3. CRTC type 4 is the same as CRTC type 3. The registers also repeat as they do on the type 3.
== UM6845R and R12/R13 ==
The UM6845R differs to other CRTC in respect of R12/R13.
When VCC=0, R12/R13 is re-read at the start of each line. R12/R13 can therefore be changed for each scanline when VCC=0.
Just like other CRTCs when RC==(R9-1), the current MA is captured for the next char-line.
In demos to make a display compatible with all CRTCs program R12/R13 when VCC!=0. This will then take effect at the next frame start.
== UM6845R status register ==