prodos-drivers/clocks/README.md

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.