CEEVGTUN — next available byte locator service

The Language Environment storage manager provides a service that returns the next available byte address for the user stack to the caller. CEEVGTUN is a S/370-specific CWI (compiler writer interface) that performs this service. CEEVGTUN isolates the user from Language Environment internals. This prevents the problem of having generated code use any of the Language Environment storage management internal control blocks and structures. Only a low-level interface is provided with the following conventions.

Register type	Register number	Register description
Input Registers	R0–R11	Not used.
	R12	Address of CAA.
	R13	Save area address of the CEEVGTUN caller's caller. Note that this save area is not modified by CEEVGTUN.
	R14	Return address to the caller.
	R15	Address of CEEVGTUN.
Output Registers	R0–R14	Unchanged.
Output Registers	R15	Next available byte in the user stack.

CEEVGTUN

Call this CWI interface as follows:

L     R15,CEECAACELV-CEECAA(,R12)
L     R15,148(,R15)
BALR  R14,R15

If CEEVGTUN encounters any errors, it abends with code 4088. The reason code associated with abend 4088 indicates the cause of the failure:

99: An exception occurred while trying to locate the NAB, or a zero back chain pointer was found before finding the Language Environment dummy DSA.

Use the code sequence shown in Figure 1 only in a library routine, not in compiler-generated code.

Figure 1. Get next available byte in user stack

 CL..0    STM   14,12,12(13)          SAVE CALLER'S REGISTERS
          L     15,CEECAACELV         GET ADDRESS OF LIBVEC
          L     15,CEECELVVGTUN-CEECELV(15)  LOAD ADDR OF GET USER NAB
                                      SERVICE
          BALR  14,15                 CALL THE SERVICE
          LR    Ra,15                 LOAD NEW DSA ADDRESS
          L     15,16(,13)            RESTORE ADDRESSABILITY
          L     0,length              LOAD DSA LENGTH
          ALR   0,Ra                  GENERATE NEW NAB ADDRESS
          ...