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Selector Channel for a Communications Pre Processor

IP.com Disclosure Number: IPCOM000074839D
Original Publication Date: 1971-Jun-01
Included in the Prior Art Database: 2005-Feb-23
Document File: 2 page(s) / 14K

Publishing Venue

IBM

Related People

Annunziata, EJ: AUTHOR [+4]

Abstract

This selector channel has a programmed facility to permit an early termination of an I/O operation in the event that a Data Check (Bad Parity on Data) is encountered. Present IBM System/360 channels require that a channel maintain the data transfer operation to the end of the record before the "Data Check" condition can be signalled. In an environment where a communications preprocessor is attached to a host processor via a channel-to-channel adapter, a condition of this type is undesirable. This is because the host processor could conceivably be tied up in an operation for a considerable length of time (to the end of a long record), before it is signalled to disconnect whereas an early termination leading to an eventual retry is desirable.

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Selector Channel for a Communications Pre Processor

This selector channel has a programmed facility to permit an early termination of an I/O operation in the event that a Data Check (Bad Parity on Data) is encountered. Present IBM System/360 channels require that a channel maintain the data transfer operation to the end of the record before the "Data Check" condition can be signalled. In an environment where a communications preprocessor is attached to a host processor via a channel-to-channel adapter, a condition of this type is undesirable. This is because the host processor could conceivably be tied up in an operation for a considerable length of time (to the end of a long record), before it is signalled to disconnect whereas an early termination leading to an eventual retry is desirable. Bit 38 has been utilized in the Flag field of the channel command word (CCW) (Data Check Terminate). The programmer has the option of allowing an I/O operation to conclude in the normal manner when encountering a Data Check by not setting this bit, or he can permit the channel to come to an immediate ending condition by setting the bit to a one.

This selector channel also has a programmed inventory control device called Block Mode. Block Mode is program compatible with System/ 360 and has the unique ability to execute Data Chaining and Block Mode operations together under CCW count control. This is also true of command chaining and Block Mode operations. The channel executes the Block Mode operation whenever bit 37 of the flag field of the CCW is set to one. A typical Block Mode Read operation proceeds as follows:

Data received from the I/O device is sent to storage starting with the address location specified by the CCW. A "Block" is 64 bytes in length and starts on an even 64-byte addressing boundary. When the end of a block is detected (address bits 26-29 all ones), a new address specifying the beginning of the next block is fetched from the Next Available Block Register (NABR) of the CPU, and its contents are concatenated to the 2 data bytes of the last word location of the block to provide for a programmers aid in the form of a linkage address to the next block of data in storage. This block address is also used by the channel to describe the beginning of the next block of 64 bytes. In the meantim...