The position of the raster interrupt is based on the horizontal sync position (6845's R2) and horizontal sync width (6845's R3). Note that for a given value of R2 (here so that HSYNC is not active on the first horizontal character of a new scan line), values of 6 or greater for the horizontal sync position do not give a change in position, therefore it is triggered on the trailing edge of the HSYNC sent to the monitor and not the trailing edge of the HSYNC from the 6845. e.g. if Horizontal Sync width was set to 14, raster interrupt would trigger on cycle 6 not 14.
The internal comparison used :
0 PRI7 PRI6 PRI5 PRI4 PRI3 PRI2 PRI1 PRI0 == VC5 VC4 VC3 VC2 VC1 VC0 RC2 RC1 RC0
PRI is the programmed value. The index denotes the bit number.