trigger IRQ</PRE>
When IRQs are enabled and the PPU starts drawing at the specified location,
an IRQ is generated.&nbsp; Horizontal IRQs are rarely used, because of
the high frequency (every scanline) of the interrupts.

<P><A NAME="Reg4210"></A><B><FONT FACE="Lucida Console">Register $4210:
NMI Register (1b/R)</FONT></B>
<PRE>n---vvvv&nbsp; n: nmi flag&nbsp; v: version number</PRE>
The full behavior of these bits is not known (at least, I don't know).&nbsp;
Yoshi doc lists "$5A22" for the v bits, whatever 