Just to add, the 6845 is not a VDP.
The purpose of the 6845 is to generate all the correct addresses in RAM to generate the video signal and to perform most of the display timing. The 6845 does not process the display data. Indeed, it never sees the display data that is read from RAM.
The 6845 is just a bunch of registers as far as the processor is concerned. Once the correct MODE settings are written to the 6845, normally there is no further need to update the 6845 registers (unless the screen is scrolled or the cursor position changes).
It’s the VideoProc (Video ULA) that receives the display data and coverts it to a video signal (except when using MODE 7). This also is just registers to the processor. When a colour palette change takes place, this is the chip that receives the new palette information.
The OS provides an abstraction layer, and that is what the VDU command is. All VDU commands are instructions to the OS ROM (or to the Graphics ROM when fitted).
Mark
The purpose of the 6845 is to generate all the correct addresses in RAM to generate the video signal and to perform most of the display timing. The 6845 does not process the display data. Indeed, it never sees the display data that is read from RAM.
The 6845 is just a bunch of registers as far as the processor is concerned. Once the correct MODE settings are written to the 6845, normally there is no further need to update the 6845 registers (unless the screen is scrolled or the cursor position changes).
It’s the VideoProc (Video ULA) that receives the display data and coverts it to a video signal (except when using MODE 7). This also is just registers to the processor. When a colour palette change takes place, this is the chip that receives the new palette information.
The OS provides an abstraction layer, and that is what the VDU command is. All VDU commands are instructions to the OS ROM (or to the Graphics ROM when fitted).
Mark
Statistics: Posted by 1024MAK — Tue Nov 12, 2024 6:27 am