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DC (director) calls are very low level, hence only three of the sixteen available are of use to applications. OS (operating system) calls are more reliable, but because of their power, they'r easily able to disrupt the system - many are not for application use at all. The GN (general) calls are safe and used for applications in that they are unlikely to corrupt the system, but a little unreliable.
Alarms
OS_Alm, alarm manipulation (OZ usage)
GN_Aab, allocate alarm block
GN_Alp, process an expired alarm
GN_Fab, free alarm block
GN_Lab, link an alarm block into the alarm chain
GN_Uab, unlink an alarm block from the alarm chain
OS_Alm, alarm manipulation (OZ usage)
Application
DC_Nam, Name current application
OS_Poll, poll for an application (OZ usage)
DC_Pol, poll for card usage
OS_Ent, enter an application (OZ usage)
DC_Ent, enter new application (low level OZ usage)
OS_Exit, exit an application (OZ usage)
DC_Bye, exiting current application (OZ usage)
OS_Bye, exit application
OS_Stk, stack current process / application (OZ usage)
OS_Use, process card usage (OZ usage)
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GN_D16, 16bit unsigned division
GN_D24, 24bit unsigned division
GN_Gdn, convert an ASCII string to a binary number
GN_M16, 16bit unsigned multiplication
GN_M24, 24bit unsigned multiplication
GN_Pdn, write number as decimal ASCII string
FP_ Floating point arithmetic
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