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Dynamic Libraries
Author: David Dyer-Bennet
MRO1-2/L14
DTN 231-4076
DYER-BENNET AT KL2137, SPAGS::DDB
Edition: 1.2, 9-Jun-83
File: DYNLIB.MEM
This document exists online as
KL2137::LANG:<DYER-BENNET.ARCHITECTURE>DYNLIB.MEM
Dynamic Libraries Page ii
Issue History
Issue History
Issue 0.1 10-May-83 Initial entry
Issue 1.0 12-May-83 Add Galactic variables
Issue 1.1 2-Jun-83 Add JSYS interface discussion
Issue 1.2 9-Jun-83 Add version matching
COPYRIGHT (C) DIGITAL EQUIPMENT CORPORATION 1983, 1986.
ALL RIGHTS RESERVED.
THIS SOFTWARE IS FURNISHED UNDER A LICENSE AND MAY BE USED AND
COPIED ONLY IN ACCORDANCE WITH THE TERMS OF SUCH LICENSE AND WITH
THE INCLUSION OF THE ABOVE COPYRIGHT NOTICE. THIS SOFTWARE OR
ANY OTHER COPIES THEREOF MAY NOT BE PROVIDED OR OTHERWISE MADE
AVAILABLE TO ANY OTHER PERSON. NO TITLE TO AND OWNERSHIP OF THE
SOFTWARE IS HEREBY TRANSFERRED.
THE INFORMATION IN THIS SOFTWARE IS SUBJECT TO CHANGE WITHOUT
NOTICE AND SHOULD NOT BE CONSTRUED AS A COMMITMENT BY DIGITAL
EQUIPMENT CORPORATION.
DIGITAL ASSUMES NO RESPONSIBILITY FOR THE USE OR RELIABILITY OF
ITS SOFTWARE ON EQUIPMENT THAT IS NOT SUPPLIED BY DIGITAL.
Dynamic Libraries Page iii
Table of Contents
1.0 INTRODUCTION . . . . . . . . . . . . . . . . . . . . 1
1.1 Purpose Of This Document . . . . . . . . . . . . . 1
1.2 History And Related Documents . . . . . . . . . . 1
1.3 Extensions To Original DYNLIB Proposal . . . . . . 1
1.3.1 Digital Transfer Vector . . . . . . . . . . . . . 1
1.3.2 Service Class . . . . . . . . . . . . . . . . . . 2
1.3.3 Busy Bit . . . . . . . . . . . . . . . . . . . . . 2
1.3.4 Master Init . . . . . . . . . . . . . . . . . . . 2
1.3.5 Save Command . . . . . . . . . . . . . . . . . . . 3
1.3.6 Galactic Variables . . . . . . . . . . . . . . . . 3
1.3.7 Library Version Matching . . . . . . . . . . . . . 3
1.3.8 Dynamic Library Mechanism Version Matching . . . . 4
1.4 Benefits Of Dynamic Libraries . . . . . . . . . . 4
1.5 Conventions Used . . . . . . . . . . . . . . . . . 4
1.5.1 Indirect Words . . . . . . . . . . . . . . . . . . 4
2.0 DYNAMIC LIBRARY FUNCTIONS . . . . . . . . . . . . . 5
2.1 Calling A Routine In A Dynamic Library . . . . . . 5
2.2 Referring To A Galactic Variable . . . . . . . . . 5
2.3 Global Master Init . . . . . . . . . . . . . . . . 6
2.4 Overloading . . . . . . . . . . . . . . . . . . . 6
2.4.1 Load MTVEC Into LTVEC . . . . . . . . . . . . . . 7
2.4.2 Load Library Into LDLBLK . . . . . . . . . . . . . 7
2.5 De-linking . . . . . . . . . . . . . . . . . . . . 7
2.6 Finding A Free Provider . . . . . . . . . . . . . 7
2.7 Library Version Checking . . . . . . . . . . . . . 7
2.8 Dynamic Library Mechanism Version Checking . . . . 8
3.0 IMPLEMENTATION . . . . . . . . . . . . . . . . . . . 9
3.1 Monitor-mode Implementation . . . . . . . . . . . 9
3.1.1 In The User Program . . . . . . . . . . . . . . . 9
3.1.2 In The Library . . . . . . . . . . . . . . . . . 10
3.1.3 JSYS Interface . . . . . . . . . . . . . . . . . 11
3.2 User-mode Implementation . . . . . . . . . . . . 11
4.0 SECTION ALLOCATION . . . . . . . . . . . . . . . . 13
5.0 DIGITAL ENTRY POINTS . . . . . . . . . . . . . . . 13
5.1 Master Init . . . . . . . . . . . . . . . . . . 13
APPENDIX A BIBLIOGRAPHY
Dynamic Libraries Page 1
INTRODUCTION
1.0 INTRODUCTION
1.1 Purpose Of This Document
This document is a proposal for a dynamically linked library system.
