Burroughs Large Systems Descriptors - Usage in Compilers

Usage in Compilers

In ALGOL, the bounds of an array were completely dynamic, could be taken from values computed at run time, which was unlike Pascal where the size of arrays was fixed at compile time. This was the main weakness of Pascal as defined in its standard, but which was removed in many commercial implementations of Pascal, notably the Burroughs implementations (both the University of Tasmania version by Arthur Sale and Roy Freak, and the Burroughs Slice implementation by Matt Miller et al.)

Note that in a program in the Burroughs environment, an array is not allocated when it is declared, but only when it is touched for the first time – thus arrays can be declared and the overhead of allocating them avoided if they are not used.

Also note that low-level memory allocation system calls such as the malloc class of calls of C and Unix are not needed – arrays are automatically allocated as used. This saves the programmer the great burden of filling programs with the error-prone activity of memory management, which is crucial in mainframe applications.

When porting programs in lower-level languages such as C, the C memory structure is dealt with by doing its own memory allocation within a large allocated B5000 block – thus the security of the rest of the B5000 system cannot be compromised by errant C programs. In fact, many buffer overruns in apparently otherwise correctly-running C programs have been caught when ported to the B5000 architecture. C, like Pascal, was also implemented using the Slice compiler system (using a common code generator and optimizer for all languages). The C compiler, run-time system, POSIX interfaces, as well as a port of many Unix tools was done by Steve Bartels. An Eiffel compiler was also developed using Slice.

For object-oriented programs which require more dynamic creation of objects than the B5000 architecture, objects are best allocated within a single B5000 block. Such object allocation is higher level than C's malloc and is best implemented with a modern efficient garbage collector.

The last p-bit scenario is when bit 47 is 0, indicating that the data is not in memory, but the address is non-zero, indicating that the data has been allocated and in this case the address represents a disk address in the virtual memory area on disk. In this case a p-bit interrupt is raised and it is noted as an 'other' p-bit.

Thus the B5000 had a virtual memory system integrated into the hardware – a virtual memory system that has to this day been unsurpassed, since all other systems must build virtual memory on top of lower-level hardware. ALGOL and the B5000 also represented a significant advance on the low-level, error-prone, and programmer intensive 'malloc' mechanisms of later systems.