The OS9 Clock module is an interesting one. In the OS9 hierachy diagram it sits together with Init at the same level as the OS9 kernel (OS9p1 & OS9p2).

The Clock module takes care of initialising the clock which is a 100Hz oscillator driven signal which, in our case, is connected to the onboard MC6821 (PIA). The PIA also provides a TimerEnable signal which is used to stop the 100Hz clock if it is logical '1'.

Also the Clock module provides seconds,minutes, hours, etc. registration based on the 'ticks' coming in from the 100Hz clock. This is done through generating interrupts by the PIA.

There is no direct start of the clock by the operating system. In fact the F$Time system call is only available when the Clock module is called using the Setime command which also starts the Clock.

Below a snippet showing how the PIA is setup and the clock is started by setting the TimerEnable signal low '0'. By default the TimerEnable is '1' due to a pullup resistor.

*****
*
*  CLOCK INITIALIZATION ENTRY
*
ClkEnt pshs cc
 lda #TickSec get ticks per second
 sta D.Tick
 lda #TickSec/10 set ticks/time slice
 sta D.TSlice
 sta D.Slice
 leax CLKSRV,pcr GET SERVICE ROUTINE
 stx D.IRQ SET INTERRUPT VECTOR
 
* PIA initialisation goes here
 ldx CLKPRT,pcr get PIA address
 orcc #IntMasks set interrupt masks
 clra
 sta 1,x clr control register
 lda #%00000001 PIA port A bit0 output, rest input
 sta ,x set data direction register
 lda #5 select pheripheral register & CRA1 interrupt enable, high to low
 sta 1,x set control register
 clra
 sta ,x clear bit 0 which enables 100Hz clock
 lda ,x clear any current interrupts
 
 puls cc restore interrupt masks
 leay TIMSVC,PCR
 OS9 F$SSvc SET TIME SERVICE ROUTINE
 
ClkEnt3 rts

Full Clock source code is attached.

Note: it is assumed that PIA A-side pins other than 'pin 0' are inputs. That is true (see schematic) for 'pin 4' or PA4 which is connected to the clock but not necessarily for the other pins. Currently these pins have no meaningfull purpose but one could consider using these and then the above PIA A-side initialisation needs to change.