Reconstructing the Leftovers on the Amiga Kickstart 1.0 Disk

Update: The source is available at github.com/mist64/extract-adf; more info here.

It is well-known that the “Kickstart” disk that came with the original Amiga 1000 in 1985 contained some fragments of source code: The floppy disk that was used as a master for duplication had been used by the developers before, and had not been erased completely.

Nobody seemed to have looked too closely at what is on this disk – so I did, for this episode of Computer Archeology. The revealed data tells us about how the Amiga operating system was brought up, how it related to Tripos, and where it was developed.

The Kickstart Disk

When the Amiga 1000 was released, the 1.0 ROM image was not finalized, so the machine shipped with a “Kickstart” floppy disk that was loaded into an extra 256 KB RAM bank which was then write-protected.

A double-density Amiga disk is exactly 880 KB, i.e. it consists of 1760 sectors, each 512 bytes in size. The Kickstart disk has a header on sector 0 (saying “KICK” followed by all zero bytes), and the raw ROM image is stored on the following sectors. The ROM is 256 KB in size, so this means that all sectors from 513 on are unused, and, in the case of the Kickstart 1.0 disk, preserved.

The following analysis has been done on the Kickstart 1.0 ADF disk image with an MD5 of 2fb28e7bbab0d2eef00e742c7259a674.

Amiga Old Filesystem

Before version 1.3 of the operating system, disks were formatted with the “Amiga Old Filesystem“, which was quite inefficient, as it stored metadata pretty redundantly. Every single sector, even sectors containing file data, start with a 24 byte header, which contains

• the type of the sector (directory entry, data…)
• a pointer to the parent
• the sequence number of the sector inside the file
• the number of valid bytes in the sector
• a pointer to the successor
• and a checksum.

Every directory entry occupies a complete sector, and deleting a file means just removing the links to the directory entry sector, without touching the data sectors or the directory entry sector at all. And what’s best: For performance reasons, directory entries are clustered around the center of the disk, i.e. sector 880, which, in our case, has not been overwritten by the ROM image, so it should be easy to get some data out of this disk.

Extraction Program

The program to extract the data from the disk uses the following algorithm: It iterates over all sectors from the end of the ROM image to the end of the disk image and looks at the sector type. If it is a data block, it looks at the parent, in the hope that it is an intact directory entry. If this is the case, it walks back the list from parent to parent to get the full path and filename of the file. If the directory entry has not survived, the file will get the index of the directory entry as a name. As we now know the filename of the file the sector belongs to and we can read the sequence number from the sector header, it is possible to write 488 bytes worth of data into the output file.

Here is the source code of the program to extract the files: extract-adf.c

Results

The disk seemed to have contained C header files and assembly includes earlier, and has then been overwritten with the current set of command line tools of AmigaOS (the name of the disk “cli26.1” reflects this), extended with some custom development tools used by Commodore-Amiga.

Since the DOS part of the Amiga operating system is actually based on the only very slightly modified Metacomco Tripos operating system, the command line tools as well as the development tools mentioned in the following sections are mostly the same as those found in Tripos; therefore the terms Tripos and AmigaDOS may be used interchangeably.

CLI Programs

The “c” subdirectory contains the following command line tools that can also be found on the Workbench 1.0 disk:

Assign
Break
CD
Copy
Date
Delete
Dir
DiskCopy

Echo
Ed
Edit
EndCLI
Execute
FailAt
Fault
Format

If
Info
Install
Join
Lab
List
MakeDir
NewCLI

Prompt
Protect
Quit
Rename
Run
Search
Skip
Sort

Stack
Status
Type
Wait
Why

The following executables can be found in the “l” directory:

Disk-Validator
Port-Handler
Ram-Handler

But these may very well be earlier versions. “DiskCopy” for example is significantly smaller than the “Diskcopy” that shipped with Workbench 1.0.

Development Tools

The following command line utilities can also be found in “c”. They do not ship with WorkBench, but are part of a separtare tools disk for developers, as described in the AmigaDOS Manual. Some also exist on pre-1.0 beta versions of Workbench, although possibly in different versions.

Alink is the Amiga-native version of the TRIPOS/Metacomco linker

According to the AmigaDOS Manual (or Tripos Technial Reference Manual), “to inspect or patch disk blocks, you may use the AmigaDOS [Tripos] disk editor, DISKED.” The version on this disk is 3.2; the 2.0+ NDKs shipped with 3.4, and cintpos, the open source release of Tripos, includes the source for 3.0.

