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Inter-Office Memorandum To Mesa Users Date May 31, 1978 From Ed Satterthwaite Location Palo Alto Subject Mesa 4.0 Compiler Update Organization SOD/SO XEROX Filed on: [IRIS]DOC>COMPILER40.BRAVO This memo describes changes to the Mesa language and compiler that have been made since the last release (October 17, 1977). As usual, the list of compiler-related change requests closed by Mesa 4.0 will appear separately as part of the Software Release Description. The language accepted by the Mesa 4.0 compiler has several significant extensions and a few minor changes. It features a process mechanism, enhanced arithmetic capabilities, long and base-relative pointers, and more general block structure. Because of changes in symbol table and BCD formats, all existing Mesa programs must be recompiled. There are minor incompatibilities with Mesa 3.0 at the source level in the areas of signed/unsigned arithmetic and the scope of OPEN in an iterative statement. These incompatibilities should have negligible impact on existing programs. The syntax and semantics of declaring (but not calling) machine-coded procedures have changed substantially. Page and section numbers in this update not otherwise qualified refer to the Mesa Language Manual, Version 3.0. The BNF descriptions of new or revised syntax follow the conventions introduced in that manual. For phrase classes used but not redefined here, see its Appendix D. Revisions of phrase class definitions are cumulative; except as noted, the appearance of "... " as an alternative indicates that an existing definition is being augmented. A definition without "... " supersedes any definition of the same phrase class in the manual. Arithmetic Mesa 4.0 supports double-precision integer arithmetic (type LONG INTEGER) and provides some help with floating-point computations (type REAL). In conjunction with these changes, the rules governing combination of signed and unsigned values have been more carefully defined (see the Appendix to this memo). Syntax PredefinedType ::= INTEGER I CARDINAL I LONG INTEGER I REAL I BOOLEAN I CHARACTER I STRING I UNSPECIFIED WORD Primary .. - "- identifier [ Expression ] I LONG [ Expression ] Mesa 4.0 Compiler Update 2 ~ Signed and Unsigned Arithmetic The rules governing the use of signed and unsigned representations in single-precision arithmetic have been reformulated. In previous versions of Mesa, conditions under which an operation was considered to overflow were not well defined. As a consequence, options such as overflow detection a