---------------------------<snip>-------------------------
Later when S/370 came along the designers had recognized the
deficiencies of SVC and they came up with PC (Program Call) as a lighter
weight and more flexible mechanism. PC functions can be instantiated
dynamically (much more so than SVCs) and they have the virtue that the
PC instruction passes control more-or-less directly to the target PC
function, avoiding a great deal of processing overhead.
Return from the (original) PC instruction was accomplished by the PT
(Program Transfer) function, which switched state and branched to the
target instruction in a single operation - way faster than returning
from an SVC.
Later in S/370-ESA the stacking PC was invented and that used the (then
new) linkage stack for saving and restoring the caller's state. When the
linkage stack is used, the PR instruction (Program Return) is used to
return control and restore the caller's state all in a single operation.
-----------------------------<unsnip>------------------------
WAY BACK WHEN I worked at a time-sharing service bureau running systems
based on CP67/CMS, we used still another mechanism. CP67 was modified to
use a BAL to a particular address in low storage, which went to a setup
routine, which in turn called the appropriate service. Reason:
interrupts took too long to process The actual service and its
parameters were determined by the contents of storage immediately
following the BAL instruction. All SVC's in the virtual machine were
processed by the CMS code in the user's virtual machine and interactions
between CMS and CP67 were handled via HVC (Hypervisor Call aka DIAGNOSE)
and its parm list.
Interrupts are expensive; avoid them whenever possible; even the
PSW-SWAP is very slow!!!
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