On all CRTCs, VBORDER is disabled when VCC=0 and enabled when VCC=R6.
On CRTCs 0/1/2, the condition VCC=R6 is considered immediately to activate the VBORDER.
On CRTCs 3/4, the condition VCC=R6 is tested only at new character line start. The update of R6 during the character line is therefore not considered.