Download is quite interesting, not only because the binary and its documentation are very hard to find, but also because they tell a lot about the history of the Amiga operating system. According to the AmigaDOS manual, it is used to download “programs written on another computer (for example, a Sun) to the Amiga.” – the counterpart on the Sun workstation would be “binload”.

An interesting side note on this version of “Download” is that it still contains all linker symbols, and has a debugging mode (“-d”) that prints out every step that it is doing, so it is pretty easy to reverse-engineer.

There is strong evidence that the Amiga operating system has been cross developed on Sun machines. After all, the official Amiga SDK is called “NDK”, “Native Development Kit”, implying that the original bringup SDK was not native.

At that time (1985), Sun Workstations were 68000- or 68010-based computers running SunOS 1.x, which was based on 4.1BSD. The C compiler that came with BSD at that time was the Bell Labs “Portable C Compiler” (PCC), which could naturally output 68000 code. The “Bell Labs C compiler” (i.e. PCC) is mentioned in a header file in the “Native Development Kit” for AmigaOS as printed in “AMIGA ROM Kernel Reference Manual: Libraries and Devices“, next to the suggested native compiler “Lattice C” and two other compilers, implying that PCC could be used for Amiga development.

The AmigaDOS manual says that “the tools available on the Sun Microsystem for cross development include the assembler, linker, and two C compilers.” One compiler is explicitly mentioned: It is the “Greenhills C compiler” (metacc; the name suggests it was Metacomco’s shipping C compiler for Tripos); the other one is presumably PCC. According to this resume, the first versions of the ROM were compiled using the Greenhills C compiler.

So Commodore-Amiga must have cross-developed AmigaOS on BSD with the Greenhills C compiler.

New Executables

“d” seems to be used to delete files, “Read” contains strings about reading from serial and parallel ports, “ReadPrefs” accesses the file “config/preferences”, which is also included on this disk. It is unknown what ImageEdit is for – it does not contain any strings, but included linker symbols imply that it deals with timers.

Devices and Libraries

The disk also comes with the Narrator device and the Translator library, which are required for speech synthesis. These two files have been overwritten once, so there are now two different copies on the disk:

The header of the libraries contain the respective versions. Note that the newer version of narrator.device has the older timestamp. “PDT” implies that AmigaOS has been written on the US West Coast – in the California office of Commodore. For comparison, Workbench 1.0 shipped with

narrator 1.6 (10 Sep 1985)
translator 1.3 (4 Sep 1985)

Misc Files

Files for Development

For many of the files on the disk, the original directory entry has been lost, but it was possible to reconstruct the filename using the printed headers in “AMIGA ROM Kernel Reference Manual: Libraries and Devices“, digital versions of older and newer versions of the headers, and the AROS versions of the headers.

Unlike the published NDK headers, these still contain the revision control and author information. All files use UNIX line breaks (LF).

Some files contain the address of Commodore-Amiga, which was:

Commodore-Amiga Incorporated
983 University Ave. Building #D
Los Gatos, California, 95030

Directory Entries with Missing Data

There are directory entries for the following header files and includes on the disk, but the data is missing

devices/audio.h
devices/audio.i
devices/bootblock.h
devices/bootblock.i
devices/console.h
devices/console.i
devices/gameport.h
devices/gameport.i
devices/inputevent.h
devices/inputevent.i
devices/keyboard.h
devices/keyboard.i
devices/keymap.h
devices/keymap.i
devices/narrator.h
devices/narrator.i
devices/parallel.h
devices/parallel.i
devices/serial.h
devices/serial.i
devices/timer.h
devices/timer.i
devices/trackdisk.h
devices/trackdisk.i
hardware/blit.i
hardware/intbits.h

There are four more directory entries for executable files:

Conclusion

The following conclusions can be drawn from this exercise:

• The reduncandy in the Amiga Old Filesystem makes this filesystem perfect for reconstructing deleted and even partially overwritten data.
• AmigaOS seems to have been bootstrapped using SunOS and the Metacomco/Tripos toolchain, and has probably not been self-hosting for 1.0.
• The disk was used to move developer tools and header files between machines.
• Version 1.0 of the OS has been developed in the California office of Commodore – the original Amiga office.

Open Questions

• What source control system was used?
• How does the version numbering (“26.1” etc.) work? How is it related to source control?
• Does the 1.0 NDK still exist somewhere? It should have shipped with Lattice C, at least. It would be very interesting to compare these headers and includes with the 1.0 NDK ones; for real differences, and for changes in the comments.
• What are the differences between these headers and the 1.0 headers? Any last minute changes?

Links

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