Sideways Address Space - Sideways Model

Sideways Model

The first few bytes of Sideways ROMs contain details that inform the OS how to handle them. These include language and service entry points, ROM type code, version number and a pointer to the copyright information. On reset the OS validates each sideways bank by checking for a copyright string. During operation the OS talks to valid ROMs by jumping to the two entry points with a specific value of the accumulator set. This provides a clean API for expanding the operating system and negotiating bank switching and RAM sharing.

ROMs have two entry points: the service entry point provides the API access to the ROM, and the language entry point is the starting point for application software contained in the ROM. "Service" ROMs need not have a language entry point, and only exist to extend the OS. "Language" ROMs are ROMs that provide application software, and gain their name from the fact that the BBC BASIC language is supplied as the default ROM in bank 15. ROMs often contain both entry points, as all user software must have a service entry point to allow the OS to call into it. Pure service ROMs typically only extend the features of the OS itself, without providing any application software.

The BBC Micro requires one language ROM be present at POST to provide the computer with a user interface, else the OS will report "Language?" and halt. The version of Acorn MOS on the BBC Master has a built-in command line and will present this if no default language ROM is configured.