IBM System/360 System Programmer's Guide

WRITING SVC ROUTINES
Because your SVC routine will be a part cf the control program, you
must follow the same programming conventions used in SVC routines
supplied with System/360 Operating System.
Four types of SVC routines are su~plied with System/360 Operating
System, and the programming conventions for each type differ. The
general characteristics of the four types are described in the following
text, and the programming conventions for all types are shown in tabular
form.
Characteristics of SVC Routines
All SVC routines operate in the supervisor state. You should keep
the following characteristics in mind when deciding what type of SVC
routine to write:
• Location of the routine - Your SVC routine can be either in main
storage at all times as part of the resident control program, or on
a direct-access device as part of the SVC library. Type 1 and 2 SVC
routines are part of the resident control program, and types 3 and 4
are in the SVC library •
•
Size of the routine - Types 1, 2, and 4 SVC routines are not limited
in size. However, you nust divide a type 4 SVC routine into load
modules of 1024 bytes or less. The size of a type 3 SVC routine
must not exceed 1024 bytes.
• Design of the routine - Type 1 SVC routines must be reenterable or
serially reusable; all other types must be reenterable .
• Interruption of the routine - When your SVC routine receives
control, the CPU is masked for all maskable interruptions but the
machine check interruption. All type 1 SVC routines must execute in
this masked state. If you want to allow interruptions to occur
during the execution of a type 2, 3, or 4 SVC routine, you must
change the appropriate niasks. If you expect that a type 2, 3, or 4
SVC routine will run for an extended period of time, it is
recommended that you allow interruptions to be processed where
possible.
Programming Conventions for SVC Routines
The programming conventions for the four types of SVC routines are
summarized in Table 1. Details about many of the conventions are in the
reference notes that follow the table. The notes are referred to by the
numbers in the last column of the table. If a reference note for a
convention does not pertain to all types of SVC routines, an asterisk
indicates the types to which the note refers.
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IBM System/360 Specifications

General

Brand	IBM
Model	System/360
Category	Desktop
Language	English