<s>
Explicitly	B-General_Concept
parallel	I-General_Concept
instruction	I-General_Concept
computing	I-General_Concept
(	O
EPIC	O
)	O
is	O
a	O
term	O
coined	O
in	O
1997	O
by	O
the	O
HP	B-General_Concept
–	I-General_Concept
Intel	I-General_Concept
alliance	I-General_Concept
to	O
describe	O
a	O
computing	O
paradigm	O
that	O
researchers	O
had	O
been	O
investigating	O
since	O
the	O
early	O
1980s	O
.	O
</s>
<s>
It	O
was	O
the	O
basis	O
for	O
Intel	O
and	O
HP	O
development	O
of	O
the	O
Intel	B-General_Concept
Itanium	I-General_Concept
architecture	O
,	O
and	O
HP	O
later	O
asserted	O
that	O
"	O
EPIC	O
"	O
was	O
merely	O
an	O
old	O
term	O
for	O
the	O
Itanium	B-General_Concept
architecture	O
.	O
</s>
<s>
EPIC	O
permits	O
microprocessors	O
to	O
execute	O
software	O
instructions	O
in	O
parallel	O
by	O
using	O
the	O
compiler	B-Language
,	O
rather	O
than	O
complex	O
on-die	O
circuitry	O
,	O
to	O
control	O
parallel	O
instruction	O
execution	O
.	O
</s>
<s>
By	O
1989	O
,	O
researchers	O
at	O
HP	O
recognized	O
that	O
reduced	B-Architecture
instruction	I-Architecture
set	I-Architecture
computer	I-Architecture
(	O
RISC	B-Architecture
)	O
architectures	O
were	O
reaching	O
a	O
limit	O
at	O
one	O
instruction	O
per	O
cycle	O
.	O
</s>
<s>
The	O
basis	O
for	O
the	O
research	O
was	O
VLIW	B-General_Concept
,	O
in	O
which	O
multiple	O
operations	O
are	O
encoded	O
in	O
every	O
instruction	O
,	O
and	O
then	O
processed	O
by	O
multiple	O
execution	B-General_Concept
units	I-General_Concept
.	O
</s>
<s>
One	O
goal	O
of	O
EPIC	O
was	O
to	O
move	O
the	O
complexity	O
of	O
instruction	O
scheduling	O
from	O
the	O
CPU	O
hardware	O
to	O
the	O
software	O
compiler	B-Language
,	O
which	O
can	O
do	O
the	O
instruction	O
scheduling	O
statically	O
(	O
with	O
help	O
of	O
trace	O
feedback	O
information	O
)	O
.	O
</s>
<s>
An	O
equally	O
important	O
goal	O
was	O
to	O
further	O
exploit	O
instruction	B-Operating_System
level	I-Operating_System
parallelism	I-Operating_System
(	O
ILP	O
)	O
by	O
using	O
the	O
compiler	B-Language
to	O
find	O
and	O
exploit	O
additional	O
opportunities	O
for	O
parallel	B-Operating_System
execution	I-Operating_System
.	O
</s>
<s>
VLIW	B-General_Concept
(	O
at	O
least	O
the	O
original	O
forms	O
)	O
has	O
several	O
short-comings	O
that	O
precluded	O
it	O
from	O
becoming	O
mainstream	O
:	O
</s>
<s>
VLIW	B-General_Concept
instruction	B-General_Concept
sets	I-General_Concept
are	O
not	O
backward	B-General_Concept
compatible	I-General_Concept
between	O
implementations	O
.	O
</s>
<s>
When	O
wider	B-Operating_System
implementations	O
(	O
more	O
execution	B-General_Concept
units	I-General_Concept
)	O
are	O
built	O
,	O
the	O
instruction	B-General_Concept
set	I-General_Concept
for	O
the	O
wider	B-Operating_System
machines	O
is	O
not	O
backward	B-General_Concept
compatible	I-General_Concept
with	O
older	O
,	O
narrower	O
implementations	O
.	O
</s>
<s>
Load	O
responses	O
from	O
a	O
memory	B-General_Concept
hierarchy	I-General_Concept
which	O
includes	O
CPU	B-General_Concept
caches	I-General_Concept
and	O
DRAM	O
do	O
not	O
have	O
a	O
deterministic	O
delay	O
.	O
</s>
<s>
This	O
makes	O
static	O
scheduling	O
of	O
load	O
instructions	O
by	O
the	O
compiler	B-Language
very	O
difficult	O
.	O
</s>
<s>
EPIC	B-General_Concept
architecture	I-General_Concept
evolved	O
from	O
VLIW	B-General_Concept
architecture	O
,	O
but	O
retained	O
many	O
concepts	O
of	O
the	O
superscalar	B-General_Concept
architecture	I-General_Concept
.	O
</s>
<s>
VLIW	B-General_Concept
:	O
</s>
<s>
The	O
dependency	O
information	O
is	O
calculated	O
by	O
the	O
compiler	B-Language
,	O
so	O
the	O
hardware	O
does	O
not	O
have	O
to	O
perform	O
operand	O
dependency	O
checking	O
.	O
</s>
<s>
The	O
EPIC	B-General_Concept
architecture	I-General_Concept
also	O
includes	O
a	O
grab-bag	O
of	O
architectural	O
concepts	O
to	O
increase	O
ILP	O
:	O
</s>
<s>
Predicated	B-General_Concept
execution	I-General_Concept
is	O
used	O
to	O
decrease	O
the	O
occurrence	O
of	O
branches	O
and	O
to	O
increase	O
the	O
speculative	B-General_Concept
execution	I-General_Concept
of	O
instructions	O
.	O
</s>
<s>
Delayed	O
exceptions	O
,	O
using	O
a	O
not	O
a	O
thing	O
bit	O
within	O
the	O
general	O
purpose	O
registers	O
,	O
allow	O
speculative	B-General_Concept
execution	I-General_Concept
past	O
possible	O
exceptions	O
.	O
</s>
<s>
Very	O
large	O
architectural	O
register	B-General_Concept
files	I-General_Concept
avoid	O
the	O
need	O
for	O
register	B-Architecture
renaming	I-Architecture
.	O
</s>
<s>
The	O
Itanium	B-General_Concept
architecture	O
also	O
added	O
rotating	O
register	B-General_Concept
files	I-General_Concept
,	O
a	O
tool	O
useful	O
for	O
software	O
pipelining	O
since	O
it	O
avoids	O
having	O
to	O
manually	O
unroll	B-Operating_System
and	O
rename	O
registers	O
.	O
</s>
<s>
There	O
have	O
been	O
other	O
investigations	O
into	O
EPIC	B-General_Concept
architectures	I-General_Concept
that	O
are	O
not	O
directly	O
tied	O
to	O
the	O
development	O
of	O
the	O
Itanium	B-General_Concept
architecture	O
:	O
</s>
<s>
Gelato	O
was	O
an	O
open	O
source	O
development	O
community	O
in	O
which	O
academic	O
and	O
commercial	O
researchers	O
worked	O
to	O
develop	O
more	O
effective	O
compilers	B-Language
for	O
Linux	O
applications	O
running	O
on	O
Itanium	B-General_Concept
servers	O
.	O
</s>
