RFC138 - Status report on proposed Data Reconfiguration Service

领测软件测试网

　　 
　　Network Working Group Bob Anderson
Request for Comments: 138 Rand
NIC 6715 Vint Cerf
UCLA
Eric Harslem
John Heafner
Rand
Jim Madden
U. of Illinois
Bob Metcalfe
MIT
Arie Shoshani
SDC

Jim White
UCSB
David Wood
Mitre
28 April 1971

STATUS REPORT ON PROPOSED DATA RECONFIGURATION SERVICE

CONTENTS
I. INTRODUCTION ................................. 2

Purpose of this RFC.......................... 2
Motivation ................................... 2

II. OVERVIEW OF DATA RECONFIGURATION SERVICE ..... 3

Elements of Data Reconfiguration Service ..... 3
Conceptual Network Connections ............... 3
Connection Protocols and Message Formats ..... 4
Example Connection Configurations ............ 6

III. THE FORM MACHINE ............................. 7

Input/Output Stream and Forms ................ 7
Form Machine BNF Syntax ...................... 7
Alternate Specification of Form Machine Syntax 8
Forms ........................................ 9
Rules ........................................ 10
Terms ........................................ 10

Term Format 1 .............................. 11
Term Format 2 .............................. 11
Term Format 3 .............................. 13
Term Format 4 .............................. 13
Application of a Term ...................... 14

Restrictions and Interpretations of
Term Functions ........................... 14
Term and Rule Sequencing ..................... 16

IV. EXAMPLES ..................................... 16

Remarks ...................................... 16
Field Insertion .............................. 17
Deletion ..................................... 17
Variable Length Records ...................... 17
String Length Computation .................... 18
Transposition ................................ 18
Character Packing and Unpacking .............. 18

V. PROPOSED USES OF DATA RECONFIGURATION SERVICE 19

VI. IMPLEMENTATION PLANS ......................... 20

Appendix A ......................................... 21

Note 1 to the DRS Working Group .............. 21
Note 2 to the DRS Working Group .............. 22

I. INTRODUCTION

PURPOSE OF THIS RFC

The purpose of this RFCis to describe, in part, a proposed Network
experiment and to solicit comments on any aspect of the experiment.
The experiment involves a software mechanism to reformat Network data
streams. The mechanism can be adapted to numerous Network
application programs. We hope that the results of the experiment
will lead to a further standard service that embodies the principles
described in this RFC. We would like comments on the
appropriateness of this work as a Network experiment and also
comments on particular Network data reformatting needs that could not
easily be accomplished using these techniques.

MOTIVATION

Application programs require specific data I/O formats yet the
formats are different from program to program. We take the position
that the Network should adapt to the individual program requirements
rather than changing each program to comply with a standard. This
position doesn't preclude the use of standards that describe the
formats of regular message contents; it is merely an interpretation
of a standard as being a desirable mode of operation but not a
necessary one.

In addition to differing program requirements, a format mismatch
problem occurs where users wish to employ many different kinds of
consoles to attach to a single service program. It is desirable to
have the Network adapt to individual console configurations rather
than requiring unique software packages for each console
transformation.

One approach to providing adaptation is for those sites with
substantial computing power to offer a data reconfiguration service;
a proposed example of such a service is described here.

The envisioned modus operandi of the service is that an applications
programmer defines _forms_ that describe data reconfigurations. The
service stores the forms by name. At a later time, a user (perhaps a
non-programmer) employs the service to accomplish a particular
transformation of a Network data stream, simply by calling the form
by name.

We have attempted to provide a notation tailored to some specifically
needed instances of data reformatting while keeping the notation and
its underlying implementation within some utility range that is
bounded on the lower end by a notation expressive enough to make the
experimental service useful, and that is bounded on the upper end by
a notation short of a general purpose programming language.

