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Dynamic Microinstruction Word Width Expansion

IP.com Disclosure Number: IPCOM000051854D
Original Publication Date: 1981-Mar-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 2 page(s) / 29K

Publishing Venue

IBM

Related People

Jones, DM: AUTHOR

Abstract

The figure shows a memory arrangement for storing variable length micro instructions for use by a microprocessor. Such arrangements are advantageously employed where the microinstructions include a small number of microinstruction types which require additional bit positions to implement additional functions.

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Dynamic Microinstruction Word Width Expansion

The figure shows a memory arrangement for storing variable length micro instructions for use by a microprocessor. Such arrangements are advantageously employed where the microinstructions include a small number of microinstruction types which require additional bit positions to implement additional functions.

The microcode memory 10 is addressed on the 12-bit microprogram address bus 11 and provides an 8-bit output signal to instruction register 12 which is 16 bits in length. The expander memory 15 is addressed by four bits which are part of the 8-bit output from memory 10 when expanded word width is required. The 4-bit address is applied to expander memory address register (E-MAR) 16 to address one location in memory 15 which reads out eight bits to the instruction register 12.

The op code of the microinstruction which may be, for example, the first four bits of the 8-bit word read out from memory 10, determines if the output of memory 15 is to be employed. If so, the four bits from memory 10 corresponding to the 4-bit address sent out to the E-MAR are ignored in decoding that instruction.

Memory 15 is initialized by an instruction which requires two serial bytes from memory 10. The first byte contains the initialization op code and the 4-bit address where the following byte will be stored. After these four bits are latched in the E-MAR, the second byte is fetched and loaded into memory 15 via its 8-bit input data bu...