PhD Thesis - staffweb - University of Greenwich

PhD Thesis by John Ewer.3.6.2 Data access mechanismsThere was no use of dynamic memory allocation (compiler dependant additions) in the legacycode and no pseudo-allocation techniques were used (for example FORTRAN developers oftendeclare and use huge data arrays that are partitioned and passed down to subroutines as separatearguments). This meant that the legacy code often needed to be modified and recompiled fordifferent problem sizes. This can greatly limit portability of the software and generally meansthat portions of the code had to be made available in source code form for re-compilation. Therewas also the problem that the legacy software might not use memory in the most efficient waypossible for the particular problem being simulated.Standard FORTRAN array based storage was used for all data with each array named separatelyas a separate variable. The legacy code did not use so called "f-array" storage techniques as in,say, Phoenics [CHAM] - This technique declares a huge data array at the main program leveland passes down "chunks" of the array to sub-procedures. The use of simple array based storageand the naming conventions used in the legacy code mean that, other than the declared sizes,there was no clear indication of how differently named variables and arrays were related to oneother. Another problem that was encountered concerns the difficulty of developing additionalfunctionality that requires new variables. Generally speaking, the addition of a new variable, tosolve or calculate, will also require a large number of additional support or storage variables.This additional storage may be needed for temporary storage during calculations, for control ofthe solution, for reporting of status or for the storage of different historical versions of theparticular variable. This is a particular problem because the data dependencies in the legacy codeare unclear and it is thus difficult to ascertain how many and of what type the required newvariables should be.There is no use made of COMMON for passing data around the system between procedures.This means that all data is passed around the system as formal parameters in argument lists.Given the high degree of data dependency between most of the numerical routines there areconsiderable numbers of arguments. All variable names had to be limited to 6 characters to avoidexceeding the continuation line limits for some of the argument lists. The greatest problem posed3-44