II. OVERVIEW OF THE DATA RECONFIGURATION SERVICE

ELEMENTS OF THE DATA RECONFIGURATION SERVICE

An implementation of the Data Reconfiguration Service (DRS) includes
modules for connection protocols, a handler of some requests that can
be made of the service, a compiler and/or interpreter (called the
Form Machine) to act on those requests, and a file storage module for
saving and retrieving definitions of data reconfigurations (forms).
This section highlights connection protocols and requests. The next
section covers the Form Machine language in some detail. File
storage is not described in this document because it is transparent
to the use of the service and its implementation is different at each
DRS host.

CONCEPTUAL NETWORK CONNECTIONS

There are three conceptual Network connections to the DRS, see Fig.
1.

1) The control connection (CC) is between an originating user
and the DRS. A form specifying data reconfiguration is

defined over this connection and is applied to data passing
over the two connections described below.
2) The user connection (UC) is between a user process and the
DRS.
3) The server connection (SC) is between the DRS and the
serving process.

Since the goal is to adapt the Network to user and server processes,
a minimum of requirements are imposed on the UC and SC.

+-------------+ CC +-----------+ SC +-----------+
| ORIGINATING +--------+ DRS +--------+ SERVER |
| USER | ^ | | ^ | PROCESS |
+-------------+ | +------+----+ | +-----------+
| / |
Telnet / <------ Simplex or Duplex
Protocol UC/ Connections
Connection /
/
+-----+-----+
| USER |
| PROCESS |
+-----------+

Figure 1. DRS Network Connections

CONNECTION PROTOCOLS AND MESSAGE FORMATS

Over a control connection the dialog is directly between an
originating user and the DRS. Here the user is defining forms or
assigning forms to connections for reformatting.

The user connects to the DRS via the initial connection protocol
(ICP) specified in NWG/RFC#80, version 1. Rather than going through
a logger, the user calls on a particular socket on which the DRS
always listens. DRS switches the user to another socket pair.

Messages sent over a control connection are of the types and formats
to be specified for TELNET. Thus, a user at a terminal should be
able to connect to a DRS via his local TELNET, for example, as shown
in Fig. 2.

+--------------+
+--------+ CC | |
+-------+ TELNET +-------+ DRS |
| +--------+ | |
| +--------------+
+----------+---------+
| USER |
|(TERMINAL OR PROGRAM|
+--------------------+

Figure 2. A TELNET Connection to DRS

When a user connects to DRS he supplies a six-character user ID (UID)
as a qualifier to guarantee the uniqueness of his form names. He
will have (at least) the following commands:

1. DEFFORM (name)
2. ENDFORM (name)

These two commands define a form, the text of which is
chronologically entered between them. The (name) is six
characters. The form is stored in the DRS local file
system.

3. PURGE (name)

The named form, as qualified by the current UID, is purged
from the DRS file system.

4. LISTNAMES (UID)

The unqualified names of all forms assigned to UID are
returned.

5. LISTFORM (name)

The source text of a named form is returned.

6. DUPLEXCONNECT (user site, user send, user receive,
user method, server site, server
send, server receive, server method,
user-to-server form, server-to-user form)

7. SIMPLEXCONNECT (send site, send socket, send
method, receive site, receive
socket, receive method, form)

Either one, both, or neither of the two parties specified in 6 or 7
may be at the same host as the party issuing the request. Sites and
sockets specify user and server for the connection. Method indicates
the way in which the connection is established. Three options are
provided:
1) Site/socket already connected to DRS as a dummy
control connection. (A dummy control connection
should not also be the real control connection.)
2) Connect via standard ICP. (Only for command no. 6.)
3) Connect directly via STR, RTS.

EXAMPLE CONNECTION CONFIGURATIONS

There are basically two modes of DRS operation: 1) the user wishes to
establish a DRS UC/SC connection(s) between two programs and 2) the
user wants to establish the same connection(s) where he (his
terminal) is at the end of the UC or the SC. The latter case is
appropriate when the user wishes to interact from his terminal with
the serving process (e.g., a logger).

