Neliac Decompiler

Tom Moran wrote:

• A working decompiler for NELIAC, an Algol 58 derivative language not too unlike C, is described (including source listing) in Appendix D in "Machine-Independent Computer Programming" by Maurice H. Halstead, copyright 1962 and undoubtedly long out of print. It cites a Navy Electronics Laboratory Technical Memorandum #427 by Joel Donnelly which might conceivably still be available.

• Halstead saw a decompiler as useful for 1) understanding/modifying a program to which you no longer had source code, and 2) porting such a program to a new machine by decompiling it and then recompiling for the new machine.

Tom Moran's first project, as a summer student worker and novice programmer, was to assist Joel Donnelly with porting this decompiler to the CDC 1604.

Anecdotes by Bill Caudle on his work with Maurice Halstead (1962-1978).

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NELIAC stands for Navy Electronics Laboratory International ALGOL Compiler. So it really is supposed to be recognisably Algol.

-- MikeVanEmmerik - 04 Feb 2005

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The following is from the book:

Maurice H. Halstead,
"Machine-Independent Computer Programming"
Spartan Books,
Washington DC 1962.

Note how programs are displayed in octal, not even indisassembled form.

Decompiling with D-Neliac

For several years it has been apparent that those computer programs written in machine-dependent languages represented large investments in time which were lost whenever a change in computers was required. Even though a change to a machine-independent language was made, all programs written before the change still require complete rewriting if they are to remain in use.

If, however, such programs could be computer-converted to the Neliac language, then they would be in a form adaptable to recompilation upon any other computer having a Neliac compiler. Furthermore, if the translator were to be capable of accepting the machine-language programs of a given computer as input, then it would not matter whether the program had originally been written in machine language, in an assembly system, or any possible problem-oriented compiler system.

While it would be premature to infer that this objective can be met with complete satisfaction, the Donnelly D-Neliac system [15] as as produced by J. K. Donnelly and Herman Englander has already established that such an approach is highly feasible, and preliminary versions now exist for both the Remington Rand Univac M-460 Countess computer and for the Control Data Corporation 1604 computer. A recent version of the former is given in complete detail in Appendix D.

To understand the workings of a decompiler, consider the following case, in which Example 69 shows the source material, a machine-language version of a program which has been written, tested, and rendered operational on the M-460 Countess.

[15] J. K. Donnelly, "A Decompiler for the Countess Computer," Navy Electronics Laboratory Technical Memorandum 427 , Sept., 1960.

Click here for Example 69.

As can be seen from a study of Appendix D, in the event that a machine-language combination is encountered for which no decompilation instructions have been written it can still convert them to the Neliac machine-language form, and then continue. Strangely enough, working with decompilers has emphasized one of the limitations in the bit-handling capabilities of the Neliac language. This limitation consists of the inability to specify any but contiguous bits in a part word, a feature sometimes used in masking for logical operations.

In addition to its primary mission of translating non-Neliac programs into the Neliac language, D-Neliac has also proved useful in another way. This secondary use has been in the area of error detection. Since a program will appear in different words, and with several of the details handled differently if it is decompiled after having been compiled, the paraphrased version is sometimes helpful in the detection of errors in logic. A classic example is a case in which an occasionally used regression-analysis program was found to fault in certain circumstances. Since the program had originally been coded in machine language by a mathematician no longer available, the fault was so subtle that it had not been found, even after considerable searching. Upon decompilation, however, it was immediately apparent that a misuse of an index occasion- ally occurred.

A final pair of examples will show the decompilation of one of the utility routines long in use on the Countess for loading flexo- writer coded punched tape. This particular routine calls upon other subroutines outside the area being decompiled, so that while the decompiler named them, it obviously could record only their locations, and not their contents. The numerical values for the Flexowriter codes in Table III correspond with results in this case, too, of course. The first example shows the raw program as fed to the D-Neliac program, the second shows the Neliac program which it produced, while the third shows the Name List which it also furnished.

Click here for Example 72.

Click here for the Univac M460 instruction set.

CategoryDecompilation