Browse Prior Art Database

Diskette Load/Dump Control

IP.com Disclosure Number: IPCOM000089042D
Original Publication Date: 1977-Sep-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 3 page(s) / 64K

Publishing Venue

IBM

Related People

Mitchell, MJ: AUTHOR

Abstract

A Diskette Load/Dump Control (DLDC) unit allows a diskette drive to be driven by Magnetic Tape support in present System software packages. As illustrated in Fig. 1, the DLDC communicates with the System channel interface on one side, and controls the diskette device on the other side. The interface to the System channel accepts tape commands and executes them in a program transparent fashion from the standpoint of the System. Device status is returned with the same indications that would occur with tape devices. For example, when the end of tape indication (reflective strip with tape) is detected, the unit exception bit is returned with the usual device status.

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Diskette Load/Dump Control

A Diskette Load/Dump Control (DLDC) unit allows a diskette drive to be driven by Magnetic Tape support in present System software packages. As illustrated in Fig. 1, the DLDC communicates with the System channel interface on one side, and controls the diskette device on the other side. The interface to the System channel accepts tape commands and executes them in a program transparent fashion from the standpoint of the System. Device status is returned with the same indications that would occur with tape devices. For example, when the end of tape indication (reflective strip with tape) is detected, the unit exception bit is returned with the usual device status.

The DLDC consists of a microprocessor and hardware logic. Ideally, the microprocessor storage is of adequate size to buffer one complete track of data as represented on the diskette. Data movement is always store and forward; that is, data transferred in either input or output direction is always buffered in microprocessor storage where it is viewed by microcode. Note that a channel data overrun cannot occur with this mode of operation. The diskette track format is exactly as described in present diskette architecture. A diskette can be formatted with tracks containing 128, 256, 512, or 1K byte sectors. The conventions for defective tracks remain unchanged.

The objective is that of mapping tape records to/from diskette records. This transformation is done by the microprocessor in the track buffer in microprocessor storage. The proposed mapping technique is illustrated in Fig. 2. Tape records can be of variable length, although optimum utilization of the diskette surface occurs when tape records are blocked consistent with the sector data field size. Note that one tape record will be written onto one or more sectors; two tape records will never occupy one diskette sector. If a tape record ends prior to the end of the data field in the sector, pad characters are added to complete that sector. The next tape record begins on the next sector. As proposed in Fig. 2, the first two bytes in the diskette data field are defined to control this convention. Ten bits (phi-7 of Byte 1 and phi, 1 of Byte 2) are defined as a binary count to identify the end of data -- beginning of pad characters boundary on the sector. Three bits (2, 3 and 4 bits of Byte 2) are defined to manage the mapping function. Their use is indicated in Table 1. The fifth bit of Byte 2 is used to...