Chapter 22 Design of the B 5000 system 271
program segmentation. Through automatic program segmentation, it is possible to have program size practically independent of the size of core memory. The systems analyst or programmer intending to do multi-processing is then no longer faced with the difficult task of planning what jobs are to be run together in order that system storage capacities are not exceeded.
In achieving independence of addressing, a solution requiring large contiguous areas of memory was not deemed satisfactory. Each segment of the program and each data area should be completely relocatable without modification to the program. It is then possible to load all the segments of a program or programs onto the drum at load time and call in the segments to any available space in core memory as needed during run time. If some segment of a program is overlaid by a subsequent segment of a program, the segment of the program destroyed in core memory is still available on the drum to be called in again if needed.
Due to the very high program densities in the B 5000, the availability of high capacity drum storage on every system and automatic segmentation, a minimum B 5000 system has the capacity for a program or programs equivalent to approximately 40,000 to 60,000 single address instructions. Of course, if an installation normally ran such large programs, the system would very likely not be a minimum system. However, the installation having an occasional need to run very large programs is not prevented from doing so by storage capacity.
Processing speed now becomes a function of the size of core memory. If large programs are run in a system with small core memory, time will be consumed in recalling program segments from drum to core. If the core memory is expanded, less time will be spent in such activity and the program or programs will be speeded up, and no reprogramming is required.
Program reference table
The means of achieving independence of addressing in the B 5000 is called a Program Reference Table (PRT). The PRT is a 1,025 word relocatable area in memory used primarily for storing control words that locate data areas or program segments. There are also control words for describing input-output operations. These control words, called descriptors, contain the base address and size of data areas, program segments and input-output areas. A descriptor specifying an input-output operation also contains the designation of the unit to be used and the type of operation to be performed. Operands may also be stored in the PRT, providing direct access to single values such as indices, counts, control totals, etc.
In the word mode of the B 5000, every item of data is considered to be either a single value or an element of an array of data. If it is a single value, it will be obtained directly by indexing a descriptor contained in the PRT.
Program segments are described by program descriptors. In addition to core base address, the program descriptor contains the location in drum storage of the program segment and an indication if the program segment is currently in core memory starting at the address specified in the descriptor. Entry to a program segment is made via its program descriptor contained in the PRT. If the program segment is in core memory, entry will be made to the program segment. However, when entry is attempted to a program segment whose descriptor indicates that the segment is not in core memory, automatic entry to the Master Control Program will occur and the desired segment will then be brought in from the drum. Notice that in moving from one segment to another, it is not necessary to know whether the segment to be entered is currently in core memory. Branching within a program segment is self-relative, i.e., the distance to jump either forward or backward is specified, not the address to be jumped to.
As a result of keeping all actual addresses of data and program in the PRT, the program itself does not contain any addresses, but only references to the PRT. To specify one of the 1,024 positions in the PRT requires only 10 bits which contributes greatly to the high program density achieved in the B 5000. Since the PRT is relocatable, references to the PRT contained in the program are to relative locations, thus completely freeing the program from any dependence whatsoever on actual memory locations.