Stack areas

Edit online

The stack layout is divided into eight areas numbered 1 to 8, starting from the bottom of the diagram to the top of the diagram.

For convenience, the stack layout has been divided into eight areas numbered 1 to 8, starting from the bottom of the diagram (high address) to the top of the diagram (low address). The sender's stack pointer is pointing to the top of area 3 when the call to the catcher function is made, which is also the same SP value that is used by the catcher function on entry to its prolog. The following is a description of the stack areas, starting from the bottom of the diagram (area 1) and moving up to the top (area 8):

Area 1: Sender's Local Variable Area
Area 1 is the local variable area for the sender function, contains all local variables and temporary space required by this function.
Area 2: Sender's Output Argument Area
Area 2 is the output argument area for the sender function. This area is at least eight words in size and must be doubleword-aligned. The first eight words are not used by the caller (the sender function) because their corresponding values are placed directly in the argument registers (GPR3:GPR10). The storage is reserved so that if the callee (the catcher function) takes the address of any of its parameters, the values passed in GPR3:GPR10 can be stored in their address locations (PW1:PW8, respectively). If the sender function is passing more than eight arguments to the catcher function, then it must reserve space for the excess parameters. The excess parameters must be stored as register images beyond the eight reserved words starting at offset 56 from the sender function's SP value.
Note: This area may also be used by language processors and is volatile across calls to other functions.
Area 3: Sender's Link Area
Area 3 is the link area for the sender function. This area consists of six words and is at offset 0 from the sender function's SP at the time the call to the catcher function is made. Certain fields in this area are used by the catcher function as part of its prolog code, those fields are marked in the Runtime Stack figure and are explained below.

The first word is the back chain, the location where the sender function saved its caller's SP value prior to modifying the SP. The second word (at offset 4) is where the catcher function can save the CR if it modifies any of the nonvolatile CR fields. The third word (offset 8) is where the catcher function can save the LR if the catcher function makes any calls.

The fourth word is reserved for compilers, and the fifth word is used by binder-generated instructions. The last word in the link area (offset 20) is where the TOC area register is saved by the global linkage (glink) interface routine. This occurs when an out-of-module call is performed, such as when a shared library function is called.
Area 4: Catcher's Floating-Point Registers Save Area
Area 4 is the floating-point register save area for the callee (the catcher function) and is doubleword-aligned. It represents the space needed to save all the nonvolatile FPRs used by the called program (the catcher function). The FPRs are saved immediately above the link area (at a lower address) at a negative displacement from the sender function's SP. The size of this area varies from zero to a maximum of 144 bytes, depending on the number of FPRs being saved (maximum number is 18 FPRs * 8 bytes each).
Area 5: Catcher's General-Purpose Registers Save Area
Area 5 is the general-purpose register save area for the catcher function and is at least word-aligned. It represents the space needed by the called program (the catcher function) to save all the nonvolatile GPRs. The GPRs are saved immediately above the FPR save area (at a lower address) at a negative displacement from the sender function's SP. The size of this area varies from zero to a maximum of 76 bytes, depending on the number of GPRs being saved (maximum number is 19 GPRs * 4 bytes each).
Note:
1. A stackless leaf procedure makes no calls and requires no local variable area, but it may use nonvolatile GPRs and FPRs.
2. The save area consists of the FPR save area (4) and the GPR save area (5), which have a combined maximum size of 220 bytes. The stack floor of the currently executing function is located at 220 bytes less than the value in the SP. The area between the value in the SP and the stack floor is the maximum save area that a stackless leaf function may use without acquiring its own stack. Functions may use this area as temporary space which is volatile across calls to other functions. Execution elements such as interrupt handlers and binder-inserted code, which cannot be seen by compiled codes as calls, must not use this area.
The system-defined stack floor includes the maximum possible save area. The formula for the size of the save area is:
```
18*8
(for FPRs)
 + 19*4
(for GPRs)
 = 220
```
Area 6: Catcher's Local Variable Area
Area 6 is the local variable area for the catcher function and contains local variables and temporary space required by this function. The catcher function addresses this area using its own SP, which points to the top of area 8, as a base register.
Area 7: Catcher's Output Argument Area
Area 7 is the output argument area for the catcher function and is at least eight words in size and must be doubleword-aligned. The first eight words are not used by the caller (the catcher function), because their corresponding values are placed directly in the argument registers (GPR3:GPR10). The storage is reserved so that if the catcher function's callee takes the address of any of its parameters, then the values passed in GPR3:GPR10 can be stored in their address locations. If the catcher function is passing more than eight arguments to its callee (PW1:PW8, respectively), it must reserve space for the excess parameters. The excess parameters must be stored as register images beyond the eight reserved words starting at offset 56 from the catcher function's SP value.
Note: This area can also be used by language processors and is volatile across calls to other functions.
Area 8: Catcher's Link Area
Area 8 is the link area for the catcher function and contains the same fields as those in the sender function's link area (area 3).