From 74746f4da558abed6b5f6a000d4e915a0a57d21e Mon Sep 17 00:00:00 2001
From: Michael Pavone <pavone@retrodev.com>
Date: Fri, 3 Apr 2020 23:47:17 -0700
Subject: Top bits of address register should be cleared on partial command
 word write. Fixes Mona in 344 bytes demo

---
 vdp.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

(limited to 'vdp.c')

diff --git a/vdp.c b/vdp.c
index 93ad895..37ad611 100644
--- a/vdp.c
+++ b/vdp.c
@@ -3732,7 +3732,9 @@ int vdp_control_port_write(vdp_context * context, uint16_t value)
 		}
 	} else {
 		uint8_t mode_5 = context->regs[REG_MODE_2] & BIT_MODE_5;
-		context->address = (context->address &0xC000) | (value & 0x3FFF);
+		//contrary to what's in Charles MacDonald's doc, it seems top 2 address bits are cleared
+		//needed for the Mona in 344 Bytes demo
+		context->address = value & 0x3FFF;
 		context->cd = (context->cd & 0x3C) | (value >> 14);
 		if ((value & 0xC000) == 0x8000) {
 			//Register write
-- 
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