<s>
It	O
used	O
a	O
tagged	O
storage	O
scheme	O
to	O
implement	O
a	O
capability	B-General_Concept
architecture	O
,	O
and	O
was	O
designed	O
for	O
the	O
safe	O
and	O
efficient	O
implementation	O
of	O
strongly	O
typed	O
procedures	O
.	O
</s>
<s>
The	O
hardware	O
was	O
custom	O
and	O
microprogrammable	O
,	O
with	O
an	O
operating	O
system	O
,	O
(	O
modular	O
)	O
compiler	O
,	O
editor	O
,	O
garbage	B-General_Concept
collector	I-General_Concept
and	O
filing	O
system	O
all	O
written	O
in	O
ALGOL	B-General_Concept
68RS	I-General_Concept
.	O
</s>
<s>
There	O
were	O
(	O
at	O
least	O
)	O
two	O
incarnations	O
of	O
Flex	O
,	O
implemented	O
using	O
hardware	O
with	O
writable	O
microcode	B-Device
.	O
</s>
<s>
The	O
first	O
was	O
supplied	O
by	O
Logica	O
to	O
an	O
RSRE	O
design	O
,	O
and	O
the	O
second	O
used	O
an	O
ICL	B-Device
PERQ	I-Device
.	O
</s>
<s>
The	O
microcode	B-Device
alone	O
was	O
responsible	O
for	O
storage	O
allocation	O
,	O
deallocation	O
and	O
garbage	B-General_Concept
collection	I-General_Concept
.	O
</s>
<s>
The	O
whole	O
scheme	O
allowed	O
abstract	O
data	O
types	O
to	O
be	O
safely	O
implemented	O
,	O
as	O
data	O
items	O
and	O
the	O
procedures	O
permitted	O
to	O
access	O
them	O
could	O
be	O
bound	O
together	O
,	O
and	O
the	O
resulting	O
capability	B-General_Concept
passed	O
freely	O
around	O
.	O
</s>
<s>
The	O
capability	B-General_Concept
would	O
grant	O
access	O
to	O
the	O
procedures	O
,	O
but	O
could	O
not	O
be	O
used	O
in	O
any	O
way	O
to	O
obtain	O
access	O
to	O
the	O
data	O
.	O
</s>
<s>
Another	O
notable	O
feature	O
of	O
Flex	O
was	O
the	O
notion	O
of	O
shaky	O
pointers	O
,	O
more	O
recently	O
often	O
called	O
weak	B-General_Concept
references	I-General_Concept
,	O
which	O
points	O
to	O
blocks	O
of	O
memory	O
that	O
could	O
be	O
freed	O
at	O
the	O
next	O
garbage	B-General_Concept
collection	I-General_Concept
.	O
</s>
<s>
It	O
made	O
feasible	O
the	O
use	O
of	O
remote	O
file-stores	O
,	O
remote	O
capabilities	O
,	O
and	O
remote	B-Operating_System
procedure	I-Operating_System
calls	I-Operating_System
.	O
</s>
