Jump to content
Sign in to follow this  
adamantyr

Disk buffers in VDP, safe to overwrite?

Recommended Posts

Hey all,

 

Here is a question... so the TI disk system reserves space at the top of VDP memory for file buffers. You can reduce this by from 3 file support to 1 by calling the FILES subprogram, but it still takes up space from about >3B00 onwards.

 

My question is... can you safely wipe out this section, say if you wanted to put the sprite pattern table at >3800 onwards? Obviously it would trash the sprite pattern table if you had to do a file load later on, but if you could restore the data from CPU memory afterwards it seems alright. Obviously it would be stupid to try and do a memory image load INTO this section...

Share this post


Link to post
Share on other sites

You are probably safe as long as you have no plans to invoke a disk DSR after you trash it. However, if you place the sprite pattern table at >3800, you have room for 124 sprite patterns (123 for nanoPEB/CF7+) without trashing the file buffers, which, after declaring 1 file support starts at >3BE4 (>3BDC for nanoPEB/CF7+).

 

...lee

Share this post


Link to post
Share on other sites

My question is... can you safely wipe out this section, say if you wanted to put the sprite pattern table at >3800 onwards? Obviously it would trash the sprite pattern table if you had to do a file load later on, but if you could restore the data from CPU memory afterwards it seems alright. Obviously it would be stupid to try and do a memory image load INTO this section...

 

Yes, we are doing that in the megademo. But I believe a more detailed analysis of what you need to save and restore could reduce the required amount of CPU considerably. We also use all of the scratch pad in the demo so we save and restore that as well, but it's probably only a few bytes that need to be restored for disk access to work.

Share this post


Link to post
Share on other sites

Good to know... sounds like some experimentation will be needed. I'm irked that the MSX had the ENTIRE VDP to use without an issue... they must keep all their disk handling stuff in CPU.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
Sign in to follow this  

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...