This proposal is being made because the original proposal [1] as
implemented in [2] does not provide all of the facilities needed to use
dynamic libraries in the layered product environment proposed in [4].
Where possible, the additional capabilities are made as extensions to
[1].
1.2 History And Related Documents
In March of 1981, Dan Murphy wrote a proposal called "Dynamically Linked
Libraries in TOPS-20" [1].
From late 1981 through August 1982, Kevin Wallace wrote "Canonical
Terminal Support in the TOPS-20 Operating System" [2]. He used an
implementation of the dynamic library concept described in [1]. [5],
[6], and [7] are further descriptions of this project.
In January of 1983, Mike Uhler wrote "Extended Addressing" [3], which is
the most current description of the way extended addressing works on KL
processors (and is proposed to work on KC processors).
In May of 1983, Peter Mierswa sent out the latest revision of "Layered
Architecture" [4]. This document is a proposal for how to use the tools
available to us, and some new ones, to make products that are truly
layered.
1.3 Extensions To Original DYNLIB Proposal
1.3.1 Digital Transfer Vector
For proper restartability of programs using dynamic libraries, it was
necessary to add a concept of "master initialization" of a library. The
dynamic library mechanism needs to know how to master-initialize a
library.
To allow for the possibility of discovering further such required or
optional functions that the library mechanism should know about, we
created the concept of "DIGITAL-specified" entry points. To make it
possible to add to these without disturbing existing libraries, they
were put into a separate transfer vector.
Dynamic Libraries Page 2
INTRODUCTION
1.3.2 Service Class
This is related to the "known library" concept described but not
implemented for the DYNLIB% jsys (existing unofficial implementation).
Certain types of dynamic libraries (particularly those that perform some
sort of process-wide resource allocation function) must exist in only
one copy in a process. On the other hand, some libraries provide only a
single "stream" of operations but maintain an internal state (for
example, record sort, in which you call sort once for each record to be
passed in, once to terminate the list of records, and then once for each
record in sorted order). If a stream is in progress when a new caller
attempts to start a stream, another copy of the library should be loaded
to satisfy the new demand.
Thus, when an unmodified local transfer vector is used, the dynamic
library mechanism must first determine if there are any copies of the
library currently in memory able to provide the service requested. We
feel that this should be done not by checking precise file identity, but
rather by having a library declare what "class" of service it performs.
This is represented by a string, and is encoded into a PDV name so that
the library mechanism can easily determine the list of providers
present.
1.3.3 Busy Bit
Some libraries have no internal state. Some provide a single stream of
operations. Some provide multiple streams. To allow the mechanism to
know if it may route a stream to a provider, a flag has been defined in
the dynamic library descriptor block to say if this copy of the library
can accept another stream. This bit should be manipulated by the
library to correctly reflect its state.
1.3.4 Master Init
To make programs restartable (important in a multi-forking environment)
and keep the cost of the restart within reasonable bounds, each library
routine must be provided with a master-init routine which restores the
library to a virgin state. This routine will be called once when the
library is first mapped in, and once thereafter each time the program is
restarted.
Other conventions, such as having the init routine of each library call
the init routine of every library it may call, are too inflexible and
expensive.
Dynamic Libraries Page 3
INTRODUCTION
1.3.5 Save Command
The existing DYNLIB% implementation includes automatic unlinking of
dynamic libraries on execution of a save command. This is specifically
undesirable for two reasons:
1. The command sequence "^C", "SAVE", "Continue" should work. If
libraries get unlinked on the save command, it won't.
2. Many sites instruct their users to save their core image when a
program exhibits "unusual" behavior (when a bug is suspected),
and to deliver the .exe file produced along with their bug
report to help the local support people see what happenned. If
the save command de-links dynamic libraries, then this
procedure becomes much less useful.
We cannot see any reason why this troublesome function is desirable or
even useful, so we request that it be removed.
1.3.6 Galactic Variables
"Galactic" variables are variables with a wider scope than "global"
variables -- in particular, variables whose scope exceeds a single .exe
file. On VMS, these are called "universal" variables, but that term
means something else entirely on TOPS-20 systems.
