does anyone know the precise measurement of the cpu speed as the various docs out there give different speeds with a variation of a couple thousand clock cycles (ok i know this isn't much) , but all agree generally agree on the 1.78 Mhz bit. A doc I read yesterday describe a calculation which lead to a speed of 1,786,840.0 hz and a frame speed of 29,780 1/3 hz (this is for ntsc)
p.s. If you know this precise speed, do you also know the pal cpu speed

For the NTSC NES, the CPU speed is 1789772.7272727272727Hz (exactly 1/2 NTSC colorburst, which is 3.5795454545454545).

For PAL, the CPU speed is 1662607Hz. Yes, it's slower than the NTSC CPU; this is why sound in PAL games is off-pitch when played in an NTSC NES (and vice-versa).

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Quietust
P.S. If you don't get this note, let me know and I'll write you another.

mmm lot of decimal places - rounded up or down I don't think any docs i have give that num

I suspect it's a fraction that produces a repeating decimal.

this may be a harder question but do you also know the precise timing of the frame period - hblank / vblank

I may be confused here but as instructions are whole numbers and ntsc cpu runs at 1789772.7272727272727Hz does this mean that cpu frame runs until it has hit negative numbers (like emu cpu core do) before it executes nmi (else as i see it would be cutting in through part of a instruction being executed). ???

You wanted the CPU's frequency, so I gave it to you.
1789772.7272727272727Hz == 1.789772727...MHz.
You can do the math from there.

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Quietust
P.S. If you don't get this note, let me know and I'll write you another.

Interrupts would happen after the current instruction finishes.

I think you'd reset your CPU timer if it ran out while an instruction was executing, since the crystal speed is independant of the CPU. Right?

Yes I did want it and I'm grateful
p.s thanks members - so the crystal speed is't directly to the core - so it is like emu's do it.