The Kickstart, a ROM chip which contains essential parts of the Operating System and an Amiga can not boot without it.
Originally, the first Amiga (A1000) had a small bootstrap ROM, which contained code to load the basic operating system from a disk, known as the "Kickstart disk" into 256 kilobytes of RAM, which was then transformed into ROM by disabling the write signal to this RAM. Of course, the contents of this pseudo-ROM was lost when the computer was turned off. The point of using RAM was that Commodore wanted to be able to continue developing the operating system without requiring people to change ROMs when a version was available; instead, they could just distribute new "Kickstart" disks, which they did three times, upgrading version 1.0 to 1.1 and later 1.2.
Version 2.0 was too large to fit into this pseudo-ROM, so the Kickstart was stored on ROM chips directly. This is the reason why Amiga 1000s cannot run more modern versions of AmigaOS.
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