Galactic variables are the "other half" of dynamic linking not mentioned
in previous proposals -- access to data in other libraries. Previous
proposals have only given access to routines in other libraries.
This is needed for "logical completeness" (never a big draw in getting
funding) and for at least one specific need in the global interrupt/trap
manager currently being designed.
This function is very cheap to provide, as explained below.
1.3.7 Library Version Matching
The library itself and the .REL file defining the LDLBLK will contain
library version information. At run-time this information is checked to
see if the version of the library actually found is acceptable to the
program requesting it. The version matching rule is specified in the
library but can be overridden in the LDLBLK.
Dynamic Libraries Page 4
INTRODUCTION
1.3.8 Dynamic Library Mechanism Version Matching
The dynamic library blocks (LDLBLK and DLBLK) contain version number
information. This can be checked to make sure that the blocks found are
compatible with the code massaging them.
1.4 Benefits Of Dynamic Libraries
Better independence of packages. Living in your own section, which you
control, gives better isolation than sharing that section with other
code.
Easier software updates. A new version of a dynamic library can be
introduced into all programs calling it by simply placing it on the
directory from which dynamic libraries are loaded. There is no need to
relink programs using it.
More consistent program behavior. If a facility is always provided by
the same package of code, then it will always be provided in the same
way (this isn't really a benefit of dynamic libraries per se, the same
thing could be achieved by linking the code that provides the facility
with the caller).
Easier maintenance. A crash of from a program structured from dynamic
libraries will show clearly and neatly what version of each library was
loaded, what the internal state of that library is, and so forth.
Within a library, things will always be in the same place.
More efficient real memory use. If a facility provided by a dynamic
library is never called during a given run of a program, that library
will never be mapped in.
1.5 Conventions Used
1.5.1 Indirect Words
ALL of the indirect words (EFIW, IFIW) mentioned in this document must
have the index register bits set to ZERO!!! No indexing should be used
in the dynamic library tables. The reason for this is simple: the
contents of the registers aren't globally constant, so the result of the
indexing may not be what was intended.
Indirection is permitted. None of the further indirect words reached
through indirection should perform indexing either.
Dynamic Libraries Page 5
DYNAMIC LIBRARY FUNCTIONS
2.0 DYNAMIC LIBRARY FUNCTIONS
The following functions must be provided by the dynamic library
mechanism:
2.1 Calling A Routine In A Dynamic Library
To be able to call routines in a dynamic library, you must:
1. Be running in a non-zero section
2. Have a global-format stack pointer in AC17
3. Have linked your program with a library-specific .REL file for
each of the libraries you may call directly (you do not need to
consider libraries that may be called by libraries you call)
A routine in a dynamic library is called by performing a PUSHJ through a
"local transfer vector". This local transfer vector is provided by the
library-specific .REL file mentioned above.
Suppose the dynamic library EXAMPL is documented as containing a routine
RNDNAM. The argument-passing rules for this routine and what registers
it preserves should be documented as well.
To call this routine from MACRO, you must set up the arguments as
specified for the routine, and then say
PUSHJ P, @RNDNAM
You may use any type and number of levels of indexing and indirection in
the instruction that calls a routine in a dynamic library. It is also
perfectly acceptable for the PUSHJ instruction to be XCT'd from
somewhere else.
2.2 Referring To A Galactic Variable
To refer to a data location in another library, you must:
1. Be running in a non-zero section
2. Have linked your program with a library-specific .REL file for
the library which defines the galactic variable to which you
wish to refer
To refer to the galactic variable, make an indirect reference through a
"local galactic vector" which is provided by the library-specific .REL
file mentioned above.
Dynamic Libraries Page 6
DYNAMIC LIBRARY FUNCTIONS
Referring to a galactic variable defined in a library not yet mapped in
will cause that library to be mapped in (and its master init routine to
be run).
2.3 Global Master Init
This routine should be called from the initialization routine of every
top-level program (since on tops-20 top-level programs are entered
through entry vectors, whereas dynamic libraries are entered through
transfer vectors pointed to by PDV's, a package can always know how it
was entered, and thus whether it is running at the top level or not).
This finds and calls the master init routine of every copy of every
dynamic library mapped into the process address space.
This is necessary to make restarting work right without incurring
unacceptable overhead.
2.4 Overloading
Merge information about several dynamic libraries into one local
transfer vector.
