Interrupt Status; I/O Unit Utilization - ZiLOG Z80-RIO Operating System User's Manual

***
NOTE
***
When external files make system calls resulting in
execution of other external files, the current state of the
memory map needs to be saved in order to determine what
space to deallocate as a result of program loading. This
map is saved on the user stack occupying a block of
44H bytes. Care must be taken to allocate sufficient
stack sizes for programs using this feature.
***
NOTE
***
RIO is not reentrant from its system entry point.
4.6 INTERRUPT STATUS
The initialization process associated with system restart
sets interrupt mode 2, and the I register to the base address
of the interrupt vector, with interrupts enabled. Proper
system operation depends on this interrupt status. With
this configuration the a-bit vector supplied by the
interrupting device is used with the contents of the I
register to form a pointer to the interrupt service routine
starting address. Zilog support devices can be programmed
to supply appropriate interrupt vectors using this space.
If program constraints make it necessary to alter the
interrupt mode or I register, they must restore the proper
conditions before making system calls that result in disk
activity. See Appendix C for the system interrupt vector
address for use in restoring the I register.
4.7 I/O UNIT UTILIZATION
The user is free to redefine all I/O logical units with the
exception of '0'. Unit
a
is restricted to use by RIO.
Units 1, 2 and 3 are predefined to be the console input
device, console output device and system volume output
device. Units 4 through 20 are initialized to be the
master device. Redefinition of these units may result in
abnormal system behavior upon return to RIO. This means
that in the standard RIO configuration which allows 0-20,
17 units may be defined concurrently, with an additional
3 units predefined by RIO for use as console or line
printer I/O devices.
E3-0072-01, Rev. B
-31-
10/31/79
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