<s>
ALGOL	B-General_Concept
68-R	I-General_Concept
was	O
the	O
first	O
implementation	O
of	O
the	O
Algorithmic	B-Language
Language	I-Language
ALGOL	B-Language
68	I-Language
.	O
</s>
<s>
In	O
December	O
1968	O
,	O
the	O
report	O
on	O
the	O
Algorithmic	B-Language
Language	I-Language
ALGOL	B-Language
68	I-Language
was	O
published	O
.	O
</s>
<s>
On	O
20	O
–	O
24	O
July	O
1970	O
a	O
working	O
conference	O
was	O
arranged	O
by	O
the	O
International	O
Federation	O
for	O
Information	O
Processing	O
(	O
IFIP	O
)	O
to	O
discuss	O
the	O
problems	O
of	O
implementing	O
the	O
language	O
,	O
a	O
small	O
team	O
from	O
the	O
Royal	O
Radar	O
Establishment	O
(	O
RRE	O
)	O
attended	O
to	O
present	O
their	O
compiler	B-Language
,	O
written	O
by	O
I	O
.	O
F	O
.	O
Currie	O
,	O
Susan	O
G	O
.	O
Bond	O
,	O
</s>
<s>
In	O
the	O
face	O
of	O
estimates	O
of	O
up	O
to	O
100	O
man-years	O
to	O
implement	O
the	O
language	O
,	O
using	O
multi-pass	B-Application
compilers	I-Application
with	O
up	O
to	O
seven	O
passes	O
,	O
they	O
described	O
how	O
they	O
had	O
already	O
implemented	O
a	O
one-pass	B-Language
compiler	I-Language
which	O
was	O
in	O
production	O
for	O
engineering	O
and	O
scientific	O
uses	O
.	O
</s>
<s>
The	O
ALGOL	B-General_Concept
68-R	I-General_Concept
compiler	B-Language
was	O
initially	O
written	O
in	O
a	O
local	O
dialect	O
of	O
ALGOL	B-Language
60	I-Language
with	O
extensions	O
for	O
address	O
manipulation	O
and	O
list	O
processing	O
.	O
</s>
<s>
The	O
parser	O
was	O
written	O
using	O
J	O
.	O
M	O
.	O
Foster	O
's	O
Syntax	O
Improving	O
Device	O
(	O
SID	O
)	O
parser	B-Language
generator	I-Language
.	O
</s>
<s>
The	O
first	O
version	O
of	O
the	O
compiler	B-Language
occupied	O
34	O
K	O
words	O
.	O
</s>
<s>
It	O
was	O
later	O
rewritten	O
in	O
ALGOL	B-General_Concept
68-R	I-General_Concept
,	O
taking	O
around	O
36	O
K	O
words	O
to	O
compile	B-Language
most	O
programs	O
.	O
</s>
<s>
ALGOL	B-General_Concept
68-R	I-General_Concept
was	O
implemented	O
under	O
the	O
George	B-General_Concept
3	I-General_Concept
operating	O
system	O
on	O
an	O
ICL	B-Device
1907F	I-Device
.	O
</s>
<s>
The	O
compiler	B-Language
was	O
distributed	O
at	O
no	O
charge	O
by	O
International	O
Computers	O
Limited	O
(	O
ICL	O
)	O
on	O
behalf	O
of	O
the	O
Royal	O
Radar	O
Establishment	O
(	O
RRE	O
)	O
.	O
</s>
<s>
To	O
allow	O
one	O
pass	O
compiling	B-Language
,	O
ALGOL	B-General_Concept
68-R	I-General_Concept
implemented	O
a	O
subset	O
of	O
the	O
language	O
defined	O
in	O
the	O
original	O
report	O
:	O
</s>
<s>
Many	O
of	O
these	O
restrictions	O
were	O
adopted	O
by	O
the	O
revised	O
report	O
on	O
ALGOL	B-Language
68	I-Language
.	O
</s>
<s>
To	O
allow	O
compiling	B-Language
in	O
one	O
pass	O
ALGOL	B-General_Concept
68-R	I-General_Concept
insisted	O
that	O
all	O
identifiers	O
were	O
specified	O
(	O
declared	O
)	O
before	O
use	O
.	O
</s>
<s>
To	O
allow	O
recursive	O
declarations	O
of	O
modes	O
(	O
types	O
)	O
a	O
special	O
stub	O
mode	O
declaration	O
was	O
used	O
to	O
inform	O
the	O
compiler	B-Language
that	O
an	O
up-coming	O
symbol	O
was	O
a	O
mode	O
rather	O
than	O
an	O
operator	O
:	O
</s>
<s>
MODE	O
A	O
=	O
STRUCT	B-Language
(	O
REF	O
B	O
b	O
)	O
;	O
</s>
<s>
The	O
ALGOL	B-General_Concept
68-R	I-General_Concept
team	O
found	O
this	O
too	O
difficult	O
to	O
handle	O
and	O
made	O
two	O
changes	O
to	O
