Take an unheadered ROM of Zelda 3 (aka LTTP)

Change $39 and $3A to 0xEAs *Original values are 80 16*.

This NOPs out a Branch Always instruction. That's it. Now load up your game in an emulator.

Press the L button to freeze the game. Music will continue running as will animations like water and flowers, but sprites and Link will be frozen on the screen.

To advance by a frame, press the R button. To unfreeze the game, simply press L again.