Programmable Interval Timer: Difference between revisions

The "Access Mode" bits tell the PIT what access mode you wish to use for the selected channel, and also specify the "counter latch" command to the CTC (more on the "counter latch" command laterlatter). These bits must be valid on every write to the mode/command register. For the "read back" command (also discussed later), these bits have a different meaning. For the remaining combinations, these bits specify what order data will be read and written to the data port for the associated PIT channel. Because the data port is an 8 bit I/O port and the values involved are all 16 bit, the PIT chip needs to know what byte each read or write to the data port wants. For "lobyte only", only the lowest 8 bits of the counter value is read or written to/from the data port. For "hibyte only", only the highest 8 bits of the counter value is read or written. For the "lobyte/hibyte" mode, 16 bits are always transferred as a pair, with the lowest 8 bits followed by the highest 8 bits (both 8 bit transfers are to the '''same''' IO port, sequentially -- a word transfer will not work).