Linkage conventions

External function modules are called by the assembler using standard OS Linkage conventions. The external function can be written in any language that:

Uses standard OS linkage conventions.
Can be called many times using the module (or phase) entry point.
Retains storage for variables across invocations and does not require a run-time environment to be maintained across invocations.

See the specific programming language Programmer's Guide to determine if you can use the programming language to write an external function for the High Level Assembler.

The contents of the registers upon entry to the external function are as follows:

Register 0: Undefined
Register 1: Address of external function parameter list
Registers 2 through 12: Undefined
Register 13: Address of the 72 byte register save area
Register 14: Return address
Register 15: Address of entry point of external function