In the first case (Fig. 1, where the originating user is either a
terminal or a program) the user issues the appropriate CONNECT
command. The UC/SC can be simplex or duplex.

The second case has two possible configurations, shown in Figs. 3 and
4.

+--------+ CC +--------+ +------+
| +------+ | SC | |
+------+ /| TELNET | UC | DRS +------+ SP |
| |/ | +------+ | | |
| USER | /+--------+ +--------+ +------+
| |/
+------+

Figure 3. Use of Dummy Control Connection

+--------+
+------+ /| USER | CC +--------+ +------+
| |/ | SIDE +------+ | SC | |
| USER | +--------+ UC | DRS +------+ SP |
| |\ | SERVING+------+ | | |
+------+ \| SIDE | +--------+ +------+
+--------+

Figure 4. Use of Server TELNET

In Fig. 3 the user instructs his TELNET to make two duplex
connections to DRS. One is used for control information (the CC) and
the other is a dummy. When he issues the CONNECT he references the
dummy duplex connection (UC) using the "already connected" option.

In Fig. 4 the user has his TELNET (user side) call the DRS. When he
issues the CONNECT the DRS calls the TELNET (server side) which
accepts the call on behalf of the console. This distinction is known
only to the user since to the DRS the configuration in Fig. 4 appears
identical to that in Fig. 1. Two points should be noted:

1) TELNET protocol is needed only to define forms and direct
connections. It is not required for the using and serving
processes.
2) The using and serving processes need only a minimum of
modification for Network use, i.e., an NCP interface.

III. THE FORM MACHINE

INPUT/OUTPUT STREAMS AND FORMS

This section describes the syntax and semantics of forms that specify
the data reconfigurations. The Form Machine gets an input stream,
reformats the input stream according to a form describing the
reconfiguration, and emits the reformatted data as an output stream.

In reading this section it will be helpful to envision the
application of a form to the data stream as depicted in Fig. 5. An
input stream pointer identifies the position of data (in the input
stream) that is being analyzed at any given time by a part of the
form. Likewise, an output stream pointer locates data being emitted
in the output stream.

/\/\ /\/\
^ | | FORM | | ^
| | | ----------------- | | |
| | | +- ----------------- -+ | | |
| | | | CURRENT PART OF | | | |
INPUT | |<= CURRENT < ----------------- > CURRENT => | | OUTPUT
STREAM | | POINTER | FORM BEING APPLIED | POINTER | | STREAM
| | +- ----------------- -+ | |
| | ----------------- | |
| | ----------------- | |
| | ----------------- | |
\/\/ \/\/
Figure 5. Application of Form to Data Streams

FORM MACHINE BNF SYNTAX

form ::= rule | rule form

rule ;;= label inputstream outputstream ;

label ::= INTEGER | <null>

inputstream ::= terms | <null>

terms ::= term | terms , term

outputstream ::= : terms | <null>

term ::= identifier | identifier descriptor |
descriptor | comparator

identifier ::= an alpha character followed by 0 to 3
alphamerics

descriptor ::= (replicationexpression , datatype ,
valueexpression , lengthexpression control)

comparator ::= (value connective value control) |
(identifier .<=>. control)

replicationexpression ::= arithmeticexpression | <null>

datatype ::= B | O | X | E | A

valueexpression ::= value | <null>

lengthexpression ::= # | arithmeticexpression | <null>

connective ::= .LE. | .LT. | .GE. | .GT. | .EQ. | .NE.

value ::= literal | arithmeticexpression

arithmeticexpression ::= primary | primary operator
arithmeticexpression

primary ::= identifier | L(identifier) | V(identifier) |
INTEGER

operator ::= + | - | * | /

literal ::= literaltype "string"

literaltype ::= B | O | X | E | A

string ::= from 0 to 256 characters

control ::= : options | <null>

options ::= S(where) | F(where) | U(where) |
S(where) , F(where) |
F(where) , S(where)

where ::= arithmeticexpression | R(arithmeticexpression)

