From 26d54c2395f2c38df662b17db7789667f92e87b3 Mon Sep 17 00:00:00 2001
From: "R. Belmont" <rb6502@users.noreply.github.com>
Date: Tue, 8 Sep 2009 01:09:30 +0000
Subject: [PATCH] Naomi/AW: Enforce read-only status of PowerVR ID registers.

---
 src/mame/video/dc.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/src/mame/video/dc.c b/src/mame/video/dc.c
index d54e469c479..265f6cccd25 100644
--- a/src/mame/video/dc.c
+++ b/src/mame/video/dc.c
@@ -1014,7 +1014,13 @@ WRITE64_HANDLER( pvr_ta_w )
 	reg = decode_reg_64(offset, mem_mask, &shift);
 	dat = (UINT32)(data >> shift);
 	old = pvrta_regs[reg];
-	pvrta_regs[reg] = dat; // 5f8000+reg*4=dat
+
+	// Dreamcast BIOS attempts to set PVRID to zero and then dies 
+	// if it succeeds.  Don't allow.
+	if ((reg != PVRID) && (reg != REVISION))
+	{
+		pvrta_regs[reg] = dat; // 5f8000+reg*4=dat
+	}
 
 	switch (reg)
 	{