The DISPTMG signal is composited from its 3 internal subcomponents: HBORDER (Horizontal), VBORDER (Vertical), IBORDER (Immediate) by using the NOR function. DISPTMG is "0" if at least one of its subcomponents is "1". These 3 subcomponents are all independent of each other.
On CRTC 2, when HSYNC is active, the condition HCC=0 does not disable HBORDER, unlike the other CRTCs.
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