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-rw-r--r--src/devices/machine/mc146818.cpp7
1 files changed, 5 insertions, 2 deletions
diff --git a/src/devices/machine/mc146818.cpp b/src/devices/machine/mc146818.cpp
index 8780643c2e3..254fa8a23b2 100644
--- a/src/devices/machine/mc146818.cpp
+++ b/src/devices/machine/mc146818.cpp
@@ -582,8 +582,11 @@ WRITE8_MEMBER( mc146818_device::write )
case REG_A:
// top bit of A is read only
- m_data[REG_A] = data & ~REG_A_UIP;
- update_timer();
+ if ((data ^ m_data[REG_A]) & ~REG_A_UIP)
+ {
+ m_data[REG_A] = data & ~REG_A_UIP;
+ update_timer();
+ }
break;
case REG_B: