IBM 1130 User Manual page 496

Overlay 3. This is the FORTRAN disk I/O package,
which may contain:
SDFIO (620 words), the disk I/O package
SDFND (80 words), the disk FIND package
SUFIO (730 words), the disk unformatted
I/O package
All three subroutines are subtype 1. The size of
this package, therefore, ranges from 0 (no disk
I/O) to 1430 words.
Note that SDFND is not included unless your
FORTRAN program contains a FIND statement.
SDFIO is included if the *IOCS (DISK) card is pre-
sent; SUFIO if the *IOCS (UDISK) card is present.
The typical program will require SDFIO and
SDFND, for an overlay size of 700 words.
Section Subsections
Page
65
10
I
30 03
The SOCAL Overlay Scheme
Just before you execute a program or store one in
core image format (DCI) , the Core Load Builder
(CLB) is given the task of building a complete core
load, or program package, which will fit into core
storage.
CLB assembles your program and all its required
subroutines, and determines how much core storage
they will require. In so doing, it considers the
subroutines that are to be LOCAL. The CLB then
tries to inc lude the last remaining elements, the three
SOCAL overlays, in four steps:
1. As a first step, CLB attempts to fit all three
overlays in core with no sharing. Using the typical
overlay sizes, this will require 520 +1750 +700 or
2970 words of core.
2. A second step is taken if there is not enough
room to hold all three packages at the same time.
This involves the sharing of core storage by overlay
1 (arithmetic) and overlay 2 (non-disk I/O). The area
they share must be large enough for the larger of the
two overlays, in this case (and almost always) the
non-disk I/O subroutines, overlay 2. The size of
the SOCAL area will now be 1750 +700 or 2450 words,
a reduction of 520 words, the size of overlay 1.
As required by the user's program, Flipper will
read each overlay from the disk whenever it is needed,
placing it on top of the last overlay. Overlay 3, the
disk I/O, will remain in core at all times. Because
Flipper is now needed, your net gain is 520-100 or
420 words.
3. The third step is taken if there is still not
enough room in core storage.
It
involves the shar-
ing of core storage by all three packages, in an area
the size of the largest of the 3 overlays. As before,
this will probably be the non-disk I/O overlay, at
1750 words.
4. If step 3 fails to provide enough room in core,
step 4 will so advise you with a message.
Summarizing the C LB makes a step-by-step
attempt to fit your program and its subprograms into
the available core storage space.
Step 1 involves the most core storage -- typically
about 2970 words.
Step 2 requires about 520-100 or 420 words less
than step 1.
Step 3 requires about 700 words less than step 2.
Figure 65.1 shows the three steps, or overlay levels,
in graphic form. Note that the discussion of this typ-
ical program did not include the program itself. Only
the subprograms have been considered.