[openMSX-commits] [SCM] openMSX (main) branch master updated. RELEASE_0_10_0_RC1-7-g6ff4592

[Wouter V. <m97...@us...>]

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- Log -----------------------------------------------------------------
commit 6ff459214a189857979f0be6203420489dcf5904
Author: wouter <ver...@gm...>
Date:   Sun Dec 29 22:13:29 2013 +0100

    Fixed VRAM corruption with 40MHz Z80
    
    See sd_snatcher's post of 29-12-2013, 01:02 here:
      http://www.msx.org/forum/msx-talk/openmsx/wanted-feedback-mac-os-x-users?page=8
    
    The (remaining) problem was not that there was too fast VRAM access (two
    accesses too fast after each other) but that the VRAM address registers were
    already changed before the request was actually executed.
    
    See comments in code for more details.

-----------------------------------------------------------------------

Summary of changes:
 src/video/VDP.cc |   23 ++++++++++++++++++++++-
 src/video/VDP.hh |    3 ++-
 2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/src/video/VDP.cc b/src/video/VDP.cc
index 948403b..e1600fb 100644
--- a/src/video/VDP.cc
+++ b/src/video/VDP.cc
@@ -834,6 +834,7 @@ void VDP::scheduleCpuVramAccess(bool isRead, byte write, EmuTime::param time)
 	// E.g. OUT (#98),A followed by IN A,(#98) returns the just written value.
 	if (!isRead) cpuVramData = write;
 	cpuVramReqIsRead = isRead;
+	cpuVramReqAddr = (controlRegs[14] << 14) | vramPointer; // see below
 	if (unlikely(cpuAccessScheduled())) {
 		// Already scheduled. Do nothing.
 		// The old request has been overwritten by the new request!
@@ -845,7 +846,24 @@ void VDP::scheduleCpuVramAccess(bool isRead, byte write, EmuTime::param time)
 
 void VDP::executeCpuVramAccess(EmuTime::param time)
 {
-	int addr = (controlRegs[14] << 14) | vramPointer;
+	// We don't know whether the vram address is calculated now or already
+	// when the vram request gets scheduled. It's also not possible to test
+	// this difference on real HW (AFAIK). Though it does make a difference
+	// when e.g. emulating an overclocked Z80 (40Mhz) in combination with
+	// 'too_fast_vram_access=ignore'. See also this forum post:
+	//     http://www.msx.org/forum/msx-talk/openmsx/wanted-feedback-mac-os-x-users?pa
+	//     sd_snatcher's post of 29-12-2013, 01:02
+	// If I have to guess then I'd say the address is only calculated at
+	// this moment because that saves some hardware registers, and looking
+	// at the rest of the VDP design, the Yamaha engineers really tried to
+	// save registers whenever possible.
+	// Still, to fix the artificial turbo-Z80 scenario, we now calculate
+	// the address early.
+	// It's possible that once we emulate proper turbo-Z80 emulation (switch
+	// back to 3.5MHz speed when accessing the VDP) we can remove this part
+	// again from the VDP emulation.
+	//int addr = (controlRegs[14] << 14) | vramPointer;
+	int addr = cpuVramReqAddr;
 	if (displayMode.isPlanar()) {
 		// note: also extended VRAM is interleaved,
 		//       because there is only 64kB it's interleaved
@@ -1558,6 +1576,9 @@ void VDP::serialize(Archive& ar, unsigned version)
 	} else {
 		ar.serialize("readAhead", cpuVramData);
 	}
+	if (ar.versionAtLeast(version, 6)) {
+		ar.serialize("cpuVramReqAddr", cpuVramReqAddr);
+	}
 	ar.serialize("cpuExtendedVram", cpuExtendedVram);
 	ar.serialize("displayEnabled", displayEnabled);
 	byte mode = displayMode.getByte();
diff --git a/src/video/VDP.hh b/src/video/VDP.hh
index 2b8ead4..9360a23 100644
--- a/src/video/VDP.hh
+++ b/src/video/VDP.hh
@@ -875,6 +875,7 @@ private:
 	  * variable indicates whether the pending request is read or write.
 	  */
 	bool cpuVramReqIsRead;
+	unsigned cpuVramReqAddr;
 
 	/** Does CPU interface access main VRAM (false) or extended VRAM (true)?
 	  * This is determined by MXC (R#45, bit 6).
@@ -902,7 +903,7 @@ private:
 	/** Cached version of tooFastAccess setting. */
 	bool allowTooFastAccess;
 };
-SERIALIZE_CLASS_VERSION(VDP, 5);
+SERIALIZE_CLASS_VERSION(VDP, 6);
 
 } // namespace openmsx
 


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