The Mark 1's firmware is the set of scripts that provide basic I/O to allow software to interact with hardware.
Firmware scripts all start with "_fw:" - for example, the "_fw:mit" script handles calls to the Multiple Interval Timer. These scripts are not mapped to the filesystem and can only be updated using a special firmware update utility (or, absent the ability to boot at all, may be replaced manually). While it is theoretically possible to replace them with custom firmware scripts, it is usually safer to install a custom operating system instead.
The default firmware includes the following interfaces: