The IDO is the IRIS Development Option. This optional package includes the header files necessary to compile programs on Silicon Graphics machines running IRIX, as well as the SGI/MIPS C compiler.
In common with several other commercial UNIX vendors, SGI shipped an "Execution Only Environment" (EOE) standard, requiring users to purchase the extra-cost IDO if they needed to compile applications on their machine. The IDO provided the headers, backend MIPS "ucode" optimizing compiler and assembler as well as a front-end C to ucode translator. The IDO backends could be used with other optional language translators as well, including Fortran and C++. Since it includes header files, the IDO is specific to each version of IRIX, often numbered similarly but not always (the best IDO to use for IRIX 4.0.5 is IDO 4.1.1).
With the release of the MIPSpro series of compilers, the IDO was split into three parts: the IDF (IRIS development foundations), IDL (IRIS development libraries), and the language front-ends themselves (MIPSpro C, MIPSpro C++, MIPSpro Fortran, etc.).
In the late 1990s, SGI decided to release certain parts of their development tools freely. These were the IDO for IRIX 5.3 (IDO 5.3) and the IDF and IDL for IRIX 6.2. IRIX 6.5 shipped with the IDF and IDL CD-ROMs in the software library, enabling hobbyists with GCC to build programs for IRIX without having to buy the IDO.