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2 3 ' 4 5 6
]'8'9'10'11'12'13'14'15
AlclRI
L/4
Reserved
Reserved
I
B
Address
A
=
Absence Bit
=
1 if segment is absent from main memory.
C
Clean bit. Used to eliminate unnecessary output swapping.
R
Reference Bit
=
1 if segment has been referenced (set by microcode).
L
Length Field
=
segment length divided by 4.
B
Bank Address. Points to memory bank (if resident in main memory) in which segment resides.
ADDR ESS
=
Absolute Segment Address within given bank in third word of segment. If segment is
absent, words 3 and 4 contain absolute disc address.
90020·6
Figure 2-7.
Data Segment Table Entry Format
mente
Each entry consists
of a four-word descriptor
which in-
cludes
the
absolute
address
of
the
related
segment and its
length.
Entry number 0 in the table is unique in that it simply
points
to the
final
entry in the table (2).
This defines the
length of
the table for the benefit of the
operating system
in
allocating space for the table itself.
Segment number 0 does not
exist.
Assume that one
user is executing a
process
which re-
quires code segments 22 thr'ough 25.
Also assume that segments 22
and 23
are in
Main Memory and that segments 24
and
25 are not
presently
needed and,
therefore,
are on disc.
The process is
presently executing instructions in code segment 23.
This means
that
the address value conta ined in the fourth word of
cs'r
~entry
23 has been
loaded into
the
PB Register.
Therefore,
the
PB
Register is pointing to PB (a)
as shown in (3),
figure 2-8.
The
PL Register, using a
value derived from the
segment length,
is
pointing to PL(a).
The P Register is advancing from PB(a) toward
PL(a) •
The last nine
locations of segment 23 are not part
of the
seg-
ment's code,
but are added by the operating system when the seg-
ment is
loaded into
virtual
memory.
These locations
contain
linking references for every procedure call (PCAL) in the Segment
Transfer Table (STT).
A PCAL is an instruction that references a
set of instructions
elsewhere in the code segment.
This set of
instructions is structured as a procedure to perform a
standard-
ized operation
or computation
and then
return
control to
the
instruction
immediately
following
the
call
instruction.
It
should be noted that entr ies in the STT are numbe red fr om the end
back towards the code.
Entry number 0 gives the
STT length
as
shown in figure 2-6.
This indicates the
number of the last STT
entry (4),
figure 2-8,
so that the hardware can make
validity
2-19
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