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Modification of Programs in Read Only Storage

IP.com Disclosure Number: IPCOM000086342D
Original Publication Date: 1976-Aug-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Quinn, RL: AUTHOR

Abstract

(READ-ONLY STORAGE) p1: Initialization routine Write E1 at address A1 P2: Patch entry Read patch tape; load patch code at address E3 Change E1 to E2 at address A1 Write E3 at address A2 E1: Main program Instructions, data, etc. Branch to location specified by contents of address A1 E2: Branch to location specified by contents of address A2 (READ-WRITE STORAGE) A1: Address constant E1 E2 A2: Address constant E3 E3: Patch routine Instructions, data, etc. Branch to location E1 (or other ROS address)

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Modification of Programs in Read Only Storage

(READ-ONLY STORAGE) p1: Initialization routine Write E1 at address A1 P2: Patch entry Read patch tape; load patch code at address E3 Change E1 to E2 at address A1 Write E3 at address A2 E1: Main program Instructions, data, etc. Branch to location specified by contents of address A1 E2: Branch to location specified by contents of address A2 (READ-WRITE STORAGE) A1: Address constant E1 E2 A2: Address constant E3 E3: Patch routine Instructions, data, etc. Branch to location E1 (or other ROS address)

It is often desirable to implement frequently-used or systems programs in read-only storage (ROS), so that they need not be reloaded into a volatile read- write storage (RWS) every time the power is turned off. On the other hand, storing programs in ROS renders it difficult to modify or correct the program.

The above simplified coding demonstrates how portions of ROS can be effectively bypassed and replaced by patch code located in RWS. An initialization routine starting at address P1 in ROS operates on power-up to write an address constant (adcon) E1 at address A1 in RWS. Another routine, at address P2, permits patch code to be read from a tape transport or other I/O device into RWS, beginning at address E3. This routine then changes the adcon at RWS address A1 from E1 to E2, and writes the value of adcon E3 at RWS address A2.

The main program begins at ROS address E1. Its last instruction is an indirect branch, i.e., a bran...