Virtual Jaguar accuracy

[Cyprian]

I have three observations where VJ differs from the Jag:

 

1. Screen positioning:
   VJ - the screen is moved to top left vs my LCD TV. We can see that on following screenshots: Jag_Interrputs / VJ_Interrupts
2. CPU/GPU accuracy:
   I did a simple counter on the CPU and the GPU. Left counter is the CPU, the right one - the GPU (value in hex) on screenshots Jag_GPU_CPU_counter / VJ_GPU_CPU_counter. And the VJ's CPU counter is 1.7 times faster, GPU's 2,4 times faster that the Jag.
3. 68000 interrupts:
   When PIT timer interrupt is near to the VBL's one then VJ drops my VBL interrupt and the screen jumps. On the screenshots Jag_Interrputs / VJ_Interrupts we see pure dump of  INT1 done during interrupt. The screen is 1bpp and INT1 is represented as 16 pixels. The VBL is lost when single dots are drawn on "VJ_Interrupts"

 

 

 

 

Jag_Interrputs:

 

VJ_Interrupts:

 

 

Jag_GPU_CPU_counter:

 

VJ_GPU_CPU_counter:

 

Edited March 28, 2021 by Cyprian

[matmook]

Yep, that' why it's important to test on the real hardware from time to time.

[Zerosquare]

#1 is possibly a VJ bug, but margins and picture centering varies from TV to TV (especially on CRT ones), so relying on it is not a good idea anyways.

 

#2 is a consequence of VJ not emulating stuff down to the cycle-level (which would make it even more complex, and slower). So if you want your game to run well in emulation, don't rely on "implicit" timing such as how fast the CPU/GPU/DSP are. Use explicit timing sources such as the VBL and timer interrupts instead, and don't depend on perfect timing accuracy. Even on real hardware, this advice is usually good, too.

 

#3 sounds like an actual bug or limitation in VJ. Try reporting it to @Shamus.

Edited March 28, 2021 by Zerosquare

[Cyprian]

#1 - for me ST/Falcon had well centered screen and they are reference. In additional, emulators like Steem have nice feature - possibility to extend visible border from normalt o large and max.

#2 - ok

#3- I've just sent him PM