PDP-11 Subprogram Calling Sequence Standards

This document, a Programming Department Memo from January 1970, proposes and evaluates various standards for subprogram calling sequences in PDP-11 software. The author highlights the importance of these standards for ensuring software interchangability and consistency, particularly for FORTRAN. The memo explores several approaches: prohibiting re-entrant calls, mandating re-entrant calls for all subroutines, a 'compromise' (Bell-Delagi) method, and utilizing the TRAP instruction. Following a comparative analysis of calling sequence lengths and complexity, the author recommends the 'All Calls Re-entrant' method as the standard, noting its minimal performance penalty and alignment with future application needs. The document also outlines conventions for register usage (R5 as the calling register), argument ordering on the stack, FORTRAN function return values, and register saving/restoring practices.