IBM 5280 Distributed Data System - Index of

When a program executing in a background partition needs to use a keyboard, itcan cause an edge indicator to be displayed on the keyboard/display screen. Thisindicator notifies the operator that a background partition needs the keyboard.The operator can interrupt the program that is using the keyboard and attach thebackground partition. When the background partition no longer needs the keyboard,the partition must be detached to give control of the keyboard back to theinterrupted program. Only one partition can be attached to a keyboard/displayat any given time.INPUT AND OUTPUT BUFFERSThere must be at least one physical buffer in main storage for each lOB in a programthat has I/O instructions. The physical buffer length must be a multiple of128 bytes. Double buffering can be used for minimal delays in interactive programs;a second physical buffer is set up so the 5280 can process data in one while an inputor output operation is being performed with the other. Double buffers are alsorequired to duplicate fields of a previous record into the same field of a currentrecord. The 5280 keeps track of the buffers and the records that are in the buffers.Data sets can be blocked for better utilization of diskette space; a logical buffer isset up and the blocking and deblocking functions are performed automatically bythe 5280. Or the logical buffer can be omitted and logical records can be blockedand deblocked directly to and from the physical buffer.EXTERNAL STATUS PROCESSINGWhile an I/O device is processing I/O, it may encounter a condition that the devicemicroprocessor cannot handle, such as an error condition or a condition thatrequires operator intervention or execution of object code instructions. When thisoccurs, the device microprocessor stops processing the I/O, places a conditioncode into the device lOB, sets an external status flag in the device lOB, and sets anattention line to the main microprocessor. The device microprocessor continuesto service the other lOBs.When the main microprocessor determines that an lOB has the external status flagset, it enters the partition and executes appropriate object code instructions toresolve the conditions. The instructions are determined by the application program.When the application program has resolved the condition, the main microprocessorresets the external status flag and goes to the next partition. The device microprocessorreturns to the lOB only when it again receives an I/O command. The I/Ocommand may be a reissue of the last I/O command.5280 Functions and Storage Overview 13