When the unknown OSWORD call is passed by the operating system to the paged ROMs, the parameters of the call held in the A, X and Y registers are stored in zero page locations &EF, &F0 and &F1. To work out what OSWORD call has come in, the Streamer ROM compares the contents of &EF with the set of calls it supports. Here we can see that the Streamer ROM traps OSWORD calls &74 and &75.

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