SCREEN MANAGER MULTIPLE VIEWPORT FOR A MULTI- TASKING DATA PROCESSING SYSTEM

A task control structure for transferring tasks from a storage device to a system memory and for controlling execution of tasks, and a document manager for loading document information in the form of document data structures from the storage device to the system memory and managing access to the data structures by the tasks are using task control blocks to manage the execution of tasks and document control blocks to manage access to the document data structures by the tasks. Each document file has a document control block and the document files and the document control blocks are designed to represent and relate to the structure of documents. Each document file has at least one page including at least one area, each containing at least one type of information. Each area including, in an area containing text information, at least one column for containing text information including at least one line, each line including a string of at least one text character, a reference to attribute applying to the characters or the string, and references to external data items associated with the line. A screen manager is used for creating visual display screens, each screen having a virtual screen with one or more viewpoints onto data to be displayed. The screen manager includes a control structure containing information relating each screen to a corresponding task, and, for each screen, information describing certain properties of the screen, and information relating the screen to the data residing in the document structure to be displayed.