Fix implementation of IAC microinstruction

1 files changed, 2 insertions, 1 deletions

diff --git a/isa/decode.cpp b/isa/decode.cpp
index 6ab90fd..df263ad 100644
--- a/isa/decode.cpp
+++ b/isa/decode.cpp
@@ -331,7 +331,8 @@ instruction_context decode(std::uint_fast32_t flags, unsigned int pc, unsigned i
                     if (cma) ctx.acc = ~ctx.acc.value() & 07777;
                     if (cml) ctx.link = !ctx.link.value();
                     if (iac) {
-                        if (++ctx.acc.value() == 0) ctx.link = !ctx.link.value();
+                        ctx.acc = (ctx.acc.value() + 1) & 07777;
+                        if (ctx.acc.value() == 0) ctx.link = !ctx.link.value();
                     }
                     if (rar && !ral) {
                         unsigned int x = (ctx.link.value() << 12) | ctx.acc.value();