Browse Prior Art Database

Dual Ported Four Sector File Buffer

IP.com Disclosure Number: IPCOM000087666D
Original Publication Date: 1977-Mar-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 3 page(s) / 75K

Publishing Venue

IBM

Related People

Cotton, RF: AUTHOR [+3]

Abstract

A dual-ported buffer allows data transfer to occur between a disk file unit and a processor interconnected by a device attachment. Control circuitry is provided for controlling the writing of data into and reading data from a buffer in conjunction with the operation of the disk file storage drive. The buffer is divided into four sections by means of sector boundary latches. The states of these latches, together with input and output mode signals, are used to signal the channel and the disk file attachment as to the availability of the buffer. For an output (write) operation, the channel can fill the buffer when any section thereof is empty, and the disk file can empty the buffer if any section thereof is full.

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Dual Ported Four Sector File Buffer

A dual-ported buffer allows data transfer to occur between a disk file unit and a processor interconnected by a device attachment. Control circuitry is provided for controlling the writing of data into and reading data from a buffer in conjunction with the operation of the disk file storage drive. The buffer is divided into four sections by means of sector boundary latches. The states of these latches, together with input and output mode signals, are used to signal the channel and the disk file attachment as to the availability of the buffer. For an output (write) operation, the channel can fill the buffer when any section thereof is empty, and the disk file can empty the buffer if any section thereof is full. On an input (read) operation, the disk file can fill the buffer if any section is empty, and the channel can empty the buffer if any section is full.

In Fig. 1, a central processing unit (CPU) 1 has an associated channel la, storage 2, and a device attachment 3 for the interconnection of a disk file unit 5. File unit 5 has a disk 6 rotating during operation and two read-write heads 7 and 8 for accessing selected tracks on disk 6. It is assumed that the data transfer rate of the disk file 5 is about 2.2 microseconds per 16-bit word. It is further assumed that the file channel attachment to the CPU 1 can normally keep up with this rate if no other devices are running. However, a practical system, running other devices, can cause the cycle steal rate to drop below that necessary to prevent overrunning the disk file 5.

The solution described here is to buffer the disk file attachment so that the channel la can get behind momentarily and catch up later. However, difficulties may be encountered because of the magnitude of overrun that can occur. A heavily loaded system can continue getting behind so that overrun would soon become inevitable if small buffers are used. Since the entire track is too large to buffer, a smaller sector-oriented buffer is proposed.

A sector buffer is the smallest size that will prevent overrun because of the nature of the file. Once a 256-byte sector of data has started transferring from the file, it must continue to completion. Furthermore, the buffer must be accessible simultaneously by both the file and channel. Hence, it is dual ported.

This dual-ported multisector approach is used in the disk file attachment buffer. A 1024-byte Functional Storage Unit (FSU) random-access memory 12 (Fig. 2), was selected as the buffer for economy. This random-access memory is large enough to allow four sectors to be buffered before becoming full and fast enough to allow dual porting through the mechanism of multiplexing. Also, it was decided to al...