the	O
language	O
.	O
</s>
<s>
The	O
ALGOL	B-General_Concept
68-R	I-General_Concept
team	O
decided	O
to	O
use	O
an	O
explicit	O
VOID	O
symbol	O
in	O
order	O
to	O
simplify	O
parsing	O
(	O
and	O
increase	O
readability	O
)	O
:	O
</s>
<s>
This	O
modification	O
to	O
the	O
language	O
was	O
adopted	O
by	O
the	O
ALGOL	B-Language
68	I-Language
revised	O
report	O
.	O
</s>
<s>
The	O
ALGOL	B-General_Concept
68-R	I-General_Concept
team	O
redefined	O
formal	O
declarers	O
to	O
be	O
the	O
same	O
as	O
virtual	O
declarers	O
which	O
include	O
no	O
bound	O
information	O
.	O
</s>
<s>
In	O
ALGOL	B-General_Concept
68-R	I-General_Concept
the	O
example	O
above	O
could	O
be	O
recoded	O
like	O
this	O
:	O
(	O
the	O
bounds	O
of	O
a	O
in	O
the	O
procedure	O
would	O
depend	O
on	O
the	O
caller	O
)	O
.	O
</s>
<s>
In	O
the	O
revised	O
report	O
on	O
ALGOL	B-Language
68	I-Language
formal	O
bounds	O
were	O
also	O
removed	O
,	O
but	O
the	O
FLEX	O
indication	O
was	O
moved	O
in	O
position	O
so	O
it	O
could	O
be	O
include	O
in	O
formal	O
declarers	O
:	O
</s>
<s>
In	O
ALGOL	B-Language
68	I-Language
code	O
can	O
be	O
run	O
in	O
parallel	O
by	O
writing	O
PAR	O
followed	O
by	O
a	O
collateral	O
clause	O
,	O
for	O
example	O
in	O
:	O
</s>
<s>
A	O
semaphore	B-Operating_System
type	O
(	O
SEMA	O
)	O
with	O
the	O
traditional	O
P	O
(	O
DOWN	O
)	O
and	O
V	O
(	O
UP	O
)	O
operators	O
is	O
provided	O
for	O
sysynchronizing	O
between	O
the	O
parts	O
of	O
the	O
parallel	O
clause	O
,	O
</s>
<s>
This	O
feature	O
was	O
not	O
implemented	O
in	O
ALGOL	B-General_Concept
68-R	I-General_Concept
.	O
</s>
<s>
An	O
extension	O
named	O
ALGOL	O
68-RT	O
was	O
written	O
which	O
used	O
the	O
subprogramming	O
feature	O
of	O
the	O
ICL	B-Device
1900	I-Device
to	O
provide	O
multithreading	B-Operating_System
facilities	O
to	O
ALGOL	B-General_Concept
68-R	I-General_Concept
programs	O
with	O
semantics	O
similar	O
to	O
modern	O
thread	B-Operating_System
libraries	O
.	O
</s>
<s>
No	O
changes	O
were	O
made	O
to	O
the	O
compiler	B-Language
,	O
only	O
the	O
runtime	B-Library
library	O
and	O
the	O
linker	O
.	O
</s>
<s>
In	O
ALGOL	B-Language
68	I-Language
the	O
GOTO	O
symbol	O
could	O
be	O
omitted	O
from	O
a	O
jump	O
:	O
</s>
<s>
As	O
ALGOL	B-General_Concept
68-R	I-General_Concept
was	O
a	O
one	O
pass	O
compiler	B-Language
this	O
was	O
too	O
difficult	O
,	O
so	O
the	O
GOTO	O
symbol	O
was	O
made	O
obligatory	O
.	O
</s>
<s>
The	O
same	O
restriction	O
was	O
made	O
in	O
the	O
official	O
sublanguage	O
,	O
ALGOL	B-General_Concept
68S	I-General_Concept
.	O
</s>
<s>
In	O
ALGOL	B-Language
68	I-Language
uniting	O
is	O
the	O
coercion	O
that	O
produces	O
a	O
UNION	O
from	O
a	O
constituent	O
mode	O
,	O
for	O
example	O
:	O
</s>
<s>
In	O
standard	O
ALGOL	B-Language
68	I-Language
uniting	O
was	O
possible	O
in	O
firm	O
or	O
strong	O
contexts	O
,	O
so	O
for	O
example	O
could	O
be	O
applied	O
to	O
the	O
operands	O
of	O
formulas	O
:	O
</s>
<s>
The	O
ALGOL	B-General_Concept
68-R	I-General_Concept
implementers	O
found	O
this	O
gave	O
too	O
many	O
ambiguous	O
situations	O
so	O
restricted	O
the	O
uniting	O
coercion	O
to	O
strong	O
contexts	O
.	O
</s>
<s>
The	O
ALGOL	B-General_Concept
68-R	I-General_Concept
compiler	B-Language
initialised	O
unused	O
memory	O
to	O
the	O
value	O
-6815700	O
.	O
</s>
<s>
In	O
ALGOL	B-Language
family	I-Language
languages	O
