272 Part 3 The instruction-set processor level: variations in the processor

The word mode of the B 5000 processor has four types of syllables. The syllable type is distinguished by the two high-order bits of each 12-bit syllable. The types of syllable and the identification bits are:

The first of these, the operator syllable, causes operations to be performed. The remaining ten bits of the operator syllable are the operation codes. There are approximately sixty different operations in the word mode. For those operations requiring an operand or operands, the processor checks for sufficient operands in the registers; if they are not there, pushups from the stack in memory occur automatically.

The literal syllable is used for placing constants in the stack to be used as operands. The ten bits of the literal syllable are transferred to the stack. This allows the program to contain integers less than 1,024 as constants.

The operand call syllable, and the descriptor call syllable address locations in the program reference table. The purpose of the operand call syllable is to place an operand in the stack; the purpose of the descriptor call syllable is to place the address of an operand, a descriptor, in the stack. There are four situations that arise, depending on the word read from the program reference table.

For (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.

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