This is an extension of the capabilities you get by manually moving some
EFIW into the local transfer vector in locations where you want to
supercede the routine provided by the library. For example, you can
cause your transfer vector to contain EFIW's to your private (presumably
improved; perhaps debugging versions?) of some routines and point to
the library copies of other routines.
The normal rules for overloading are as follows:
1. LTVEC entries not containing their default initial state will
not be altered. The default initial state is "IFIW
7777,,LDLBLK" for the monitor implementation, or "IFIW
LDLBLK-1" for the user-mode implementation.
2. If an MTVEC entry contains 0, the corresponding LTVEC entry is
not altered.
3. If the LTVEC is longer than the MTVEC, excess LTVEC locations
are not altered.
4. If the MTVEC is longer than the LTVEC, the additional MTVEC
entries are ignored.
Dynamic Libraries Page 7
DYNAMIC LIBRARY FUNCTIONS
2.4.1 Load MTVEC Into LTVEC
You specify a master transfer vector and a local transfer vector. The
master transfer vector is loaded over the local transfer vector
according to the usual rules.
2.4.2 Load Library Into LDLBLK
You specify a LDLBLK whose transfer vectors you want to load over, and
an LDLBLK specifying the library you want to load from. The overloading
is done according to the usual rules.
2.5 De-linking
The monitor DYNLIB% jsys provides functions for de-linking a dynamic
library. We think that this function is not fully thought out. In
particular, it is not clear how the monitor will find all the LTVEC's
pointing to any given MTVEC.
We do not see any need for this function.
2.6 Finding A Free Provider
When a streaming (single or multi) library is entered at the point that
causes a stream to be initiated, if the library is not capable of
initiating another stream, it should return an error. Newly-written
dynamic libraries should all be non-streaming or multi-streaming, so
this should not come up in practice.
If this became an important consideration, it might be possible to write
a routine that the library stream-initialization routine could call
which would find the LTVEC the call came through (by analyzing the call
instruction, whose successor address is on the stack), find a free
provider, and re-map the LTVEC to point to this provider. Note that
this would lose the effects of any overloading performed on that LTVEC.
This would need a lot more thought before I wanted to commit to it.
2.7 Library Version Checking
The master DLBLK in the library .EXE file contains the library version
number in location .DYVER. It contains the default version checking
rule for that library within the flag word (.DYFLG) in field DY%VER.
Dynamic Libraries Page 8
DYNAMIC LIBRARY FUNCTIONS
The LDLBLK in the calling program contains the library version number
that it corresponds to in location .LDVER. It may optionally contain a
version checking rule (indicated by LD%VMA in .LDVER) in field LD%VER of
.LDFLG, which overrides the default rule specified in the DLBLK.
Based on this information, a check is performed when the library is
loaded to see if it matches the version the calling program was compiled
against well enough to be used. If it does not, an error is printed and
the program terminated.
The version numbers are in standard TOPS-20 version word format. For
version matching purposes, only VI%MAJ and VI%MIN are considered.
If the major and minor versions from both places are the same, then the
library is accepted.
In addition, the library may optionally be accepted if:
o Library major version greater than program major version
(VM%MAG)
o Library major version less than program major version (VM%MAL)
o Major versions equal and library minor version greater than
program minor version (VM%MIG)
o Major versions equal and library minor version less than
program minor version (VM%MIL)
Any combination may be set.
2.8 Dynamic Library Mechanism Version Checking
The master DLBLK in the library .EXE file contains the library mechanism
version number in location .DYFVN.
The LDLBLK in the calling program contains the library mechanism version
number in location .LDFVN.
Based on this information, a check is performed when the library is
loaded to see if it matches the version of the dynamic library mechanism
(FAKDYN in the user mode implementation) that is being used. If it does
not, an error is printed and the program terminated.
The version numbers are in standard TOPS-20 version word format. For
version matching purposes, only VI%MAJ and VI%MIN are considered.
If the major and minor versions from both places are the same, then the
library is accepted.
In addition, the library may optionally be accepted if:
Dynamic Libraries Page 9
DYNAMIC LIBRARY FUNCTIONS
o Library major version greater than mechanism major version
(VM%MAG)
o Library major version less than mechanism major version
(VM%MAL)
o Major versions equal and library minor version greater than
mechanism minor version (VM%MIG)
o Major versions equal and library minor version less than
mechanism minor version (VM%MIL)
Any combination may be set.
This is applied both to the LDLBLK and the DLBLK.
3.0 IMPLEMENTATION
First I will describe the proposed changes for monitor-implemented
dynamic libraries.
Later, I describe a way to implement the same capabilities in user code,
leaving an easy way to take advantage of monitor capabilities when they
become available.
