Abstract: A microcoded pulse sequencer for real time control of a nuclear magnetic resonance (NMR) magnetic resonance imaging (MRI) system includes a microcode store which is continually loadable. Microinstructions are loaded into a one portion of the store while the sequencer executes micro-instructions out of another portion of the store. Means are provided to ensure that the sequencer never executes micro-instructions currently being loaded, and also ensures (through timing features) that the sequencer can never, over time, execute micro-instructions more rapidly than new micro-instructions can be loaded (i.e., so that the sequencer never "runs out of" micro-instructions). Since the host processor and the microcode sequencer provide for continual loading of the sequencer store while the sequencer is running, the user/programmer may view the control store as an ideal virtual control store having an unlimited size.