Browse Prior Art Database

PC Fixed Disk Usage During Emulation

IP.com Disclosure Number: IPCOM000060772D
Original Publication Date: 1986-May-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 3 page(s) / 55K

Publishing Venue

IBM

Related People

Hansen, BW: AUTHOR [+2]

Abstract

The IBM PC (Personal Computer) hardware interrupt level five is shared between the session for emulation of the IBM 5250 and the PC session. This allows concurrent usage of the fixed disk for application programs while 5250 emulation is running. The 5250 emulation program does not use any interrupt vectors other than those normally used by DOS and BIOS in the PC. This offers increased compatibility with other PC programs by virtue of using the existing interrupt vectors. The use of the fixed disk in the PC is interleaved with the 5250 emulation feature in a transparent fashion. The Enhanced PC/5250 Emulation Adapter can generate a hardware interrupt request. The request must enter the PC's 8259 interrupt controller chip. The IBM PC has reserved all eight 8259 interrupts for other purposes.

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PC Fixed Disk Usage During Emulation

The IBM PC (Personal Computer) hardware interrupt level five is shared between the session for emulation of the IBM 5250 and the PC session. This allows concurrent usage of the fixed disk for application programs while 5250 emulation is running. The 5250 emulation program does not use any interrupt vectors other than those normally used by DOS and BIOS in the PC. This offers increased compatibility with other PC programs by virtue of using the existing interrupt vectors. The use of the fixed disk in the PC is interleaved with the 5250 emulation feature in a transparent fashion. The Enhanced PC/5250 Emulation Adapter can generate a hardware interrupt request. The request must enter the PC's 8259 interrupt controller chip. The IBM PC has reserved all eight 8259 interrupts for other purposes. It is desirable to share a hardware interrupt request level that causes the least amount of interference to the IBM PC-XT, and provide the greatest amount of compatibility with other existing PC features. In this instance interrupt level 5 (IL5) is shared with the fixed disk adapter. Sharing IL5 allows it to run in the PC without displacing other devices, or consuming one of the valuable interrupt levels. IL5 has the unique characteristic that disk activity need not occur concurrently with twinax activity, i.e., the user apparently has concurrent use, but they actually do run alternately. For example, transferring data from the host to disk alternates between using the twinax to move data into a buffer and then momentarily suspending twinax activity to move the buffer to disk. The fixed-disk service routines have the proper characteristics which enable sharing with twinax-generated interrupts. Specifically: Short service routines. The fixed-disk routines complete in typically 50 - 100 msec. The twinax interrupt can go without servicing for up to 400 msec. Predictable interrupt arrival. The fixed-disk does not generate a hardware interrupt in the period between finishing one disk request and starting the next request. If there are no disk operations requested from BIOS, then there will never be a hardware interrupt request. When an application program running in the PC wants to access data on the fixed-disk, the sequence of events, shown in the drawing, is as follows: 1. As the PC is first powered on, the fixed-disk interrupt vectors (interrupt x'OD' and x'13') in low PC storage are initialized to the proper points in ROM for the BIOS interrupt handlers. 2. When the Enhanced PC/5250 Emulation program is loaded, it saves the contents of interrupt x'OD' (hardware IL5) and interrupt x'13' (software fixed disk I/O interface). It writes over the vectors in low storage so they now point to handler code within the 5250 emulation program. 3. The 5250 emulation program commands its hardware to use IL5 for twinax interrupts. Both the 5250 adapter and the fixed-disk adapter have tri-state drivers on the IL5 req...