Old PCsenvironmental hazard

Kickstart, in the Amiga context, is the ROM-based 1 portion of AmigaOS. It's not only a bootstrap ROM but actually contains the most significant and essential parts of the operating system itself - namely, the AmigaOS kernel and some of the most important shared libraries:

exec.library - tasks (processes), memory management, messages (events), i-o, interrupts, etc.

graphics.library - low-level graphics functions

dos.library - file access (through filesystems), mounting unmounting disk-based devices

input.library - mouse, keyboard, and joystick handling

intuition.library - the graphical user interface (screens, windows, menus, buttons, etc.)

workbench.library - the GUI desktop

Kickstart also contains a low-level device driver for accessing the floppy drive(s), the filesystem driver - AmigaOS has a custom filesystem with 31-character filenames, file comments, etc. (or actually two such "official" filesystems plus some more as 3rd party implementations) - in some models, an SCSI or IDE driver, the necessary code for initializing expansion cards and booting from a floppy or a hard drive, a windowed command-line interface etc., etc.

Curiously, even with all this stuff packaged in it is still necessary to boot an Amiga from a floppy or from a hard drive. You need nothing more than a standard AmigaDOS boot block on a floppy disk to get into a shell-alike window with a command-line interface - all this coming from the Kickstart ROM, not from the disk. Likewise, you can get to the "Workbench" - the standard GUI desktop of AmigaOS - simply by running an executable that makes a call in the workbench.library (again, all the desktop GUI code residing in the ROM and not on the floppy). But Kickstart doesn't let you access any of this functionality directly - you must boot from an HDD or from a floppy disk first.

Then what are the parts of AmigaOS that don't reside on the ROM? Additional (non-essential) filesystem and device drivers, additional fonts, international keyboard layouts, commands that you use in the shell environment, applications for setting up your preferences (as regarding to colors, mouse acceleration and the like), various tools and utilities, etc., etc. In Linux terms, it's like you had the kernel, an X11 environment, a simple shell, and a simple desktop-window manager on ROM - but everything else is loaded from the HDD, or from a floppy.

In case bugs should be found from the ROM-based code there was a well-defined mechanism for loading in "patches" - disk-based snippets of code that would functionally replace the buggy parts of the ROM code. (I'm not sure about the exact details, but I seem to recall that even the ROM-based system libraries had modifiable jump tables in RAM, or something like that, so it was easy to patch in replacements for them.) This mechanism was also widely used by 3rd party coders for extending or enhancing the basic functionality of the OS in various ways.

Major new operating system releases, of course, required installing a new Kickstart ROM chip on the motherboard. The chip was socketed. (Flash memory, for some reason, was never used for storing the Kickstart code in any of the Amiga models. Perhaps it was too expensive back then.) There was also a way to load a new version of Kickstart in software (essentially overriding the ROM-based Kickstart in full), but this method was not officially supported, not really practical if you didn't own a hard drive, and ate up its own share of your precious RAM memory. (It was a bit controversial, too, since "soft-kicking" wasn't considered to be much of interest for others than those who pirated the new versions of the AmigaOS instead of buying them.)

That would be "Startup-Sequence" - a shell script that the Kickstart tries to find and run from the boot floppy-parbreastion. Much like the SysV rc scripts or the autoexec.bat in MS-DOS. (There was also "Shell-Startup", which would be closer to .profile or .bashrc in its function.)

AmigaOS had two shells - a simple ROM-based Command Line Interface (CLI), and a more advanced AmigaShell, parts of which were loaded from the disk. The latter had a better command-line editor and command-line history, etc. There were also several 3rd party add-ons for tab-completion, more advanced scroll buffer functionality, and the like, and several 3rd party ports of traditional UNIX shells or their close equivalents.

The Amiga hardware doesn't have a notion of a "text mode". Everything is always in graphics mode. Since the Kickstart ROM also contains the built-in equivalent of a window manager - instead of requiring one to load separate GUI subsystems from the disk - shells were always windowed (though you could turn them into full-screen windows with no borders or breastle bars if you wanted to.)

1 Granted, the Kickstart wasn't ROM-based in the earliest Amiga model ever - the A1000 (which had a special Kickstart disk that loaded the Kickstart into a protected memory area) - but it was delivered as a ROM chip in all of the subsequent models.

-- znark