Other I/O and Second Processor Support
The OS has calls to handle reading and writing to all I/O (ports and screen memory) and programmers are strongly advised to use these. The reason for this being is that when a second processor is installed, user software is run from the separate memory map on the far side of the Tube processor bus, and direct access to memory-mapped I/O registers and video memory is impossible. However, for the sake of performance, many apps including probably all assembled games write directly to main memory for I/O, and hence crash or give you a blank screen if a 6502 second processor is attached. One issue involved here is a lack of sprite support in the OS and a need to handle this manually from user code. The driver code for the Tube interface itself is not present in the MOS, usually being supplied by an external service ROM.
The MOS contains two built-in file systems: cassette and ROM. These are quite similar (try *ROM, *OPT 1 2, *CAT with a suitable ROM installed) and share a great deal of code. They feature a rudimentary copy protection mechanism where a file with a certain flag set cannot be loaded except to execute it. The Advanced Disc Filing System (ADFS), installed as standard in the Master series, has a similar mechanism.
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