Forums

Sega Master System / Mark III / Game Gear
SG-1000 / SC-3000 / SF-7000 / OMV
Home - Forums - Games - Scans - Maps - Cheats - Credits
Music - Videos - Development - Hacks - Translations - Homebrew

View topic - VRAM writing during display

Reply to topic
Author Message
  • Joined: 10 Feb 2020
  • Posts: 1
Reply with quote
VRAM writing during display
Post Posted: Mon Feb 10, 2020 2:51 pm
Hello,

I read in the technical doc that VRAM reading is buffered.
But what about RAM writing during the display ? It is highly probable that VRAM also needs to be accessed to print pixels.

The pixel clock seems to be half of the VDP clock. Is there a cycle "dedicated" for VRAM updating (if needed...) and the other for the display ?

Thanks !
  View user's profile Send private message
  • Site Admin
  • Joined: 19 Oct 1999
  • Posts: 13096
  • Location: London
Reply with quote
Post Posted: Mon Feb 10, 2020 5:19 pm
Yes, there’s a free “slot” for CPU VRAM access every so often during active display - I think every 32 pixels or so, but the details are complicated.
  View user's profile Send private message Visit poster's website
  • Joined: 05 Sep 2013
  • Posts: 2586
Reply with quote
Post Posted: Mon Feb 10, 2020 6:25 pm
I'd say probably once every 38 pixels, as if you write to VRAM every 25 CPU cycles you can corrupt VRAM, if you write every 26 CPU cycles you won't.

but it's likely more complicated than that. You probably can write slightly faster during h_blank, but I'm just supposing that.

or it could be the other way around
  View user's profile Send private message Visit poster's website
Reply to topic



Back to the top of this page

Back to SMS Power!