TetrOS

TetrOS is a small feature rich Tetris clone which is written in Assembly. It fits completely into a 512 byte boot sector as it requires only 446 bytes (which is the maximum allowed size of the first stage boot loader in the master boot record (MBR) of a drive) and is executed during the boot sequence before any operating system is loaded. Actually, it does not need any existing operating system. TetrOS is an operating system, hence the suffix OS in its name.”

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