From 6fcb3077eb5d2d91f30b0152fc48a208077cd816 Mon Sep 17 00:00:00 2001
From: David Banks <dave@hoglet.com>
Date: Sun, 30 Sep 2018 12:29:08 +0100
Subject: [PATCH] Z80: In INTACK cycle, force 0xFF onto the databus

---
 chip-z80/support.js | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/chip-z80/support.js b/chip-z80/support.js
index b5a2666..f8859dd 100644
--- a/chip-z80/support.js
+++ b/chip-z80/support.js
@@ -110,7 +110,14 @@ var state        = 0;
 var last_rd_done = 1;
 
 function handleBusRead(){
-    if(!isNodeHigh(nodenames['_rd'])){
+    if(!isNodeHigh(nodenames['_m1']) && !isNodeHigh(nodenames['_iorq'])) {
+        // Interrupt acknownledge cycle, force 0xFF onto the bus
+        // In IM0 this is seen as RST 0x38
+        // In IM1 this is ignored
+        // In IM2 this is used as the low byte of the vector
+        // TODO: ideally this "vector" would be a configurable parameter
+        writeDataBus(0xff);
+    } else if(!isNodeHigh(nodenames['_rd'])){
         var a = readAddressBus();
         var d = eval(readTriggers[a]);
         if(d == undefined)