VAX MACRO

VAX MACRO is the computer assembly language implementing the VAX instruction set architecture for the OpenVMS operating system, originally released by Digital Equipment Corporation in 1977. A significant amount of VMS is written in VAX MACRO.
The syntax, directives, macro language, and lexical substitution operators of VAX MACRO formerly appeared in MACRO-11, the assembler for the PDP-11 series of computers. The MACRO-32 assembler supported the VAX processors developed and manufactured by DEC. It ran under the VMS operating system and produced object files suitable for the VMS linker. The MACRO-32 assembler and linker were bundled with the operating system.

VAX MACRO on other architectures

The port of VMS to the DEC Alpha architecture led to the creation of a MACRO-32 compiler, which treated VAX MACRO as an input language and generated optimized Alpha object code. The MACRO-32 compiler was necessary since it was not feasible to rewrite the large quantity of VAX MACRO code present in the VMS operating system, layered products, and third-party software. The MACRO-32 compiler was subsequently ported to Itanium and x86-64 as OpenVMS was ported to those architectures.
Due to the difficulty of mapping low-level VAX semantics onto other architectures, most VAX MACRO programs written for the native VAX assembler require modification before they can be compiled with the MACRO-32 compiler. These changes include adding directives to explicitly define certain behaviours of the code, and removing code which relies on non-portable constructs. Furthermore, the MACRO-32 compiler allows direct access to certain Alpha architecture features, including Alpha's 32 registers, 64-bit addressing, and a subset of Alpha instructions. These are mapped onto equivalents on Itanium and x86-64, but are not supported by the original VAX MACRO assembler.
The OpenVMS assembler for Alpha assembly code is named MACRO-64. Despite sharing a similar name and macro syntax, MACRO-64 is otherwise unrelated to MACRO-32.