The full address that is displayed is defined by the soft scroll register, 6845's internal scan line counter and the programmed address.
The When VCC=R4 and RCC=R9 the programmed address is stored when 6845's Horizontal counter matches R0 on the line programmed, at other times the programmed address is stored when 6845's Horizontal counter matches Horizontal Displayed (R1) on the line programmed.
It The value is then loaded into 6845never used when VCC=0 and RCC=0, R12/R13 are always used here so you can's MA counter t trigger the split to happen on the scanline before a new frame and change the MA of the first scanline of a frame. If you do the value is used at the next available line after. (e.g. VCC=0, RCC=1). At all other times the programmed split value is used to set MA on the next scanline. Note that because the address is loaded into MA it effects the rest of the screen until the next time it is re-programmed or the display restarts.
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.
