1 files changed, 19 insertions, 0 deletions

diff --git a/isa/decode.cpp b/isa/decode.cpp
index 1d46375..1979982 100644
--- a/isa/decode.cpp
+++ b/isa/decode.cpp
@@ -155,6 +155,25 @@ instruction_context decode(unsigned int dfifb, unsigned int pc, unsigned int bit
                     if (skip)
                         ctx.next_pc = (ctx.next_pc & 070000) | ((ctx.next_pc + 1) & 007777);
                 };
+            } else if ((bits & 0401) == 0401) {
+                bool cla = bits & 0200;
+                bool mqa = bits & 0100;
+                bool mql = bits & 0020;
+                bool extended_arith = bits & 0056;
+                inst.need_read_acc = mqa || mql;
+                inst.need_read_mq = mqa;
+                inst.need_write_acc = cla || mqa;
+                inst.need_write_mq = mql;
+                inst.ef = [cla, mqa, mql, extended_arith](auto &ctx) {
+                    assert(!extended_arith);
+                    if (cla) ctx.acc = 0;
+                    auto new_acc = ctx.acc;
+                    auto new_mq = ctx.mq;
+                    if (mqa) new_acc = ctx.acc.value() | ctx.mq.value();
+                    if (mql) new_mq = ctx.acc.value();
+                    ctx.acc = new_acc;
+                    ctx.mq = new_mq;
+                };
             } else {
                 assert(false);
             }