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# ProDOS Clock Drivers
The ProDOS operating system for the Apple II personal computer line natively supports the Thunderclock real-time clock card, but there is a protocol for custom clock drivers to be installed:
* Check `MACHID` bit 0 to see if a clock is already active; abort if so.
* Optional: Probe hardware to determine if the clock is present; abort if not.
* Relocate the clock driver to LC bank 1, at the address at `DATETIME`+1
* Update `DATETIME` to be a `JMP` instruction.
* Optional: Chain to the next `.SYSTEM` file.
In addition:
* The clock driver must fit into 125 bytes.
* The driver may dirty $200-$207 but other memory must be restored if modified.
* When invoked, the clock driver should read the clock hardware and encode the date and time into `DATELO`/`DATEHI` and `TIMELO`/`TIMEHI`.
* ProDOS calls the clock driver when `GET_TIME` is called, and on every call (`CREATE`, `RENAME`, etc) that might need the date and time.
See https://prodos8.com/docs/techref/adding-routines-to-prodos/ for more information.
## Included drivers
This directory includes drivers for the following real-time clocks:
* No-Slot Clock
* Cricket!
* Applied Engineering DClock
* ROMX Real-Time Clock
* FujiNet Clock
All follow the above protocol: install only if there is not already a clock, probe for the clock before installing, and chain to the next driver.