Sprite Expand Bug in Defender for C64 by AtariSoft

The official Defender game cartridge for the Commodore 64, released by AtariSoft in 1983 and developed by Joe Simko, features a unique bug that results in oversized graphics. This occurs when the player presses the Restore key at the peak of their ship’s explosion, causing the next life to display enlarged sprites for the player’s ship, alien enemies, and the humans being defended. This glitch is due to a minor initialization bug in the original 6502 assembly code. Robin first demonstrates the VIC-II sprite expansion capabilities using BASIC, then examines the bug on a real C64 using the CRT image emulator format. He employs the Super Snapshot machine language monitor to pinpoint the bug’s origin and implement a fix. The patch is refined using the LOAD ADDRESS program from the 1541 Test/Demo Diskette, flashed onto an Easy Flash 3 cartridge, and thoroughly tested in gameplay. Additionally, Robin investigates another recently discovered Defender bug, which will be covered in a future video.