One of the trickier things to manage on a NES (without additional hardware on the cartridge) was maintaining a static information panel on the bottom of the display - for the score and so forth - while the portion of the display devoted to gameplay scrolled freely around both vertically and horizontally.

The trick we used at Zippo (Wizard and Warriors II and 3, Solar Jetman etc.) a trick given to us by Rare, was to change the scroll registers and I think the character look up location at the moment the screen refresh cycle reached the appropriate point on the display. These registers would then need to be reset during the vertical blanking interval. So all in all either 100 or 120 times a second depending on the TV system.

Since we couldn't afford to keep the CPU hanging around doing nothing while we waited for the cathode ray tube gun to hit the right point on the screen the trick was a two parter.

First you would get the sound chip - such as it was - to play an inaudible sample of a determined length which would then trigger a CPU interrupt at more or less the right time, within say two or three scanlines of the position of our static panel. You would then position a spare sprite on top of a visible pixel at precisely the right point on the screen so that when it flipped its hardware collision detection bit this was precisely the right time to switch the scroll registers etc.

These were the kinds of shenanigans that made programming the NES intricate and time consuming, and also in later years I suspect made the job of emulator writers something of a misery.