Constant VI_FIELD_HZ 

pub const VI_FIELD_HZ: u64 = 60;

Nominal NTSC field rate anchoring the VI scan cadence.

The VI dot clock is off a separate crystal (~48.68 MHz) that the N64brew wiki gives only roughly, so rather than fit an imprecise dot-clock frequency, the field cadence is anchored to the standard NTSC 60 Hz and the per-half-line period derived from the software-programmed VI_V_TOTAL. PAL uses VI_FIELD_HZ_PAL, selected by Vi::field_hz from the field length.