Changes
/* Split Screen facility */
The other register pair at 6802h and 6803h define the start address in memory (same number form as R12 and R13 respectively in the 6845, and therefore high byte first) which partly represents the location in memory from which to start displaying data for the lower screen (the soft scroll register and scan line counter are used to define the exact value in addition to the start address). This value is read when the 6845's Horizontal counter matches Horizontal Displayed (R1) on the line programmed it is then stored to be used on the next line.
To get an exact required byte address, use of split screen and soft scroll must be used.
This feature allows the lower part of the picture to come from a separate memory area and be separately scrolled. The start address is loaded into 6845's MA counter. However, note Note that soft scrolling (Section 2.5 below) acts on because the address is loaded into MA it effects the rest of the whole screenunless a new split screen value is programmed.
Note that care should be taken with programming this facility such that the screen split does not alter the function of address bits A1-A8 and the dynamic memory refresh is not upset. This can be accomplished by setting the start of the second screen to lie on 16k boundary. The reason is that the dynamic memory refresh is derived from the memory address that the 6845 describes.
The value in register pair 6802h/6803h is the first displayed line, and not the start address of the 16k block.
Also, during vertical retrace, the value in register 6801h should not be set to 257 less the total number of scan lines on the screen.
