134 Part 1½
Fundamentals Section 3 ½
Computers of Historical Significance
3 The word is a descriptor containing the base address of the data area in which the operand resides.
4 The word is a program descriptor containing the base address of a subroutine.
Fir (1), the operand call syllable has completed its action by placing an operand in the stack. The descriptor call syllable will cause the construction of a descriptor of the operand, replacing the operand by the constructed descriptor.
For (2), the operand call syllable then reads the operand from the cell addressed. The descriptor call syllable has completed its action.
For (3), indexing of the descriptor by the item that is now the second item in the stack occurs. For an operand call syllable, the operand is obtained from the indexed address; for the descriptor call syllable, action is complete after the indexing.
In the case of (4), subroutine entry occurs to the subroutine addressed. A word of the three previous types may be left in the registers upon return from the subroutine, in which instance the actions described above will take place, depending upon the type of syllable which initiated the subroutine.
Essentially, the four types of action that occur for an operand call syllable are obtaining an operand directly, indirectly, from an array, or by computation. Sometimes in the use of the call syllables, it is not known which type of action will occur for a particular syllable when the program is created. This is particularly true for call syllables in subroutines.
Programs in the word mode consist of strings of syllables which follow the rules of Polish notation. Variable length strings of call syllables and literal syllables, which place items of information in the stack, are followed by operator syllables which perform their operations on information in the stack.
The indexing features of the B 5000 allow generalized indexing and at the same time provide complete storage protection. Data areas and program segments of different programs may be intermingled, but a program is prevented from storing outside of its data areas. The method of indexing allows any of the 1,024 words of the program reference table to be considered index registers. Multilevel indexing is provided, i.e., indices of arrays can themselves be elements of arrays.
The subroutine control provided in the B 5000 allows nesting of subroutines-even recursive nesting (a subroutine is a subroutine of itself) arbitrarily deep. Dynamic allocation of storage for parameter lists and temporary working storage simplify the use of subroutines. Storage is automatically allocated and deallocated as required.
Character Made Program
In the character mode of the B 5000 Processor, there is only one type of syllable, called the operator syllable. Program segments in the character mode are constructed of strings of these syllables. The character mode is designed to provide editing, formatting, comparison, and other forms of data manipulation. In doing so, the processor uses two areas of memory-the source and destination areas. When a program switches from word mode to character mode, two descriptors containing the base addresses of these areas are supplied. The source area or destination area may be changed at any time during character mode so that the program may act on several areas.
The character mode operator syllable is split into two 6-bit parts; the last part specifies the operation to be performed and the first part specifies the number of times the operation is to be performed. Operations are provided for the transferring, deletion, comparison, and insertion of characters or bits. Also, there are operations which allow the repetition of syllable strings. This is quite useful for complex table look-up operations and for editing information which contains repeated patterns.
Conclusion
The Burroughs B 5000 system has been designed as an integrated hardware-software package which offers such benefits as savings in the memory space required to store equivalent object pro grams; multi-processing and parallel processing; and running identical programs on systems with different size memories and different system configurations with no loss in individual system efficiency.
References
Lonergan and King [1961]; Barton [1961]; Bock [1963]; Carlson [1963]; Maher [1961].