From 82c461307eba25265867f233a27060621ac6cba4 Mon Sep 17 00:00:00 2001
From: Julian Blake Kongslie
Date: Mon, 31 Oct 2022 19:03:38 -0700
Subject: TAD is 12-bit addition with carry-complements-link, not 13-bit
 addition

---
 isa/decode.cpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

(limited to 'isa/decode.cpp')

diff --git a/isa/decode.cpp b/isa/decode.cpp
index c35118b..af38c64 100644
--- a/isa/decode.cpp
+++ b/isa/decode.cpp
@@ -34,8 +34,8 @@ instruction_context decode(unsigned int dfifb, unsigned int pc, unsigned int bit
             inst.need_write_acc = true;
             inst.need_write_link = true;
             inst.ef = [](auto &ctx) {
-                unsigned int sum = (ctx.link.value() << 12) + ctx.acc.value() + ctx.data.value();
-                ctx.link = (sum >> 12) & 1;
+                unsigned int sum = ctx.acc.value() + ctx.data.value();
+                ctx.link = ctx.link.value() ^ (sum >> 12);
                 ctx.acc = sum & 07777;
             };
             break;
-- 
cgit v1.2.3