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VDP timing

Discussion in 'Technical Discussion' started by saxman, Nov 3, 2012.

  1. saxman

    saxman

    Oldbie Tech Member
    I've been all over the net, and I can't find an answer to this one very vital question. When does the Hint counter actually refresh?

    Like, say I want to change register 10 and set it to 150. Previously, it was set to 223 (so an H interrupt would occur right below the screen area). So, first Hint happens at the end of line 223, then I set register 10 to a value of 150. Will the very next Hint occur at line 151, or will I have to wait a cycle for the the interrupt to occur there?

    Also, I have always been under the impression, based on documentation I've read, that the line counter gets reset at line 0. This would effectively sync the counter with the top of the screen. Does anyone know for certain that this is actually what happens, or can the line counter value from the bottom of the screen get carried over to the top of the next frame?
     
  2. TmEE

    TmEE

    Master of OPL3-SA2/3 Tech Member
    1,726
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    Estonia, Rapla City
    T-04YBSC-A !
    From the docs of Charles MacDonald :

    There is 262 lines in 60Hz and 313 lines in 50Hz. Line count nhould start from first active display line.

    EDIT: Midframe changes only happen when the counter expires, on large enough value it means next frame if the write is done after the int happens. But when it is done before then when the int fires you get your newly written count used.
     
  3. LocalH

    LocalH

    roxoring your soxors Tech Member
    Do we know where on the designated lines it is reloaded?