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23:17, 22 January 2021 RSX (Resident System eXtensions) were a way to extend the command set of Locomotive BASIC, e.g. for accessing external peripherals like disk drives, light pens, speech synthesizers and so on.
RSX commands can be distinguished from normal commands by the preceding pipe symbol "|" (also called "RSX bar", "RSX pipe" or "RSX slash").
Some of the best known RSX commands are the ones provided by AMSDOS, as they were present on all CPC664, CPC6128, CPC6128+ and CPC464 with DDI-1: