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Padding a Microcode Control Word

IP.com Disclosure Number: IPCOM000040178D
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 2 page(s) / 45K

Publishing Venue

IBM

Related People

Frye, HE: AUTHOR [+3]

Abstract

Described is a mechanism whereby a Horizontal Microcode control word can specify the state of high-order bits to be used in halfword or fullword ALU (Arithmetic Logic Unit) operations when only an 8-bit immediate field is used. An IA (Immediate Arithmetic) HMC (Horizontal Microcode) control word has 8 bits of immediate information which is used to fully specify the low 8 bits of data in either byte, halfword or fullword mode. It (Image Omitted) also uses one additional pad. This bit specifies the value to be used for the high byte when in halfword mode and the high three bytes when in fullword mode. When this bit is zero, the high byte or high three bytes of ALU input are forced to zero. In like manner, when this bit is one, all of the bits of the high byte(s) of ALU input are forced one.

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Padding a Microcode Control Word

Described is a mechanism whereby a Horizontal Microcode control word can specify the state of high-order bits to be used in halfword or fullword ALU (Arithmetic Logic Unit) operations when only an 8-bit immediate field is used. An IA (Immediate Arithmetic) HMC (Horizontal Microcode) control word has 8 bits of immediate information which is used to fully specify the low 8 bits of data in either byte, halfword or fullword mode. It

(Image Omitted)

also uses one additional pad. This bit specifies the value to be used for the high byte when in halfword mode and the high three bytes when in fullword mode. When this bit is zero, the high byte or high three bytes of ALU input are forced to zero. In like manner, when this bit is one, all of the bits of the high byte(s) of ALU input are forced one. By using an AND function in the ALU and specifying that the high bits of data should be all ones, this allows resetting any combination of the low 8 bits of a halfword or fullword. See the halfword example in Fig. 1. By using an OR function in the ALU and specifying that the high bits of data should be all zeros, this allows setting any combination of the low 8 bits of a halfword or fullword. See the halfword example in Fig. 2. This is somewhat more flexible than other methods of data compression that have been used which involve sign extending an 8-bit byte to form a 16-bit halfword or 32-bit fullword. In particular, sign extension would not all...