ALTERNATE SPECIFICATION OF FORM MACHINE SYNTAX

infinity
form ::= {rule}
1
1 1 1
rule ::= {INTEGER} {terms} {:terms} ;
0 0 0
infinity
terms ::= term {,term}
0
1
term ::= identifier | {identifier} descriptor
0
| comparator
1
descriptor ::= ({arithmeticexpression} , datatype ,
0
1 1 1
{value} , {lengthexpression} {:options}
0 0 0
1
comparator ::= (value connective value {:options} ) |
0
1
(identifier .<=. value {:options} )
0
connective ::= .LE. | .LT. | .GE. | .GT. | .EQ. | .NE.

lengthexpression ::= # | arithmeticexpression

datatype ::= B | O | X | E | A

value ::= literal | arithmeticexpression

infinity
arithmeticexpression ::= primary {operator primary}
0
operator ::= + | - | * | /

primary ::= identifier | L(identifier) |
V(identifier) | INTEGER
256
literal ::= literaltype "{CHARACTER} "
0
literaltype ::= B | O | X | A | E
1
options ::= S(where) {,F(where)} |
0
1
F(where) {,S(where)} | U(where)
0

where ::= arithmeticexpression |
R(arithmeticexpression)
3
identifier ::= ALPHABETIC {ALPHAMERIC}
0

FORMS

A form is an ordered set of rules.

form ::= rule | rule form

The current rule is applied to the current position of the input
stream. If the (input stream part of a) rule fails to correctly
describe the contents of the current input then another rule is made
current and applied to the current position of the input stream. The
next rule to be made current is either explicitly specified by the
current term in the current rule or it is the next sequential rule by
default. Flow of control is more fully described under TERM AND RULE
SEQUENCING.

If the (input stream part of a) rule succeeds in correctly describing
the current input stream, then some data may be emitted at the
current position in the output stream according to the rule. The
input and output stream pointers are advanced over the described and
emitted data, respectively, and the next rule is applied to the now
current position of the input stream.

Application of the form is terminated when an explicit return
(R(arithmeticexpression)) is encountered in a rule. The user and

server connections are closed and the return code
(arithmeticexpression) is sent to the originating user.

RULES

A rule is a replacement, comparison, and/or an assignment operation
of the form shown below.

rule ::= label inputstream outputstream ;

A label is the name of a rule and it exists so that the rule may be
referenced elsewhere in the form for explicit rule transfer of
control. Labels are of the form below.

label ::= INTEGER | <null>

The optional integer labels are in the range 0 >= INTEGER >= 9999.
The rules need not be labeled in ascending numerical order.

TERMS

The inputstream (describing the input stream to be matched) and the
outputstream (describing data to be emitted in the output stream)
consist of zero or more terms and are of the form shown below.

inputstream ::= terms | <null>
outputstream ::= :terms | <null>
terms ::= term | terms , term

Terms are of one of four formats as indicated below.

term ::= identifier | identifier descriptor |
descriptor | comparator

Term Format 1

The first term format is shown below.

identifier

The identifier is a symbolic reference to a previously identified
term (term format 2) in the form. It takes on the same attributes
(value, length, type) as the term by that name. Term format 1 is
normally used to emit data in the output stream.

Identifiers are formed by an alpha character followed by 0 to 3
alphameric characters.

Term Format 2

The second term format is shown below.

identifier descriptor

Term format 2 is generally used as an input stream term but can be
used as an output stream term.

A descriptor is defined as shown below.

descriptor ::= (replicationexpression, datatype,
valueexpression, lengthexpression
control)

The identifier is the symbolic name of the term in the usual
programming language sense. It takes on the type, length, and value
attributes of the term and it may be referenced elsewhere in the
form.

