Motorola EXORdisk II User Manual page 262

SYSTEM DESCRIPTION
24.4 -- System Files
24.4.1 System overlays
The system overlay files are loaded into memory into one
of the Tour overlay regions discussed in the subsequent
section.
The
overlay handler only brings an overlay into
memory if it is not already in memory at the time a specific
function is required.
If
an
ove~lay
remains in memory;
access to its function is Taster than if it has
be
to loaded
from the
disk~tte.
The
functions contained in the seven
overlay files are shown in the following table:
Overlay Function
MDosavo.SY
Diskette allocation
and
deallocation.
MDOSOV1.SY
P~ocessing
standard rile names,
allocating contiguous memory,
reserving a device, releaSing
a
device, writing standard records,
writing FDRs, writing end-or-file
records. console reader/punch device
hand
1
ing.
MDOSOV2. SY Reading standard records,
FDRs. ,
i'eading
MDOSOV3. SY Closing a file/device, rewinding
diskette filesl changing file names
and attributes.
MDOSOV4.SY Opening a
~ile/device.
MDOSOV5. SY CHAIN file execution.
MDOSOV6.SY Command line interpretation.
When MDOS is initialized} the directorq is searched ror
the seven overlays
by
name. The physical diskette addresses
are then retained so that a subsequent rererence to an
overlay runction does not involve another directory search.
Thusl
MDOS
must be reinitialized each time the diskette in
drive zero is changed so that the overlays can be located
again.
Overlays MDOSOVO and MOOSaVi use overlay region one.
Overlays MDOSOV2 and MDOSOV3 use overlay region two.
Overla~s
MDOSOV4 and MDOSOV5 use overlay region three
I
'and
overlay MDOSOV6 uses the User Program Area into which the
MDOS commands also are loaded. The overlay regions are shown
in
the
memory map diagram of section
24.
5.
_~
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