How to Turn a Commodore Disk Drive Into a Standalone 8-Bit Computer
Dave McMurtrie turned a Commodore 1541 disk drive into a standalone computer by swapping its ROM chip and using a terminal to program it.
The Commodore 1541 disk drive was a common sight on desks of the 1980s and early 1990s, sitting next to Commodore 64 computers. While it was used to load programs and data from floppy disks, it has long been understood that the 1541 was something more than just a disk drive. It comes equipped with a CPU and memory of its own, and that fact has long been leveraged by utilities like fast loaders that took advantage of this extra hardware.
Dave McMurtrie of the Commodore History YouTube channel wondered if this overpowered disk drive could be turned into a full-fledged computer. With the same MOS 6502 CPU that is found in the Commodore 64, RAM, ROM, and a pair of 6522 Versatile Interface Adapter chips, the answer is obviously yes. But the question still remains: how?
Despite all of the hardware that the 1541 contains, it is still missing the video and sound chips found in a Commodore 64, as well as the keyboard and most of the I/O ports. For this reason, McMurtrie sees the 1541 as being more like the Commodore KIM-1 of the 1970s. This computer was extremely limited in terms of I/O, with just a numeric keypad and seven-segment display for a user interface.
As such, McMurtrie decided to turn the 1541 into a KIM-1-like computer. These machines were often interfaced with via terminals to avoid using the onboard keypad. So McMurtrie planned to use a modern terminal emulator to interface with the 1541. That solved the I/O problem, but the 1541 was still a disk drive, and nothing but a disk drive.
To fix that, the drive’s ROM chip was replaced with one that was programmed with KIM-1 firmware. Next, an adapter was used to connect the 1541’s serial port to a modern computer. From there, a terminal emulator was used to supply the 1541 with a series of 6502 opcodes that specify the program it is to run. These were essentially the same as KIM-1 programs, with a few small modifications to account for hardware differences.
After laboriously typing out a long series of opcodes (without a single typo!), McMurtrie was rewarded with a “HELLO WORLD” response. And that, my friends, is how you turn a 1541 disk drive into a computer.