The border has higher priority than pixels but lower priority than the black colour output when HSYNC/VSYNC are active.
This The DISPTMG signal is composited from its 3 internal subcomponents: HBORDER (Horizontal), VBORDER (Vertical), IBORDER (Immediate) by using the NOR function.
The 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 the DISPTMG on CRTC 2.
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