The Data Register (Port &FB7F) is used to write Commands and Parameters, to read/write data bytes, and to receive result bytes. These 3 operations are called Command-, Execution-, and Result-Phase.
<br>Command Phase: A command consists of a command byte (eventually including the MT, MF, SK bits), and up to 8 parameter bytes.
=== Command Execution Phase ===: During this phase, the actual data is transferred (if any). Usually that are the data bytes for the read/written sector(s), except for the Format Track Command, in that case 4 bytes for each sector are transferred. During data transfers between the FDC and the processor, the FDC must be serviced every 26µs (for MFM mode with CPC timings) or the FDC terminates the FDC command.
A command consists of a command byte (eventually including the MT, MF, SK bits), and up to 8 parameter bytes. <br> === Execution Phase === During this phase, the actual data is transferred (if any). Usually that are the data bytes for the read/written sector(s), except for the Format Track Command, in that case 4 bytes for each sector are transferred. During data transfers between the FDC and the processor, the FDC must be serviced every 26µs (for MFM mode with CPC timings) or the FDC terminates the FDC command. <br> === Result Phase === : Returns up to 7 result bytes (depending on the command) that are containing status information. The Recalibrate and Seek Track commands do not return result bytes directly, instead the program must wait until the Main Status Register signalizes that the command has been completed, and then it must (!) send a Sense Interrupt State command to 'terminate' the Seek/Recalibrate command. During the result phase, all the result bytes must be read. The FDC will not accept a new command until all the result bytes are read.
<br>
