UM6845:
VL6845: R31 reading leaves data bus undriven, R18-R30 absent
Status register bit7 goes to '0' when R31 written to.
Status b6: set to 0 when R16/R17 read, set to 1 when LPEN strobe
Status b5: 1=current in vertical blanking
R8 adds:
b0/b1=interlace
b2= 0:binary addressing, 1:row/column addressing
b3= 0:shared memory, 1:transparent memory
b4/b5=cursor skew
b6= 0:p34 is memory address RA4, 1:p34 is update strobe
b7= 0:updates during blanking, 1:updates during phi2
R18: update location H (next memory address about to be accessed)
R19: update location L