Browse Prior Art Database

Early Detection of I/O Instructions

IP.com Disclosure Number: IPCOM000105129D
Original Publication Date: 1993-Jun-01
Included in the Prior Art Database: 2005-Mar-19
Document File: 2 page(s) / 56K

Publishing Venue

IBM

Related People

Ray, DS: AUTHOR

Abstract

An implementation for early detection of I/O instructions is disclosed. A copy of the 'I/O tag bit' is kept in a fast latch for lookup early in the cycle.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 54% of the total text.

Early Detection of I/O Instructions

       An implementation for early detection of I/O instructions
is disclosed.  A copy of the 'I/O tag bit' is kept in a fast latch
for lookup early in the cycle.

      In some architecture's, such as the Power Architecture, there
are not specific load and store instructions which operate in I/O
address space.  Instead, during execution, a lookup in the segment
registers is done to see if a t-bit is active in that particular
segment register.  If the t-bit is a one, then the effective address
is intended for I/O space.  If the t-bit is a zero, then the
effective address is intended for memory address space.  In hardware
implementations, large data structures like the segment registers,

(there are 16 segment registers which are 32 bits wide), are kept in
register files.  These register files conserve silicon area at the
cost of having slower access times as opposed to registers or
latches.  Because of this, detection of t-bits, (thus, I/O
instructions), are not known until too late in the cycle to do any
processing.  Thus, control logic becomes complex because a certain
amount of 'cleanup and backout' has to be done in the following cycle
for proper execution of an I/O instruction.

      Fig. 1 shows a typical implementation of a t-bit lookup in the
segment registers.  When a load or store instruction is in execute,
an effective address is generated out of the adder.  The top 4 bits
of the adder are used to address...