IBM OS/390 manual Proga Start Progb Csect Progc Csect MVS, 11,SAVEA 11,SAVEB 11,SAVEC, Savea DC

contained in register 13. Therefore, you must specify a save area to receive the registers.

PROGA START		PROGB CSECT		PROGC CSECT
(MVS)		(MVS)		(MVS)
.		.		.
.		.		.
STM 14,12,12(13)		STM 14,12,12(13)		STM 14,12,12(13)
ST	13,SAVEA+4	ST	13,SAVEB+4	ST	13,SAVEC+4
LA	11,SAVEA	LA	11,SAVEB	LA	11,SAVEC
ST	11,8(13)	ST	11,8(13)	ST	11,8(13)
LR	13,11	LR	13,11	LR	13,11
.		.		.
.		.		.
Application		Application		Application
Program		Program		Program
Logic		Logic		Logic
.		.		.
.		.		.
CALL PROGB		CALL PROGC
L	13,4(,13)	L	13,4(,13)	L	13,4(,13)
Return (MVS)		Return (PROGA)		Return (PROGB)
SAVEA DC	18F′ 0 ′	SAVEB DC	18F′ 0 ′	SAVEC DC	18F′ 0 ′
.		.		.
END		END		END
Figure 28.	MVS Subroutine Linkage

If a standard save area of 18 fullwords is reserved in the calling program, the save area contains the following information at completion of the called program′s initialization logic.

Word 1	Used by LE-compliant languages
Word 2	Address of the caller′ s save area (the backward chain).
Word 3	Address of the save area of the called program (the forward chain).
Word 4	Register 14.	Return address within the calling module.
Word 5	Register 15.	Entry point address of called module.
Words 6-18	Registers 0 through 12, respectively, of the calling program.

Consider three programs using the concept of forward and backward chains with standard linkage conventions. Under VSE, these could be three application programs, while under MVS, the highest-level program that must be considered is the MVS control program because it calls the MVS highest-level application program.

Linkage Macros

CALL, SAVE, and RETURN macros are available under VSE and MVS. This set of macros performs the general housekeeping required to maintain subroutine conventions within the CSECTs of a simple program structure. In general, these MVS macros provide additional functions not available in VSE. You can use the VSE versions of these macros under MVS without any modification.

Chapter 13. A s s e m b l e r 271