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Save Restore ALU Condition Register

IP.com Disclosure Number: IPCOM000088126D
Original Publication Date: 1977-Apr-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 2 page(s) / 68K

Publishing Venue

IBM

Related People

Booth, RC: AUTHOR [+3]

Abstract

The contents of a program condition register (PCR) having no direct external controls are saved in a hardware register upon occurrence of an interrupt in a microprocessor and restored prior to return to the main program level by using microinstructions to recreate the conditions in the PCR.

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Save Restore ALU Condition Register

The contents of a program condition register (PCR) having no direct external controls are saved in a hardware register upon occurrence of an interrupt in a microprocessor and restored prior to return to the main program level by using microinstructions to recreate the conditions in the PCR.

PCR 30 (Fig. 1) has three bit positions for representing carry, zero and nonzero conditions which may occur as a result of an arithmetic or logic operation performed by arithmetic and logic unit (ALU) 20. ALU 20 performs arithmetic and logic operations on operands contained in registers 10 and 15. The contents of PCR 30 can only be changed as a result of an ALU operation. Bits cannot be directly set into PCR 30. Therefore, when an interrupt occurs, the contents of PCR 30 are saved in PCR SAVE register 40, and prior to return to the main program level, they are restored to PCR 30.

The contents of PCR SAVE register 40 are sensed via AND circuits 41 under control of an I/O SENSE microinstruction and transferred into registers 10 and
15. A series of microinstructions are then executed to control ALU 20, whereby the status of the carry, zero and nonzero bits is recreated in PCR 30. Thus, PCR 30 is restored before the main program level execution continues. The sequence is illustrated in Fig. 2.

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