1. The way you figure out the amount of CPU cycles executed during VBlank is as follows:

A scanline is equivalent to 341 PPU cycles. During the VBlank period, the CPU runs 20 scanlines worth of cycles. Thus the amount of PPU cycles executed during this period is 341*20 = 6820. 6820 PPU cycles = 2273.333... CPU cycles, which is the number that you are looking for.

2. Here is a diagram that might help you...

Scanline	NTSC NES's doings
0	VBlank starts
20	VBlank ends. Visible portion of a frame begins here (assuming BG and SP are enabled)
21	First scanline to be rendered (any data processed here is not visible)
260	Last scanline to be rendered
261	PPU does nothing here (no data is actually processed)

Thus the VBlank flag should be cleared during scanline 20. For more info on this read Brad Taylor's docs on the PPU.

You should assume that the NES starts on scanline 20 and not on scanline 0 (VBlank) as soon as you turn it on.

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