Usually programmers use GCC to generate object files or executable programs without knowing how it works. What is normally unseen is that GCC generates GNU assembler code and then invokes the GNU assembler and the linker. It is actually the assembler which produces the object files and the linker which produces executable programs. GCC can be told to avoid invoking the assembler and linker and instead to retain the GNU assembler source file. This is where the Tachyon z/Assembler can be used to generate object code for OS/390 or z/OS instead of Linux for S/390 and zSeries.

GCC is distributed in both source and executable form. The same source can be used to build versions for many different platforms and architectures. GCC can even be built as a cross compiler, running on one platform but generating code for another platform. If you want to generate code for an S/390 platform, you can either run the GCC provided with Linux for S/390 and zSeries or you can build a cross compiler to run on Linux/x86, Windows or any other supported platform.

GCC produces excellent, well-optimized machine instructions. Hartmut Penner, who maintains the S/390 and zSeries versions of GCC, works for IBM Germany and understands how to generate instructions which cause a minimum of pipeline stalls on S/390 and zSeries machines.

More information about GCC and the generated GNU assembler code was presented at at SHARE 98 session 8131. The session handout is available in PDF format. Further information about assembler programming for Linux for S/390 and zSeries will be presented at SHARE 99 session 8139.

To build OS/390 and z/OS programs using GCC and the Tachyon z/Assembler, you need:

GCC

Using version 3.0.2 or later of GCC is recommended. The 2.9 versions of GCC which were distributed with early versions of Linux for S/390 do not generate nearly as optimized code as the 3.0 and later versions. You can download current and old versions of GCC by following links from the GCC home page.

Tachyon z/Assembler

Version 2.2 of the Tachyon z/Assembler an later includes the tas program, which is available free-of-charge. The tas program can be used to invoke the Tachyon z/Assembler to create ELF and GOFF object files from GCC-generated GNU assembler source. You can download the Tachyon z/Assembler for GCC and use the tas program on any of the following platforms: Linux for S/390 and zSeries, Linux/x86, Windows, AIX or Solaris. You will need version 2.2.15 or later of the Tachyon z/Assembler to assemble the output of GCC version 3.3.

LIB390

On LINUX, the GCC runtime library is provided by GLIBC. Since GLIBC does not include an OS/390 interface, another library is needed. Tachyon Software has provided the start of an open-source OS/390 runtime library for GCC called LIB390. Tachyon Software will continue to enhance LIB390 and encourages contributions from others interested in extending the library.

To compile a C program for OS/390 or z/OS using GCC do the following:

1. Run the gcc program using the -S option to cause it to create a GNU assembler source file. For instance, to compile the program hello.c and create the GNU assembler source file hello.s, you can use the following command:
   gcc -S -O3 hello.c
2. Run the tas program to assemble the GNU assembler file and create a GOFF object file. For instance, to assemble hello.s and create an object file named hello.obj and a listing file named hello.lst, you can use the following command:
   tas -o hello.obj -goff -a hello.s
3. Upload the object files to OS/390 or z/OS and link them with the LIB390 object files. Alternatively, you can use the Tachyon File Tools to link the object files and create a TSO XMIT file which can then be uploaded to OS/390 and z/OS.