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Conditionally Executable Instructions

IP.com Disclosure Number: IPCOM000075953D
Original Publication Date: 1971-Dec-01
Included in the Prior Art Database: 2005-Feb-24
Document File: 1 page(s) / 11K

Publishing Venue

IBM

Related People

Brown, JR: AUTHOR [+2]

Abstract

In data processors of the "mini-computer" size, it is desirable to limit the number of instructions used in the machine to the minimum number which can perform the required operations. One substantial reduction in the number of instructions required can be made by eliminating a distinction between conditionally executable instructions and the unconditional ones. This is done by providing each instruction of the instruction set with a separate pair of prefix bits to control the execution of the instruction. The first bit is used to indicate if the instruction is to be conditionally or unconditionally executed, and the second bit will indicate the state of a condition latch for which a conditional instruction is to be executed.

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Conditionally Executable Instructions

In data processors of the "mini-computer" size, it is desirable to limit the number of instructions used in the machine to the minimum number which can perform the required operations. One substantial reduction in the number of instructions required can be made by eliminating a distinction between conditionally executable instructions and the unconditional ones. This is done by providing each instruction of the instruction set with a separate pair of prefix bits to control the execution of the instruction. The first bit is used to indicate if the instruction is to be conditionally or unconditionally executed, and the second bit will indicate the state of a condition latch for which a conditional instruction is to be executed. That is: Prefix Operation

00 Execute if condition latch is reset

01 Execute if condition latch is set

10 Execute unconditionally

11 Not used.

Thus, the two prefix bits will provide for all conditional instructions and it is not necessary to have any instruction decoding for conditional instructions. Any instruction for which the condition is not met will be bypassed as a nonoperation and there will be no machine control by the operation code part of the instruction.

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