Three new memory mapped registers have been added within the ASIC, to provided a horizontally split screen facility. One at address 6801h defines the scan line after which the screen split occurs. A value of zero (as at power on reset) will turn this feature off.
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 full byte address is defined by the soft scroll register, 6845's internal scan counter and the split screen start address). This value is read when the 6845's Horizontal counter matches Horizontal Displayed (R1) on the line programmed it . This value is then storedto be used on the next line.
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 on the line after. Note that because the address is loaded into MA it effects the rest of the screen unless a new split screen value is programmed.