In particular it should be noted that the code described, I-code, was designed specifically for the language IMP-77, a systems implementation language which contains many of the constructions which pose problems for optimisation [Robertson, 1979]. It in no way attempts to be a "universal" intermediate code. Notwithstanding, the code, with a small number of minor extensions to cover non-IMP features, has been used successfully in an ALGOL 60 compiler and is currently proving viable in projects for writing Pascal and Fortran 77 compilers.
The intermediate code as finally designed is completely machine independent, except inasmuch as the source program it describes is machine dependent, demonstrating that the problems may not be as intractable as thought by Branquart et al. who state that "clearly complete machine independency is never reached" [Branquart, 1973]
In addition to the problems of machine independence there is also the question of operating system independence, as nowadays it is common for machines to have several systems available. For this reason the task of producing a compiler is far from finished when it can generate machine code [Richards, 1977]. To simplify the generation of versions of a compiler for different operating systems a third phase of compilation was added, although it soon became clear that the extra phase could be used for other purposes as well, as will be shown later (section 4).