Z3-Fast and RTG RAM is supported on the 68020, 68020+FPU and 68040 only.
Depending on the processor variant, you can expand the RAM up to 10 MB or theoretically up to 2 GB. The limit of 2 GB is valid for the Amiga models A3000 and A4000. On the mainboard of these computers you can plug in up to 18 MB RAM, and for every of the four Zorro-III bus expansion you can expand the memory to the maximum!
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 operative system without requiring people to change ROMs when the new version was available; instead, they could just distribute new "Kickstart" disks, which they did three times, upgrading version 1.0 to 1.1, 1.2. Version 2.0 was too large to fit into this pseudo-ROM, and that's the reason why Amiga 1000s cannot run the modern versions of the Amiga OS.
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