Decrunch the executable file "tetris" (63,284 bytes) with Imploder or xfd. The decrunched file will have a size of 235,300 bytes. Load the decrunched file into a hex editor. Search for the text string "CREDITS3" (it's at position $EF1). Change the character 3 to a value between 4 and 9 inclusive). Save the result. You can use this file as it is or recrunch it with Imploder. When you next start the game using this file, you'll see that you have as many credits as you have chosen according to the options screen. You can start the game with the number of credits shown or if you press 'Fire' to change the credits, you can have up to 256 continues.