = FLAGS;
                             inst.write_ctlreg = FLAGS;
-                            inst.ef = [](auto &ctx) {
-                                ctx.ctlval.value() |= FLAG_INT_ENABLE_DELAY;
+                            inst.ef = [flags](auto &ctx) {
+                                ctx.ctlval = flags | FLAG_INT_ENABLE_DELAY;
                             };
                             break;
                         case 2:
                             // IOF clear int_enable and int_enable_delay
-                            inst.read_ctlreg = FLAGS;
                             inst.write_ctlreg = FLAGS;
-                            inst.ef = [](auto &ctx) {
-                                ctx.ctlval.value() &= ~FLAG_INT_ENABLE_DELAY & ~FLAG_INT_ENABLE;
+                            inst.ef = [flags](auto &ctx) {
+                                ctx.ctlval = flags & ~FLAG_INT_ENABLE_DELAY & ~FLAG_INT_ENABLE;
                             };
                             break;
                         case 3:
                             // SRQ skip if pending interrupt
-                            inst.read_ctlreg = FLAGS;
                             inst.possibly_redirects = true;
-                            inst.ef = [](auto &ctx) {
-                                if (ctx.ctlval.value() & FLAG_INT_REQUEST)
+                            inst.ef = [ir](auto &ctx) {
+                                if (ir)
                                     ctx.next_pc = (ctx.next_pc & 07777) | ((ctx.next_pc + 1) & 07777);
                             };
                             break;
                         case 4:
                             // GTF get flags
-                            inst.read_ctlreg = FLAGS;
+                            inst.read_ctlreg = FLAGS_SAVED;
                             inst.need_read_link = true;
                             inst.ef = [](auto &ctx) {
                                 auto flags = ctx.ctlval.value();
@@ -130,12 +126,11 @@ instruction_context decode(std::uint_fast32_t flags, unsigned int pc, unsigned i
                             break;
                         case 5:
                             // RTF restore flags
-                            inst.read_ctlreg = FLAGS;
                             inst.need_read_acc = true;
                             inst.need_write_link = true;
                             inst.write_ctlreg = FLAGS;
-                            inst.ef = [](auto &ctx) {
-                                if ((ctx.ctlval.value() & FLAG_INT_ENABLE) && (ctx.acc.value() & FLAG_INT_ENABLE)) {
+                            inst.ef = [ie](auto &ctx) {
+                                if (ie && (ctx.acc.value() & FLAG_INT_ENABLE)) {
                                     ctx.ctlval = ctx.acc.value() | FLAG_INT_ENABLE_DELAY;
                                 } else if (ctx.acc.value() & FLAG_INT_ENABLE) {
                                     ctx.ctlval = (ctx.acc.value() | FLAG_INT_ENABLE_DELAY) & ~FLAG_INT_ENABLE;
@@ -283,11 +278,9 @@ instruction_context decode(std::uint_fast32_t flags, unsigned int pc, unsigned i
                         switch (bits & 07) {
                             case 01:
                                 // CDF change data field
-                                inst.read_ctlreg = FLAGS;
                                 inst.write_ctlreg = FLAGS;
-                                inst.ef = [field](auto &ctx) {
-                                    ctx.ctlval.value() &= ~FLAG_DF;
-                                    ctx.ctlval.value() |= field << FLAG_DF_SHIFT;
+                                inst.ef = [flags, field](auto &ctx) {
+                                    ctx.ctlval = (flags & ~FLAG_DF) | (field << FLAG_DF_SHIFT);
                                 };
                                 break;
                             default:
-- 
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