The replication expression is defined below.

replicationexpression ::= arithmeticexpression | <null>
arithmeticexpression ::= primary | primary operator
arithmeticexpression
operator ::= + | - | * | /
primary ::= identifier | L(identifier) | V(identifier) |
INTEGER

The replication expression is a repeat function applied to the
combined data type and value expression. It expresses the number of
times that the value (of the data type/value expression) is to be
repeated within the field length denoted by the data type/length
expression.

A null replication expression has the value of one. Arithmetic
expressions are evaluated from left-to-right with no precedence. (It
is anticipated that this interpretation might be changed, if
necessary.)

The L(identifier) is a length operator that generates a 32-bit binary
integer corresponding to the length of the term named. The
V(identifier) is a value operator that generates a 32-bit binary
integer corresponding to the value of the term named. (See
Restrictions and Interpretations of Term Functions.) The value
operator is intended to convert character strings to their numerical
correspondents.

The data type is defined below.

datatype ::= B | O | X | E | A

The data type describes the kind of data that the term represents.
(It is expected that additional data types, such as floating point
and user-defined types, will be added as needed.)

Data Type Meaning Unit Length

B Bit string 1 bit
O Bit string 3 bits
X Bit string 4 bits
E EBCDIC character 8 bits
A Network ASCII character 8 bits

The value expression is defined below.

valueexpression ::= value | <null>
value ::= literal | arithmeticexpression
literal ::= literaltype "string"
literaltype ::= B | O | X | E | A

The value expression is the unit value of a term expressed in the
format indicated by the data type. It is repeated according to the
replication expression, in a field whose length is constrained by the
length expression.

A null value expression in the input stream defaults to the data
present in the input stream. The data must comply with the datatype
attribute, however.

A null value expression generates padding according to Restrictions
and Interpretations of Term Functions.

The length expression is defined below.

lengthexpression ::= # | arithmeticexpression | <null>

The length expression states the length of the field containing the
value expression as expanded by the replication expression. If the
value of the length expression is less then the length implied by the
expanded value expression, then the expanded value expression is
truncated, see Restrictions and Interpretations of Term Functions.

The terminal symbol # means an arbitrary length, explicitly
terminated by the value of the next term. The # is legal only in the
input stream and not in the output stream.

If the length expression is less than or equal to zero, the term
succeeds but the appropriate stream pointer is not advanced.
Positive lengths cause the appropriate stream pointer to be advanced
if the term otherwise succeeds.

Control is defined under TERM AND RULE SEQUENCING.

Term Format 3

Term format 3 is shown below.

descriptor

It is identical to term format 2 with the omission of the identifier.
Term format 3 is generally used in the output stream. It is used in
the input stream where input data is to be passed over but not
retained for emission or later reference.

Term Format 4

The fourth term format is shown below.

comparator ::= (value connective value control) |
(identifier .<=. value control)
value ::= literal | arithmeticexpression
literal ::= literaltype "string"
literaltype ::= B | O | X | E | A
string ::= from 0 to 256 characters
connective ::= .LE. | .LT. | .GE. | .GT. | .EQ. | .NE.

The fourth term format is used for assignment and comparison.

The assignment operator .<=. assigns the value to the identifier.
The connectives have their usual meaning. Values to be compared must
have the same type and length attributes or an error condition arises
and the form fails.

The Application of a Term

The elements of a term are applied by the following sequence of
steps.

1. The data type and value expression together specify a unit
value, call it x.

2. The replication expression specifies the number of times x
is to be repeated (or copied) in concatenated fashion. The
value of the concatenated xs becomes, say, y of length L1.

3. The data type and the length expression together specify a
field length of the input area (call it L2) that begins at
the current stream pointer position.

4. The value of y is truncated to y' if L1 > L2. Call the
truncated length L1'.

5. If the term is an input stream term, then the value y' of
length L1' is compared to the input value beginning at the
current input pointer position.

