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-rw-r--r--src/emu/machine/z8536.c1
1 files changed, 1 insertions, 0 deletions
diff --git a/src/emu/machine/z8536.c b/src/emu/machine/z8536.c
index c1393b2988d..a497f3aefc4 100644
--- a/src/emu/machine/z8536.c
+++ b/src/emu/machine/z8536.c
@@ -828,6 +828,7 @@ void z8536_device::external_port_w(int port, int bit, int state)
case PORT_A:
case PORT_B:
{
+ assert((PORT_A_DATA_DIRECTION + (port << 3)) >= 0 && (PORT_A_DATA_DIRECTION + (port << 3)) < ARRAY_LENGTH(m_register));
UINT8 ddr = m_register[PORT_A_DATA_DIRECTION + (port << 3)];
if (!BIT(ddr, bit)) return;