I think your mental model is wrong; the 6845 doesn't process VDU commands. The MOS processes them and changes OS state. (Under some circumstances you can write to the 6845 registers, but that's not normal).
When you redefine characters with VDU 23 (eg "VDU 23,224,1,2,3,4,5,6,7,8") then the 6845 isn't even in the loop; this is just setting memory contents (UDG character RAM)
Later when you write a character (eg VDU 224) then the OS reads those memory contents, transforms them and writes the results to _new_ memory locations (screen memory RAM). Again the 6845 isn't in the loop.
Out of band to all of this (almost literally; it happens on the opposite CPU clock cycle) the 6845 reads the screen memory and generates the necessary video output.
So none of your UDG VDU commands goes anywhere near the 6845.
When you redefine characters with VDU 23 (eg "VDU 23,224,1,2,3,4,5,6,7,8") then the 6845 isn't even in the loop; this is just setting memory contents (UDG character RAM)
Later when you write a character (eg VDU 224) then the OS reads those memory contents, transforms them and writes the results to _new_ memory locations (screen memory RAM). Again the 6845 isn't in the loop.
Out of band to all of this (almost literally; it happens on the opposite CPU clock cycle) the 6845 reads the screen memory and generates the necessary video output.
So none of your UDG VDU commands goes anywhere near the 6845.
Statistics: Posted by sweh — Tue Nov 12, 2024 1:54 am