thus, we chose FORTRAN IV as the language with which to implement the 
Bugsystem. FORTRAN IV has become a standard language among 
minicomputers and has simplified the tasks ot maintaining Bugsysem software 
and training new programmers to implement new BRL operators. The 
importance of these aspects cannot be overestimated since, all in all, ten 
different programmers have added software to the system during its three years 
of development, each requiring instruction on the software conventions and 
use of system utility subrouting packages. But at least they knew the 
FORTRAN language. 
The software was first operational on a PDP 11/45 computer at 
Southeastern Massachusetts University under the DOS-9 operating system. It 
was then implemented on a Data General ECLIPSE S/200 under the RDOS 
operating system. Some assembler language subroutines had to be recoded, 
including the software drivers that handle the direct memory channel to the 
Bugwatcher. But the transition to the new computer went fairly smoothly. The 
PDP 11 system was maintained for development purposes after the ECLIPSE 
was sent to its Narragansett home at the EPA lab. Due to malfunctioning 
DOS-9 software, the PDP 11 operating system was changed to RT 11—a change 
that required as much or more development effort than changing computers! 
Virtual File Structure 
Certainly one of the major disadvantages to using minicomputers for large 
application software projects is the address space limitation imposed by its 
small word size. This system was no exception. To overcome this limitation so 
as to allow both programs and data to fit within allocated memory, both 
program structures and data structures were designed so that only pieces of 
either resided in memory at any given time. Manufacturers of minicomputers 
recognize this problem and provide software support for manipulating 
overlayed programs; i.e., programs consisting of parts which are swapped in and 
out of main memory. However, software support was not available for similarly 
overlaying data sets. The earlier prototype system did not allow data sets to be 
any larger than the memory buffer on the IBM 1800—a simple solution, but 
not acceptable in the newer system. Certain Bugsystem applications require the 
acquisition and analysis of behavioral records consisting of many frames of 
video data or the trajectories of hundreds of organisms. The system required 
the potential to manipulate data structures ten to hundreds of times the size of 
main memory available for data. 
One of the early software design goals was to simplify the process of adding 
new BRL operators. Each operator was to be implemented in FORTRAN as a 
program overlay. An operator would be required to access as many as three 
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