3.1 Monitor-mode Implementation
3.1.1 In The User Program
I suggest that all dynamic libraries consist of two user-visible parts:
the dynamic library .exe file, and a .rel file that defines the proper
local dynamic library block for that dynamic library. Having the local
dynamic library block available from a rel file removes the
responsibility of constructing it (and the possibility of making it
wrong) from the users of dynamic libraries. It also makes it easier to
change the local dynamic library block, which will probably be necessary
to do when making the transition from user-code to monitor-supported
dynamic libraries.
I propose that the local dynamic library block should look like this
(this is a considerable change in format from our current version):
LDLBLK: length (must be 11 currently)
Block format version number (to be used in the future to
provide fancier kinds of dynamic library services such as
calls by name instead of by offset)
Flag word (DIGITAL-defined only)
Initialized: library has been loaded and vectors updated
User word
Pointer to "service class" string
Pointer to file spec string
Library version number
IFIW Address of DIGITAL-specified entry-point vector (LDTVEC)
Dynamic Libraries Page 10
IMPLEMENTATION
IFIW Address of library-specific entry-point vector (LCTVEC)
Reserved to DIGITAL
IFIW Address of library-specific galactic vector (LCGVEC)
[The possibility of additional entries is reserved to DIGITAL]
Digital-specified entry points will likely include several flavors of
initialization and cleanup. They have been made a separate vector so
that it will be possible to add additional ones in the future if needed.
Note that there is nothing requiring that each offset in each transfer
vector point to a unique routine.
LDTVEC: diglen+1
7777,,LDLBLK
. . .
7777,,LDLBLK
LCTVEC: cuslen+1
7777,,LDLBLK
. . .
7777,,LDLBLK
LCGVEC: cgalen+1
7777,,LDLBLK
. . .
7777,,LDLBLK
3.1.2 In The Library
I propose that the dynamic library block (DLBLK) in the dynamic library
itself should change similarly:
DLBLK: length (must be 9 currently)
Block format version number
Flag word (DIGITAL defined only)
Busy: library can't start a stream
Version match: what are acceptable matches between DLBLK
version and LDLBLK version?
Library-use word
Library version number
IFIW Address of DIGITAL-specified entry-point vector (DTVEC)
IFIW Address of library-specific entry-point vector (CTVEC)
Reserved to DIGITAL
IFIW Address of library-specific galactic variable vector (CGVEC)
[The possibility of additional entries is reserved to DIGITAL]
DTVEC: diglen+1
IFIW routine entry-point
. . .
IFIW routine entry-point
CTVEC: cuslen+1
IFIW routine entry-point
. . .
Dynamic Libraries Page 11
IMPLEMENTATION
IFIW routine entry-point
CGVEC: cgalen+1
IFIW address
. . .
IFIW address
In addition to the PDV "Dynamic Library" which points to the DLBLK, each
dynamic library will have a PDV named "DYNLIB$class", where "class" is
the name of the service class provided. Class names containing a "%"
are reserved to DIGITAL. For example, the sort service dynamic library
would have a PDV "Dynamic Library", and a PDV "DYNLIB$SORT%".
The service-class PDV should point to the DLBLK just as the "Dynamic
Library" PDV does, so that the DLBLK can be found directly from the
service class name. It should point to THE DLBLK, not to another copy.
There is only one DLBLK in a dynamic library.
Note: some convention for tops-20 name-space should be adopted and
applied to these PDV names as well as to all global symbol names. The
exact standard adopted is much less important than the adoption of SOME
standard. The details above are primarily intended as examples.
Note: The PDV name "Dynamic Library" should belong to DIGITAL name
space, not customer name space. Rather than explicitly listing it as an
exception to the rules, the name should be changed so that it falls in
DIGITAL name space.
3.1.3 JSYS Interface
Not yet defined. The details of this aren't nearly as important to
settle as the capabilities and data structures for normal use. Note
that explicit calls to the JSYS interface will be very much the
exception, not the norm.
3.2 User-mode Implementation
One goal of our FAKDYN procedure must be that it can be replaced by a
monitor-supported dynamic library procedure with minimum effort. This
FAKDYN proposal could be converted to a monitor-supported DYNLIB
(assuming the monitor buys the changes proposed above, which are
independent of FAKDYN anyway) by re-linking existing programs with a new
LDLBLK rel file. No code changes in any program would be required.
To use FAKDYN, there will be the additional requirement that all
packages calling dynamic libraries be linked with FAKDYN.REL.