6. If the values are identical over the length L1' then the
input value of length L2 (may be greater than L1') starting
at the current pointer position in the input area, becomes
the value of the term.

In an output stream term, the procedure is the same except that the
source of input is the value of the term(s) named in the value
expression and the data is emitted in the output stream.

The above procedure is modified to include a one term look-ahead
where lengths are indefinite because of the arbitrary symbol, #.

Restrictions and Interpretations of Term Functions

1. Terms specifying indefinite lengths, through the use of the #
symbol must be separated by some type-specific data such as a
literal. (A literal isn't specifically required, however. An
arbitrary number of ASCII characters could be terminated by a
non-ASCII character.) # is legal only in the input stream, not
in the output stream.

2. Truncation and padding is as follows:
a) Character to character (A <--> E) conversion is left
justified and truncated or padded on the right with blanks.
b) Character to numeric and numeric to numeric conversions are
right-justified and truncated or padded on the left with
zeros.
c) Numeric to character conversion is right-justified and
left-padded with blanks.

3. The following are ignored in a form definition over the control
connection.
a) TAB, carriage return, etc.
b) blanks except within quotes.
c) /* string */ is treated as comments except within quotes.

4. The following defaults prevail where the term part is omitted.

a) The replication expression defaults to one.
b) The data type defaults to type B.
c) The value expression of an input stream term defaults to
the value found in the input stream, but the input stream
must conform to data type and length expression. The value
expression of an output stream term defaults to padding
only.
d) The length expression defaults to the size of the quantity
determined by replication expression, data type, and value
expression.
e) Control defaults to the next sequential term if a term is
successfully applied; else control defaults to the next
sequential rule. If _where_ evaluates to an undefined
_label_ the form fails.

5. Arithmetic expressions are evaluated left-to-right with no
precedence.

6. The following limits prevail.

a) Binary lengths are <= 32 bits
b) Character strings are <= 256 8-bit characters
c) Identifier names are <= 4 characters
d) Maximum number of identifiers is <= 256
e) Label integers are >= 0 and <= 9999

7. Value and length operators product 32-bit binary integers. The
value operator is currently intended for converting A or E type
decimal character strings to their binary correspondents. For
example, the value of E'12' would be 0......01100. The value
of E'AB' would cause the form to fail.

TERM AND RULE SEQUENCING

Sequencing may be explicitly controlled by including control in a
term.

control ::= :options | <null>
options ::= S(where) | F(where) | U(where)
S(where) , F(where) |
F(where) , S(where)

where ::= arithmeticexpression | R(arithmeticexpression)

S, F, and U denote success, fail, and unconditional transfers,
respectively. _Where_ evaluates to a _rule_ label, thus transfer can
be effected from within a rule (at the end of a term) to the
beginning of another rule. R means terminate the form and return the
evaluated expression to the initiator over the control connection (if
still open).

If terms are not explicitly sequenced, the following defaults
prevail.

1) When a term fails go to the next sequential rule.
2) When a term succeeds go to the next sequential
term within the rule.
(3) At the end of a rule, go to the next sequential
rule.

Note in the following example, the correlation between transfer of
control and movement of the input pointer.

1 XYZ(,B,,8:S(2),F(3)) : XYZ ;
2 . . . . . . .
3 . . . . . . .

The value of XYZ will never be emitted in the output stream since
control is transferred out of the rule upon either success or
failure. If the term succeeds, the 8 bits of input will be assigned
as the value of XYZ and rule 2 will then be applied to the same input
stream data. That is, since the complete rule 1 was not successfully
applied, then the input stream pointer is not advanced.

IV. EXAMPLES

REMARKS

The following examples (forms and also single rules) are simple
representative uses of the Form Machine. The examples are expressed
in a term-per-line format only to aid the explanation. Typically, a
single rule might be written as a single line.

