Primary mode programs receiving control

When a primary mode program receives control, it can save the calling program's registers on the linkage stack or in the caller-provided save area. If using the caller-provided save area, then distinctions are made as to whether the primary mode program is or is not changing the 64–bit GPRs. Each possibility is discussed below.

Note that the linkage conventions assume that a primary mode program does not use ARs. By leaving the contents of the ARs untouched, the program preserves ARs 2-13 across program linkages.

A primary mode program that uses the linkage stack must:

Issue a BAKR instruction which will save the caller's ARs and 64-bit GPRs on the linkage stack.
Establish a GPR as a base register.
Set GPR 13 to indicate that the caller's registers are saved on the linkage stack:
- If the program creates a save area, obtain a 72-byte save area on a word boundary (or 144-byte or larger on a doubleword boundary if routines called by the program need that extra space) in the primary address space:
  1. Set the second word of the save area to the character string 'F1SA' if obtaining a 72-byte save area or 'F6SA' if obtaining a 144-byte or larger save area
  2. Load GPR 13 with the save area address.
- If the program does not creates a save area, do one of following actions:
  - Load 0 into GPR 13.
  - Set the second word of a two-word area in the primary address space to the character string ‘F1SA’. Load the address of the two-word area into GPR 13.

Conditions described for a primary mode program that uses the caller-provided save area:

Does not change 64–bit GPRs and that uses the 72–byte caller-provided save area
Changes 64-bit GPRs and that uses the 72-byte caller-provided save area
Changes 64-bit GPRs and that uses the 144-byte caller-provided save area

Does not change 64–bit GPRs and that uses a 72–byte caller-provided save area, the program must:

Save 32-bit GPRs 0 - 12, 14, and 15 into the caller-provided save area pointed to by GPR 13.
Establish a base register.
Obtain a 72-byte save area on a word boundary (or 144-byte or larger on a doubleword boundary if routines called by the program need that extra space) in the primary address space.
Store the address of the caller's save area into the back chain field of its own save area.
Store the address of its save area into the forward chain field of the caller's save area.
See Example.

Changes 64-bit GPRs and that uses the 72-byte caller-provided save area, the program must:

Save the low halves (bits 32 through 63) of GPRs 0 - 12, 14, and 15 into the caller-provided save area pointed to by GPR 13.
Obtain a 216-byte (if using F5SA) or 288-byte (if using F8SA) save area on a doubleword boundary in the primary address space
Save the high halves (bits 0 through 31) of GPRs 0 - 15 into its own save area.
Store the address of the caller's save area into the back chain field of its own save area.
Store the address of its save area into the forward chain field of the caller's save area.
Set the word at offset 4 to "F5SA" or "F8SA" to indicate how the caller's registers were saved.
See Example of F5SA and Example of F8SA.

Changes 64-bit GPRs and that uses the 144-byte caller-provided save area, the program must:

Save 64-bit GPRs 0 - 12, 14, and 15 into the caller-provided save area pointed to by GPR 13 in F4SA format.
Obtain a 144-byte save area on a doubleword boundary (or larger if routines called by the program need that extra space) in the primary address space.
Store the address of the caller's save area into the back chain field of its own save area.
Store the address of its save area into the forward chain field of the caller's save area.
Set the word at offset 4 to "F4SA" to indicate how the caller's registers were saved.
See Example.