The Data Register (Port FB7Fh) is used to write Commands and Parameters, to read/write data bytes, and to receive result bytes. These three operations are called Command-, Execution-, and Result-Phase.
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=== Command Phase ===
A command consists of a command byte (eventually including the MT, MF, SK bits), and up to 8 parameter bytes.
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=== Execution Phase ===
During data transfers between the FDC and the processor, the FDC must me serviced every 26us (for MFM mode with CPC timings) or the FDC terminates the FDC command.
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=== Result Phase ===