Browse Prior Art Database

Artificial Rename Assignment for Serialized Floating-Point Unit Instruction

IP.com Disclosure Number: IPCOM000118287D
Original Publication Date: 1996-Dec-01
Included in the Prior Art Database: 2005-Apr-01
Document File: 2 page(s) / 45K

Publishing Venue

IBM

Related People

Burgess, B: AUTHOR [+4]

Abstract

On some microprocessor designs, the MTFSF instruction is dispatch serialized. (Instructions that move data from a Floating Point Register (FPR) to a floating point control register is hereby identified as MTFSF). This type of serialization ensures that the completion buffer is empty and that all rename buffers are available before dispatch occurs. Consequently, on those designs, even though a specific tag assigning a target rename is not generated due to the serialization, rename buffer #0 is available for use because there are no outstanding dispatched instructions. This rename buffer is used by the Floating-Point Unit (FPU) to normalize a single-precision denormalized number, prior to writing the Floating Point Status and Control Register (FPSCR).

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Artificial Rename Assignment for Serialized Floating-Point Unit Instruction

      On some microprocessor designs, the MTFSF instruction is
dispatch serialized.  (Instructions that move data from a Floating
Point Register (FPR) to a floating point control register is hereby
identified as MTFSF).  This type of serialization ensures that the
completion buffer is empty and that all rename buffers are available
before dispatch occurs.  Consequently, on those designs, even though
a specific tag assigning a target rename is not generated due to the
serialization, rename buffer #0 is available for use because there
are no outstanding dispatched instructions.  This rename buffer is
used by the Floating-Point Unit (FPU) to normalize a single-precision
denormalized number, prior to writing the Floating Point Status and
Control Register (FPSCR).

      By removing the dispatch serialization of the MTFSF instruction
and instead making it execution serialized, the "implicit" rename
buffer #0 may no longer be available for the normalization operation
during its execution.  To resolve this issue, a rename buffer is
assigned to the FPU at the time of dispatch; but to avoid corruption
of the rename allocation logic and to ensure that a FPR is not
inadvertently updated, a "false" FPR update signal is generated by
the Completion Unit when the MTFSF instruction completes.  This
"false" FPR update de-allocates the rename bus used by the MTFSF
instruction, thereby freeing it up for...