Copyright © 1970, 1972, 1974 P.J. Brown
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This implementation of ML/I contains all the features described in the ML/I User's Manual, 4th Edition, August 1970, plus New Features 1 to 5 as described in supplements to that manual.
The initial input stream is set by the call of ML/I, but it can
thereafter be changed by re-setting the variable S10.
There are up to three output streams, which are controlled by
S20, S21 and S22.
S20 controls whether the output is to be listed on the
lineprinter: if S20 is zero, no listing is produced; if
S20 is one, a listing without line numbers is produced; if
S20 is two, a listing with line numbers is produced. These line
numbers are a count of output lines and will not, in general, correspond
with the count of source lines. Initially S20 has the value two.
S21 and S22 control the two main output streams. In each
case the value zero inhibits output to the stream and the value one
allows it. Thus if S21 and S22 are both one, two copies of
the output are made. Alternatively S21 and S22 can be
alternately switched on and off thus sending some output to one stream
and some to the other. Initially S21 is one and S22 is
zero.
The following is an example of the use of output streams:
TEXT1 MCSET S22=1 MCSET S21=0 TEXT2 MCSET S22=0 MCSET S21=1 MCSET S20=0 TEXT3
In this case TEXT1 and TEXT3 go to the stream controlled
by S21, TEXT2 goes to the stream controlled by S22,
and TEXT1 and TEXT2 are listed on the line-printer.
The way in which streams are tied to physical devices and files is described in later Sections.
This implementation of ML/I runs under the NICE executive
under any of the operating systems found on the ICL 4120/4130.
It is entered by the command &ML1 followed by a parameter list.
The majority of users will be interested in the special features for the
disc-based DES operating systems, so these will be described
first.
If the first parameter of ML/I is DES, all I/O is performed
through ASSIGNed channels (using the ACIO routine).
Input is from channel 36 and the streams corresponding to S21 and
S22 go to channels 31 and 33, respectively. The ACIO
routine is such that by default channel 36 is the card reader, channel
31 is paper tape punch 1 and channel 33 is the control teleprinter. Only
the first of these defaults is likely to be used in practice,
the other two channels normally being explicitly assigned.
Although input initially comes from channel 36 it can subsequently be
switched to any other assigned channel. If S10 is negative, this
means use an assigned channel, the value giving the channel number
(e.g. setting S10 to -97 would cause subsequent input to
be taken from channel 97).
The following example shows the use of assigned channels:
&ASSIGN;31;DV;0;WORKFILE 5; &ASSIGN;50;DC;2;MY,FILE; &ML1;DES; MCDEF PIG AS COW MCSET S20=0 MCSET S10=-50 ^^^ &NEAT; &ASSIGN;1;DV;0;WORKFILE 5;
This would cause every occurrence of PIG in MY FILE to be
replaced by COW. The file would then be assembled by
NEAT. The ML/I output listing is inhibited (by
MCSET S20=0) and channel 33 is not used and therefore not
assigned.
If the input channel is assigned to cards spaces are normally deleted from the end of each card before processing. A line containing three uparrows acts as a terminator (there are some special options for "unusual" card forms, see Section E.2.4.4).
When the end of an input channel is reached, if the channel is 36 (i.e. the initial input channel) ML/I quits; otherwise the channel is closed, a newline fed to ML/I and input is resumed from where it left off in channel 36 (the exact mechanism uses the "revert device" as described in Section E.2.4.7.2). Thus if channel 36 is a card stream it should always have a terminator (3 uparrows) at the end, even if input is switched to another channel.
The ACIO routine requires a buffer area, the size of which depends on the number and type of channels that are open at any one time. By default ML/I allocates 1068 words for this, which is ample for normal work. However in exceptional circumstances the message:
ACIO OVERFLOW
will be given and the buffer area should then be increased by specifying
the size of buffer required after the DES parameter to
ML1, e.g.:
&ML1;DES;2000;
The source text may, if desired, be listed on the lineprinter. This is
done by setting S14 to one. If S14 is zero, as it is
initially, no source listing is produced. Lines of source listing are
preceded by twenty-two asterisks and a line number. For card input the
terminating card is included in the listing and for paper tape or
teleprinter input the last line is listed even if it consists only of a
halt code (which will appear as blank). Thus the number of source lines
actually processed is normally one fewer than the count of source lines.
If S14 has the value two, lines are only listed from the original
input channel (specifically when S30 equals S10 -- see
Section E.2.4.7.2).
It is possible to designate that one character be translated into another on input. This makes it possible to input a character that a device does not support, for example a tab from cards. However only one character code can be translated in this way.
If it is desired to perform a translation, S16 should be set to
the 7-bit internal code of the character to be translated and S17
to the 7-bit internal code that is to replace it. For example if
% (internal code 5) was to represent a tab (internal code 1),
S16 and S17 should be set in the following way:
MCSET S16=5 MCSET S17=1
Initially S16 has the value -1, which, since it does not
correspond to a valid internal code, will not cause any translations to
be made.
Note that the settings of S16 and S17 apply to all input
devices and the settings may be changed dynamically if required. The
translation facility is applied before the source listing is produced
and is applied even to the newlines that are, in some cases,
artificially introduced by ML/I at the end of a piece of input (e.g.
at the end of a channel).
