On all CRTCs, only the 5 least significant bits of the selected register number are considered to write to a register. The other bits are ignored.
On CRTCs 0/1/2, only the 5 least significant bits of the selected register number are considered to read a register.:* On CRTCs 0/2, registers 18-31 read as 0. * On CRTC 1, registers 18-30 read as 0, register 31 reads as &FF.
On CRTCs 3/4, only the 3 least significant bits of the selected register number are considered to read a register, according to the following table: