The computer that loaded its system from a floppy

When the Amiga 1000 reached the shops in 1985, AmigaOS still had plenty of bugs. Instead of burning it into ROM straight away, Commodore did something else: it put a daughterboard with 256 KB of RAM in the machine, the writable control store, and loaded the core of the system, Kickstart, from a floppy disk ^[1].

At power-on, a small boot ROM, of just 8 KB, puts a request for the Kickstart disk on the screen. It copies 256 KB from the disk to the RAM chips on the daughterboard, write-protects that memory and jumps to it ^[2]. In practice there are two small ROMs, one for the low 8 bits and one for the high 8 bits of the data bus, and Kickstart sits in the same address space where the other Amigas have their ROM, from $F80000 to $FFFFFF ^[4].

You write it once and then only read it: Europeans called it WOM, a pun on ROM.

In Europe this memory became known as WOM, Write Once Memory, a pun on ROM: you write it once and then only read it ^[1]. Because it is locked, a reset does not force you to load Kickstart again; only switching the machine off erases it ^[1]. That is why the A1000 asks for two disks at power-on: first Kickstart, then Workbench.

The advantage was that the system could be updated with a new disk. That is how the A1000 ran Kickstart 1.0 and 1.1, from 1985 and 1986, and then 1.2 and 1.3 ^[3]. Some programs even took advantage of the architecture: on the A1000, Dragon's Lair had a version of the code that used those 256 KB for data ^[3]. On the Amigas that followed, such as the A500 and A2000, Kickstart moved into ROM and booting got faster ^[3].

The idea did not die with the A1000. The first Amiga 3000s also loaded Kickstart from disk and let you choose the system version at boot, with no extra tools ^[3]. And many A1000 owners went the other way, with boards that put Kickstart in ROM. The Kickstart Eliminator, recreated today as open source, uses four 27C512 EPROMs and a programmed GAL, and even gives the 256 KB of WOM back as free memory for programs or for a RAM disk ^[2].

On the bench, these boards have their quirks. One owner who fitted such an adapter to an A1000 described misrouted address traces on the board and a /BYTE line he had to tie to 5 V with a wire for the adapter to work in 16-bit mode ^[4].