Inex Code @inexcode@social.inex.rocks

KnightOS is a Unix-like operating system for Texas Instruments calculators that I started 10 years ago. It had a Unix-like filesystem, preemptive multiprocessing AND muliththreading, assembly and C programming environments, and supported 8 different calculator models based on the 8-bit z80 CPU.

Some of the constraints it worked within made for interesting challenges:

- There were 64KiB of usable RAM. The kernel stored *all* of its state in just 1024 bytes of statically allocated memory. Many subroutines had overlapping storage areas and everything was carefully planned to consider this. The userspace memory allocator was a very simple linked list, with minimal storage overhead to maximize available memory for userspace programs.

- The filesystem was stored in (worst case) 128 KiB of NOR flash, which was shared with the kernel - whose core code was <4KiB. The filesystem was carefully designed to maximize space and Flash wear cycles (unlike modern SSDs, our kernel directly managed our Flash storage), which meant designing all everyday filesystem operations to only RESET bits in the underlying storage medium. It also used minimal metadata, to maximize usable storage space for the users.

- When writing to the filesystem, the kernel code resident in Flash became unreadable - so we had to copy bits of the driver code to part of our 1024-byte statically allocated RAM and execute them from there.

- We had some basic networking working based on the I/O port, which is a 1.5mm jack similar to audio jacks (we also had a music player if you had a 1.5mm -> 2.5mm adapter, btw. MIDI and WAV). We directly controlled the voltages on the ring and tip of this port and had to design a and implement very low-level Layer 1 protocol for it.

One thing KnightOS couldn't do is math - once you installed it you basically gave up using your calculator as a calculator.

Bash tricks by Julia Evans

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