Dynamic Libraries Page 12
IMPLEMENTATION
There is one restriction under FAKDYN: a library cannot be loaded in
response to a reference to a galactic variable defined in that library.
The recommended workaround is to call a routine in the library before
attempting reference to a galactic variable. Testing the "initialized"
bit in the LDLBLK could increase the efficiency of this.
First, extend the LDLBLK with a FAKDYN kludge word:
| PUSHJ P, FAKDYN
LDLBLK: length (must be 11 currently)
Block format version number
Flag word (DIGITAL-defined only)
User word (Can be address of user data block)
Pointer to "service class" string
Pointer to file spec string
Library version number
IFIW Address of DIGITAL-specified entry-point vector (LDTVEC)
IFIW Address of library-specific entry-point vector (LCTVEC)
Reserved to DIGITAL
IFIW Address of library-specific galactic vector (LCGVEC)
[The possibility of additional words is reserved to DIGITAL]
LDTVEC: diglen+1
IFIW LDLBLK-1
. . .
IFIW LDLBLK-1
LCTVEC: cuslen+1
IFIW LDLBLK-1
. . .
IFIW LDLBLK-1
LCGVEC: cgalen+1
IFIW LDLBLK-1
. . .
IFIW LDLBLK-1
At FAKDYN entry, stack will contain:
LDLBLK -1(SP)
user return 0(SP)
LDLBLK contains all the information necessary to map in (if necessary) a
dynamic library, and to update the local transfer vectors. After doing
this, FAKDYN decrements the user return to point back to the instruction
that called the routine, and returns to re-execute that instruction.
Since the local transfer vectors have now been updated, the re-execution
of the instruction will result in calling the specified routine.
Dynamic Libraries Page 13
SECTION ALLOCATION
4.0 SECTION ALLOCATION
Another area that needs addressing is section allocation. Since which
sections are in use has a serious impact on performance, the section
allocator should be a piece of digital-supplied code, preferably in the
monitor (where it will know what processor it is running on and can
allocate accordingly). (Another advantage of putting this code in the
monitor is that an EXE file can be transported to another system with a
different processor and start allocating sections in a way appropriate
for the processor it is running on.)
5.0 DIGITAL ENTRY POINTS
A need for the following DIGITAL-specified entry points has been
determined.
5.1 Master Init
DIGITAL entry point 1 is "master init". Master init is called
automatically by FAKDYN (and eventually by the monitor) when a library
is first mapped in. If a library does not require master
initialization, that entry in the DTVEC should point to a routine that
simply returns. The entry should not be 0. (This restriction
eliminates the need to check before each and every call whether the
entry vector is valid.)
The master init routine does not accept any arguments. It must preserve
all registers. It may assume that there is a global stack pointer in
AC17. What, if any, assumptions may be made about the amount of space
available on the stack have not yet been decided.
(In addition to the master init entry point, most libraries will have an
ordinary init or a stream-init entry point. Do not confuse these
limited initializations with the master init performed once per run or
start command.)
A routine will be provided which runs the master init on every library
currently mapped in. This routine should be called by every top-level
program that uses dynamic libraries in its initialization section
(before it calls on any dynamic library services).
This takes care of the need to re-initialize on restarts without having
to re-map all the libraries in use, and without having to know in
advance which libraries are going to be used.
Note that, on TOPS-20, a program must be written to run either as a
top-level, or as a subroutine. A program can be written to be invoked
either way, but in that case it always knows which way it was in fact
invoked on this run. Thus, it is meaningful to specify actions to be
taken only if you were called as a top level program.
APPENDIX A
BIBLIOGRAPHY
[1]
Murphy, Dan
Dynamically Linked Libraries in TOPS-20
DIGITAL interoffice memorandum, March 1981
[2]
Wallace, Kevin Girard
Canonical Terminal Support in the TOPS-20 Operating System
Master's thesis, MIT, May 1982
[3]
Uhler, Mike
Extended Addressing
DIGITAL interoffice memorandum, January 1983
[4]
Mierswa, Peter
Layered Architecture
DIGITAL interoffice memorandum, May 1983
[5]
Wallace, Kevin G.
More on Dynamically Linked Libraries
DIGITAL interoffice memorandum, December 1981
[6]
Wallace, Kevin G.
Functional Specification for Canonical Terminal Support
DIGITAL interoffice memorandum, August 1982
[7]
Wallace, Kevin G.
Design Specification for Canonical Terminal Support
DIGITAL interoffice memorandum, August 1982