246 Part 2 ½ Regions of Computer Space
Section 2½ Memory Hierarchies and Multiple Processes

(DD) is used to fetch data to the stack or store data from the stack into an array which resides outside the job's stack area. The format of Data and Segment Descriptors are illustrated in Fig. 1. The ADDRESS field of both descriptors is 20 bits in length and contains the absolute address of an array in either main system memory or in the back-up disk store. The Presence bit (P) indicates whether the referenced data are present in main system memory or in the back-up disk store, and is set equal to ONE when the referenced data are present in main system memory.

A Presence Bit Interrupt is incurred when the job makes reference to data via a descriptor which has a P bit equal to ZERO. The Presence Bit Interrupt stimulates the operating system (called the Master Control Program, or MCP) to move the data from disk to main memory. The data location on disk is contained in the ADDRESS field of the DD when the P bit is equal to ZERO. After transferring the data array into the main memory, the operating system (MCP) marks the descriptor present by setting the P bit equal to ONE, and places the current memory address into the ADDRESS field of the descriptor. The interrupted job is then reactivated.

A Data Descriptor may describe either an entire array of data words, or a particular element within an array of data words. If the descriptor describes an entire array, the Indexed bit (I-bit) in the descriptor is ZERO, indicating that the descriptor has not yet been indexed. The LENGTH field of the descriptor defines the length of the data array.

A particular element of an array may be described by indexing an array descriptor. Memory protection is insured during indexing operations by performing a comparison between the LENGTH field of the descriptor and the index being applied to it. An Invalid Index Interrupt is incurred if the index value exceeds the length of the memory area defined by the descriptor.

If the value being used to index the descriptor is valid, the LENGTH field of the descriptor is replaced by the index value. At this time the I-bit in the descriptor is set to ONE to indicate that indexing has taken place. The ADDRESS and LENGTH fields are added together to generate an absolute machine address whenever a present, indexed Data Descriptor is used to fetch or store data.

The Double Precision bit (D) is used to identify the referenced data as being either single or double precision and, as a result, is also associated with the indexing operation. The D bit being equal to ONE signifies double precision and implies that the index value be multiplied by two before indexing.

The Read-Only bit (R) specifies that the memory area described by the Data Descriptor is a read-only area. An interrupt is incurred upon referencing an area through a descriptor with the intention to write if the R bit is equal to ONE.

The Copy bit (C) identifies a descriptor as being a copy of a master descriptor and is related to the present bit action. The intent of the copy action is to keep multiple copies of an absent descriptor linked back to one master descriptor. Copy action is incurred when a job attempts to pass by name an absent Data Descriptor. When this occurs, the hardware manufactures a copy of the master descriptor, forces the C bit equal to ONE and inserts into the ADDRESS field the address of the master descriptor. Thus, multiple copies of absent descriptors are all linked back to the master descriptor.
 
 

Non-Local Addressing

The most important single aspect of the B6500/B7500 stack is its facility for storing the dynamic history of a program under execution. Two lists of program information are saved in the B6500/B7500 stack, the stack history list and the addressing environment list. The stack history list is dynamic in nature, varying as the job is driven through different program paths with changing sets of data. Both lists are generated and maintained by the B6500/B7500 hardware system.

The stack history list is formed from a list of Mark Stack Control Words (MSCW) which are linked together by their DF fields (Fig. 3). A MSCW is inserted into the stack as a procedure is entered, and is extracted as that procedure is exited. Therefore, the stack