External Interruptions
An external interruption causes control to be passed to the external
interrupt handler If the user has issued the HNDEXT macro to
trap external interrupts, DMSITE passes contrel to the user's exit
routine. If the interrupt was caused by the timer, DMSITE resets the
timer and types the BLIP character at the terminal. the standard BLIP timer setting is two seconds, and the standard BLIP character is
uppercase, followed by the lowercase (it moves the typeball without
printing). Otherwise, control is passed to the tEBUG routine.
Machine Check Interruptions
Hard machine check interruptions on the real processor are not reflected
to a virtual user by CPa A message prints on the console indicating
the failure. The user is then disabled and must IPL CMS again in order
to continue. Part 3. Conversational Monitor System 239

Functional Information
The most important thing to remember about C"S, from a debugging
standpoint, is that it is a one-user system. The supervisor manages one user and keeps track of only one user
1
s file and storage
chains. Thus, everything in a dump of a particular machine relates only
to that virtual machine's activity. You should be familiar with register usage, save area structuring,
and control block relationships before attempting to debug or alter CMS. Register Usage When a C"S routine is called, R1 must point to a valid parameter list (PLIST) for that program. On return. RO mayor may not contain
meaningful information (for eXample, on return from a call to FILEDEF
with no change, RO will contain a negative address if a new FCB has been
set up; otherwise, a positive address of the already existing FCE). R15
will contain the return code. if any. The use of Registers 0 and 2
through 11 varies. On entry to a command or routine called by SVC 202 the following are
in effect: 1
12
13
14
15
Contents The-address of the PLIST supplied by the caller.
The address entry point of the called routine.
The address of a work area (12 doublewords) supplied by SVCINT. The return address to the SVCINT routine.
The entry point (same as register 12). On return from a routine, Register 15 contains:
Return o <0 )0 No error occurred
Called routine not found
Error occurred
If a C"S routine is called by an SVC 202, registers 0 through 14 are
saved and restored by C"S. Most CMS routines use register 12 as a base register.
Structure of DMSN UC D"SNUC is the portion of storage in a CMS virtual machine that contains
system control blocks, flags, constants, and pointers.
The CSECTs in DMSNUC contain only symbolic references. This means
that an update or modification to C"S, which changes a CSECT in DMSNUC. does not automatically force all eMS modules to be recompiled. Only those modules that refer to the area that was redefined must be
recompiled. 240 IBM VM/370 System Programmer's Guide