,	O
it	O
is	O
necessary	O
to	O
distinguish	O
between	O
identifiers	O
and	O
basic	O
symbols	O
of	O
the	O
language	O
.	O
</s>
<s>
In	O
source	O
code	O
programs	O
,	O
some	O
stropping	B-Application
technique	O
had	O
to	O
be	O
used	O
.	O
</s>
<s>
In	O
many	O
ALGOL	B-Language
like	O
languages	O
,	O
before	O
ALGOL68-R	O
,	O
this	O
was	O
accomplished	O
by	O
enclosing	O
basic	O
symbols	O
in	O
single	O
quote	O
characters	O
( 	O
 '	O
begin	O
 '	O
for	O
example	O
)	O
.	O
</s>
<s>
As	O
ALGOL	B-General_Concept
68-R	I-General_Concept
was	O
implemented	O
on	O
a	O
machine	O
with	O
6-bit	O
bytes	B-Application
(	O
and	O
hence	O
a	O
64	O
character	O
set	O
)	O
this	O
was	O
quite	O
complex	O
and	O
,	O
at	O
least	O
initially	O
,	O
programs	O
had	O
to	O
be	O
composed	O
on	O
paper	O
punched	O
tape	O
using	O
a	O
Friden	B-Application
Flexowriter	I-Application
.	O
</s>
<s>
Partly	O
based	O
on	O
the	O
experience	O
of	O
ALGOL	B-General_Concept
68-R	I-General_Concept
,	O
the	O
revised	O
report	O
on	O
ALGOL68	B-Language
specified	O
hardware	O
representations	O
for	O
the	O
language	O
,	O
including	O
UPPER	O
stropping	B-Application
.	O
</s>
<s>
ALGOL	B-General_Concept
68-R	I-General_Concept
included	O
extensions	O
for	O
separate	B-Library
compiling	I-Library
and	O
low-level	O
access	O
to	O
the	O
machine	O
.	O
</s>
<s>
Since	O
ALGOL	B-Language
68	I-Language
is	O
a	O
strongly	O
typed	O
language	O
,	O
the	O
simple	O
library	O
facilities	O
used	O
by	O
other	O
languages	O
on	O
the	O
ICL	B-Device
1900	I-Device
system	O
were	O
insufficient	O
.	O
</s>
<s>
ALGOL	B-General_Concept
68-R	I-General_Concept
was	O
delivered	O
with	O
its	O
own	O
library	O
format	O
and	O
utilities	O
which	O
allowed	O
sharing	O
of	O
modes	O
,	O
functions	O
,	O
variables	O
,	O
and	O
operators	O
between	O
separately	O
compiled	B-Language
segments	O
of	O
code	O
which	O
could	O
be	O
stored	O
in	O
albums	O
.	O
</s>
<s>
MODE	O
GRAPHDATA	O
=	O
STRUCT	B-Language
(	O
...	O
)	O
;	O
</s>
<s>
As	O
a	O
strongly	O
typed	O
high	O
level	O
language	O
,	O
ALGOL	B-Language
68	I-Language
prevents	O
programs	O
from	O
directly	O
accessing	O
the	O
low	O
level	O
hardware	O
.	O
</s>
<s>
Since	O
ALGOL	B-General_Concept
68-R	I-General_Concept
did	O
n't	O
compile	B-Language
to	O
standard	O
ICL	O
semicompiled	O
(	O
link-ready	O
)	O
format	O
,	O
it	O
was	O
necessary	O
to	O
extend	O
the	O
language	O
to	O
provide	O
features	O
in	O
ALGOL	B-General_Concept
68-R	I-General_Concept
to	O
write	O
code	O
that	O
would	O
normally	O
be	O
written	O
in	O
assembly	B-Language
language	I-Language
.	O
</s>
<s>
Machine	O
instructions	O
could	O
be	O
written	O
inline	B-Language
,	O
inside	O
CODE	O
...	O
EDOC	O
sections	O
and	O
the	O
address	O
manipulation	O
operators	O
INC	O
,	O
DEC	O
,	O
DIF	O
,	O
AS	O
were	O
added	O
.	O
</s>
<s>
An	O
example	O
,	O
using	O
a	O
George	B-General_Concept
peri	O
operation	O
to	O
issue	O
a	O
command	O
:	O
</s>
<s>
A	O
copy	O
of	O
the	O
ALGOL	B-General_Concept
68-R	I-General_Concept
compiler	B-Language
,	O
runnable	O
under	O
the	O
George	B-General_Concept
3	I-General_Concept
operating	O
system	O
emulator	O
,	O
by	O
David	O
Holdsworth	O
(	O
University	O
of	O
Leeds	O
)	O
,	O
is	O
available	O
,	O
with	O
source	O
code	O
,	O
under	O
a	O
GNU	B-License
General	I-License
Public	I-License
License	I-License
(	O
GPL	B-License
)	O
.	O
</s>
