The programmer indicates an error return by the sign of the halfword
code. If an error return is desired, then the code is negative. If the
code is positive, then no error return ismade. The sign of the
halfword code has no effect on determining the routine that is to be
called, sinceDftSITS takes the absolute value of the code to determine
the routine called.
Since only the second byte of the absolute value of the code is
examinedby DftSITS, seven bits (bits 1-7) are available as flags or for
otherThus. for example
i DMSFREE uses these seven bits to indicate such things as conditional requests and variable requests. When an SVC 203 is invoked, DMSITS stores the halfword code into the KUCOK location CODE203, so that the called routine can examine the seven
bits made available to it.
All callsmade by means of SVC 203 should be made by macros, with the macro expansion computing and specifying the correct halfword code.
The programmermay use the HNDSVC macro to specify the address of a
routine that will handle anySVC call other than for SVC 202 and SVC 203. In this case, the linkage conventions are as required by the
user-specifiedSVC-handling routine. CMS supports selected SVC calls generated by as and DOS/VS macros, by simulating the effect of these macro calls. DftSITS is the initial SVC interrupt handler. If the SET DOS command has been issued, a flag in NUCON will indicate that DOS/VS macro simulation is to be used. Control
is then passed toDftSDOS. Otherwise, as macro simulation is assumed and DftSITS passes control to the appropriate OS simulation routine.
There are several types of invalidSVC calls recognized by DftSITS. 1. Invalid SVC number. If the SVC number does not fit into any of the
four classes described above, then it is not handled byIn appropriate error message is displayed at the terminal, and control
is returned directly to the caller.
258IBM VM/370 System programmer's Guide
code. If an error return is desired, then the code is negative. If the
code is positive, then no error return is
halfword code has no effect on determining the routine that is to be
called, since
the routine called.
Since only the second byte of the absolute value of the code is
examined
other
i
bits made available to it.
All calls
The programmer
routine that will handle any
user-specified
is then passed to
There are several types of invalid
four classes described above, then it is not handled by
is returned directly to the caller.
258