FIELD INSERTION

To insert a field, separate the input into the two terms to allow the
inserted field between them. For example, to do line numbering for a
121 character/line printer with a leading carriage control character,
use the following form.

(NUMB.<=>.1); /*initialize line number counter to one*/
1 CC(,E,,1:F(R(99))), /*pick up control character and save
as CC*/
/*return a code of 99 upon exhaustion*/
LINE(,E,,121 : F(R(98))) /*save text as LINE*/
:CC, /*emit control character*/
(,E,NUMB,2), /*emit counter in first two columns*/
(,E,E".",1), /*emit period after line number*/
(,E,LINE,117), /*emit text, truncated in 117 byte field*/
(NUMB.<=.NUMB+1:U(1)); /*increment line counter and go to
rule one*/;;

DELETION

Data to be deleted should be isolated as separate terms on the left,
so they may be omitted (by not emitting them) on the right.

(,B,,8), /*isolate 8 bits to ignore*/
SAVE(,A,,10) /*extract 10 ASCII characters from
input stream*/
:(,E,SAVE,); /*emit the characters in SAVE as EBCDIC
characters whose length defaults to the
length of SAVE, i.e., 10, and advance to
the next rule*/

In the above example, if either input stream term fails,
the next sequential rule is applied.

VARIABLE LENGTH RECORDS

Some devices, terminals and programs generate variable length
records. To following rule picks up variable length EBCDIC records

and translates them to ASCII.

CHAR(,E,,#), /*pick up all (an arbitrary number of)
EBCDIC characters in the input stream*/
(,X,X"FF",2) /*followed by a hexadecimal literal,
FF (terminal signal)*/
:(,A,CHAR,), /*emit them as ASCII*/
(,X,X"25",2); /*emit an ASCII carriage return*/

STRING LENGTH COMPUTATION

It is often necessary to prefix a length field to an arbitrarily long
character string. The following rule prefixes an EBCDIC string with
a one-byte length field.

Q(,E,,#), /*pick up all EBCDIC characters*/
TS(,X,X"FF",2) /*followed by a hexadecimal literal, FF*/
:(,B,L(Q)+2,8), /*emit the length of the characters
plus the length of the literal plus
the length of the count field itself,
in an 8-bit field*/
Q, */emit the characters*/
TS; */emit the terminal*/

TRANSPOSITION

It is often desirable to reorder fields, such as the following
example.

Q(,E,,20), R(,E,,10) , S(,E,,15), T(,E,,5) : R, T, S, Q ;

The terms are emitted in a different order.

CHARACTER PACKING AND UNPACKING

In systems such as HASP, repeated sequences of characters are packed
into a count followed by the character, for more efficient storage
and transmission. The first form packs multiple characters and the

second unpacks them.
/*form to pack EBCDIC streams*/
/*returns 99 if OK, input exhausted*/
/*returns 98 if illegal EBCDIC*/
/*look for terminal signal FF which is not a legal EBCDIC*/
/*duplication count must be 0-254*/
1 (,X,X"FF",2 : S(R(99))) ;
/*pick up the EBCDIC and initialize count/*
CHAR(,E,,1 : F(R(98))) , (CNT .<=. 1) ;
/*count consecutive EBCDICs like CHAR*/
2 (,E,CHAR,1 : F(3)) , (CNT .<=. CNT+1 : U(2)) ;
/*emit count and current character*/
3 : (,B,CNT,8), CHAR, (:U(1));
/*end of form*/;;

/*form to unpack EBCDIC streams*/
/*look for terminal*/
1 (,X,X"FF",2 : S(R(99))) ;
/*emit character the number of times indicated*/
/*by the counter contents*/
CNT(,B,,8), CHAR(,E,,1) : (CNT,E,CHAR,CNT:U(1));
/*failure of form*/
(:U(R(98))) ;;

V. PROPOSED USES OF DATA RECONFIGURATION SERVICE

The following are some proposed uses of the DRS that were submitted
by the sites indicated.

