The DISPTMG signal is composited from its 3 internal subcomponents: HBORDER (Horizontal), VBORDER (Vertical), IBORDER (Immediate) by using the NOR function.
=== Immediate Border ===
DISPTMG can be immediately forced to 0 by using R8 (DISPTMG Skew) on CRTCs 0/3/4 or by setting R6=0 on CRTC 1. It is not possible to force DISPTMG on CRTC 2.