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Saving Bits in a Control Store Word By Redefining Branch Fields

IP.com Disclosure Number: IPCOM000048440D
Original Publication Date: 1982-Jan-01
Included in the Prior Art Database: 2005-Feb-08
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Cox, FJ: AUTHOR [+3]

Abstract

Bit width of the control store word in a horizontal microprogram-driven CPU can be minimized by alternate definition of the fields controlling branching.

This text was extracted from a PDF file.
This is the abbreviated version, containing approximately 86% of the total text.

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Saving Bits in a Control Store Word By Redefining Branch Fields

Bit width of the control store word in a horizontal microprogram-driven CPU can be minimized by alternate definition of the fields controlling branching.

In a horizontal microprogram-driven machine, a key design objective is keeping the width of the control store word to a reasonable minimum. One approach to meeting this objective is to redefine a number of the control fields such that the code points in these fields can convey different meanings, depending on which definition is currently active.

In past machines, miscellaneous control fields were primary candidates for this alternate definition scheme. In addition to alternately defining miscellaneous control fields, the fields which control microprogram branching can be alternately defined without risking field conflicts.

In past machines, controls such as indication of 'end' have occupied a stand- alone bit in the microword. The 'end of instruction' control can be specified by a code point of one of the microcode branch fields. A decode of 'end' causes another branch field to assume a secondary definition, which can be the setting of various instruction condition codes.

In a system where the OP code of any instruction is never required to be uniquely decoded to provide any control information, a unique group of 'set condition code' micro-orders is required. A stand-alone field could have been defined for this purpose in the control store word. Al...