UCLA
1. Pack/unpack text files.
2. Preprocessor to scan META compiler input.
3. Perhaps graphics.

MIT
1. Reformatting within file transfer service.
2. Character conversions.
3. Possible graphics service (Evans and Sutherland output
format).
4. Reformat arguments of subroutines remote to each other.

U. OF ILLINOIS
1. Dependent upon remote use of DRS for many remote
services.

SDC
1. Would be essential to data transfer in general.

2. Could be used in relation to data management language.

UCSB
1. Checkout of I/O formats of file system.
2. Debugging Network services in general.
3. Mapping their services into future standards.

RAND
1. To describe RJO/RJE message formats at UCSB.
2. To describe RJS message formats at UCLA.
3. To adapt Network to existing services, in general.

MITRE
1. Character conversions.
2. Testing data formats going into data bases for correct
field formatting.

VI. IMPLEMENTATION PLANS

Four sites currently plan to implement and offer the service on an
experimental basis.

1. MIT Implementation of forms interpreter in MIDAS
(assembly). Perhaps Tree Meta compiler of
forms. Implementation on PDP-10.

2. UCLA Implementation on SIGMA-7 employing META-7
to compile forms.

3. UCSB Implementation on 360/75.

4. RAND Initial implementation on 360/65; compiler to be written
in graphics CPS; compiled intermediate forms to be
interpreted by assembler language subroutine. Later
implemented on PDP-10.

In addition to the above sites, the University of Illinois and Mitre
plan to experiment with the service.

APPENDIX A

Note 1 to the DRS Working Group

As you recall, we spent considerable time in discussing the use and
meaning of the arbitrary symbol, #. To summarize, it was clear that
inclusion of the # in both replication and length expressions led to
ambiguities. We settled on its restricted use in the length
expression of an input term, although no one was entirely satisfied
with this definition.

Recently, Jim White has again commented on the #. Jim feels that it
is curious that one can pick up an arbitrary number of EBCDIC
characters, for example, but can't pick up an arbitrary number of
specific EBCDIC characters such as EBCDIC A's. Jim feels that a more
natural way to interpret the length, value, and replication
expressions would be as the IBM OS assembler interprets the
attributes of the pseudo instruction, define constant (CD).

The IBM OS assembler uses the following format.

1 2 3 4
duplication type modifiers nominal value
factor

The duplication factor, if specified, causes the constant to be
generated the number of times indicated by the factor. The type
defines the type of constant being specified. Modifiers describe the
length, scaling, and exponent of the constant. Nominal value
supplies the constant described by the subfields that precede it.

Assume that we use the # only as a duplication factor (replication
expression). Hence, in the example of the form to pack EBCDIC
characters, the counter and looping can be eliminated.

CHAR(,E,,1) ;
LEN(#,#,CHAR,1) : (,B,L(LEN)+1,*) , CHAR ;

The interpretation is that the data type, length expression, and
value expression make up the unit value. This quantity can then be
replicated. As our document now stands, only the data type and value
expression make up the unit value.

The application of a term according to Jim's suggestion is as
follows.
1. The data type, value expression, and length expression together
specify a unit value, call it x.

2. The replication expression specifies the number of times x is to
be repeated. The value of the concatenated xs becomes y of
length L.
3. If the term is an input stream term then the value beginning at
the current input pointer position.
4. If the input value satisfies the constraints of y over length L
then the input value of length L becomes the value of the term.

Note 2 to the DRS Working Group

There has been recent debate of whether the input pointer should be
advanced upon successful completion of a rule (as it now is defined)
or upon successful completion of each term. See the example on page
22. If the input pointer is advanced upon successful completion of a
term, then rules become equivalent to terms.

I would like to for us to discuss at the SJCC both the term
attributes and the input pointer advance issues.

John

[ This RFCwas put into machine readable form for entry ]
[ into the online RFCarchives by Katsunori Tanaka 4/99 ]

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