Tomasulo’s Algorithm
and IBM 360
Srivathsan Soundararajan
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What we have seen till now!!!
Single-cycle datapath.
Multi-cycle datapath.
N-stage Pipelined datapath.
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Tomasulo's algorithm
A hardware algorithm for controlling the
execution of multiple functional units with
varying latencies in a pipelined CPU
micro-architecture.
A general mechanism for register
forwarding and hazard detection.
The key idea is to virtually execute each
instruction in a single cycle.
Out-of-order execution of instructions.
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So what is so special???
Let instructions behind stall proceed.
Decode instructions and check for
structural hazard.
Wait until no data hazard and then read
operands.
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Three Stages
Issue – if reservation station free (i.e. no
structural hazard), control issues instruction and
sends operands (renames registers).
Execution – if both operands ready, then
execute. If not, watch common data bus for
result.
Write result – if CDB available, write on common
data bus to all awaiting units; mark reservation
status available
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Virtual result – a promissory note
All registers are modified so that they can either
hold a true result or a virtual result.
When an instruction is issued, a virtual result is
placed in the instruction's destination register.
A functional unit is assigned to compute the real
result.
The virtual result is replaced by the real result
when the functional unit has completed its
computation.
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Tomasulo Organization
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The concept
Each instruction, as it arrives, fetches its
operands from a special register file.
Each register in this file holds either an actual
value, or a “tag” indicating the reservation
station that will produce the register value when
it completes.
The instruction and its operands (either values
or tags) are stored in a reservation station (RS).
The RS watches the results returning from the
execution pipelines, and when a result's tag
matches one of its operands, it records the value
in place of the tag
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A good example
http://www.cs.umd.edu/users/saltz/cmsc41
1-s97/tomasulo.htm
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Why Tomasulo’s Algorithm
Hazard detection.
Dynamic Scheduling (i.e. Hardware
reorganizes instructions)
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IBM 360
The IBM 360 introduced many new
concepts, including dynamic detection of
memory hazards, generalized forwarding,
and reservation stations.
The approach is normally named
Tomasulo’s algorithm
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Installation of the IBM 360/91 in the Columbia Computer Center
machine room in February or March 1969
Photo: AIS archive.
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IBM 360
Was introduced by the team led by Michael Flynn in
1966.
The internal organization of the 360/91 shares many
features with the Pentium III and Pentium 4, as well as
several other microprocessors.
One major difference was that there was no branch
prediction in the 360/91 and hence no speculation.
Another major difference was that there was no commit
unit, so once the instructions finished execution, they
updated the registers. Out-of-order instruction commit
led to imprecise interrupts, which proved to be unpopular
and led to the commit units in dynamically scheduled
pipelined processors since that time.
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IBM 360
Although the 360/91 was not a success, the key
ideas were resurrected later and exist in some
form in the majority of microprocessors like
Pentium II, Power PC 604 etc...
It ran under Operating System/360 -- a powerful
programming package of approximately 1.5
million instructions that enabled the system to
operate with virtually no manual intervention.
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IBM 360
Within the central processing unit (CPU), there
were five highly autonomous execution units
which allowed the machine to overlap operations
and process many instructions simultaneously.
The five units were processor storage, storage
bus control, instruction processor, fixed-point
processor and floating-point processor. Not only
could these units operate concurrently, they
could also perform several functions at the same
time.
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Some uses
The IBM-360 family of computers ranged
from the model 20 minicomputer (which
typically had 24 KB of memory) to the
model 91 supercomputer which was built
for the North American missile defense
system.
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Quite huge isnt it???
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References
http://www.d.umn.edu/~gshute/cs2521/arch/tomasulo.html
http://euler.ecs.umass.edu/arch/parts/Part6-tomasulo.pdf
http://www.cs.indiana.edu/classes/p415sjoh/readings/smv/CadenceSMVdocs/smv/tutorial/node36.html#tomasulo1
http://www.beagle-ears.com/lars/engineer/comphist/ibm360.htm
http://www.eecs.ucf.edu/~lboloni/Teaching/EEL5708_2006/slides/To
masulo.ppt#268,9,Three Stages of Tomasulo Algorithm
http://www.cs.unc.edu/~montek/teaching/fall-03/lectures/lecture11.ppt#723,4,Tomasulo: Organization
http://www.ece.cmu.edu/~jhoe/distribution/2005/741/proj3.pdf
http://www.columbia.edu/acis/history/36091.html
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