The order in which the input routines perform the following operations:
S16 and S17;
is not generally defined as it depends on the device used. It is, of course, only in very unusual cases that the ordering of these operations makes any difference.
The lineprinter is used for error messages and may also be used for source and output listings. These are all interspersed with one another. If both source and output are listed simultaneously an extra newline is sent to the printer before each line of the source listing in order to separate source from output.
DESS and DESC parametersTwo minor extra facilities are available to save a little writing.
&ML1;DESS;
is equivalent to
&ML1;DES;
except that S14 is initialised to one and S20 to zero
(i.e. source is listed rather than output).
&ML1;DESC;
is similar to the above except that S14 is initialised to two,
not one. This is often useful for listing only that source text that
comes from cards.
(The reader may skip to Section 3 if he a) wishes to use ML/I only under DES and b) is happy to confine himself to the facilities described above).
This Section:
ASSIGNed channel facilities are not available.
All the facilities described here are available under the DES operating system, and similarly all the facilities of Section E.2.3 are available under other operating systems except those facilities concerned with assigned channels (which require ACIO, CFILE and DFILE).
ML/I needs a NICE-based operating system including
LP and the device routines and supporting software for any
other types of I/O that are used (e.g. DRIP, MT,
LIBRARY). ML/I occupies about 5K words and requires at least 2K
of workspace. It uses all the area between LOWADD and
TOPADD for workspace.
When channels cannot be ASSIGNed it is necessary to have
alternative means of specifying I/O devices. ML/I does this by means of
S-variables, parameters and fixed assignments. In addition to the use of
physical devices, it allows, on input, the use of items from a standard
4130 library file. The library is accessed, exactly as in
ALGOL, using the LIBRARY module. The library can be
on disc or magnetic tape, depending on which version of
LIBRARY is used.
The initial input device is specified by the parameter to ML/I as follows:
C | means cards
|
P | means paper tape
|
T | means control teleprinter (note that this version of
ML/I does not support remote teleprinter I/O).
|
M decimal digit | means library item from the
magnetic tape (or disc) handler (or volume) given by the digit.
|
DES | means channel 36.
|
The first parameter to ML/I may be followed by a list of names of up to five library items that are to be inserted.
If the parameter list to ML/I is null, card input is assumed. The following are therefore some sample calls of ML/I (written in card batch command format rather than teletype format):
&ML1; &ML1;C; &ML1;M2,LIB1,LIB2,LIB3; &ML1;DES,LIB1;
The parameter to ML/I specifies the initial input (i.e. the initial
value of S10). Dynamic changes can be made by resetting
S10. The relevant codes are listed in Section E.7. In many cases
the input routines add an extra newline at a switch of stream or at the
start or end of input, thus causing line numbers sometimes to differ
from those on other listings.
The following Sections give further information about the individual devices.
A card may contain any character that is acceptable by the card reader device routine. The following optional facilties are available with card input:
The input routine adds a newline character to the end of each card.
S11 and S12 can be set to restrict the columns of a card
that ML/I takes its input from. The rule is that ML/I ignores a
character on a card unless
S11 <= column number <= S12
The columns of the card satisfying this relation are called the
significant part of the card. Column numbers go from 1 to 80, and
initially S11 is 1 and S12 is 80, thus causing the entire
card to be examined.
If input is being listed the entire card is always printed, even if some of the columns are being ignored.
It is possible to cause the input routine to remove all spaces at the
end of the significant part of a card. The deletion of spaces is
controlled by the value of S15 as follows:
S15 = 0 | means delete all spaces at end of card.
|
S15 = 1 | means delete all but one space at end of
card.
|
S15 = 2 | means leave card untouched.
|
Of course if there are no spaces the the end of the card the value of
S15 is immaterial.
Initially S15 is 0, and this is the recommended option.
When card input is via ACIO a terminator card consisting of
three uparrows must be supplied. When input is not via ACIO
the standard terminator is a card containing three uparrows in
successive columns (normally the first three columns as for
ACIO); in addition the user may, using S13, specify the
7-bit code of any character which, when found in column 1, is to act as
a terminator. Initially S13 is set to the code for &.
However `&' cards should not be used as terminators under
batch operating systems since such cards would not be fed to ML/I but
taken as batch commands, thus prematurely ending an ML/I run.
In all cases a terminator card acts as a "halt code" to ML/I and the card itself is not subject to ML/I processing.
(This Section does not apply to ASSIGNed input).
A paper tape may contain any character that is acceptable by the paper tape input device routine (DRIP). Carriage returns and run out are ignored and line-feeds are treated as newline. If an illegal character is encountered the message
ERCHAR1 n
of DRIP is typed, and the character is ignored.
ML/I starts reading a paper tape from the first character, i.e., it does not, in contrast, for example, to MTEDIT, ignore characters up to the first line feed. The paper tape should end with a halt code. On encountering the halt code, ML/I closes the paper tape reader and outputs, on the control teleprinter, the message
TYPE C, H or T:
The user must then type one of the above letters. Their meanings are as follows:
C means continue. The user should reload the reader before typing
C. On typing C the paper tape reader is re-opened (thus,
on the interrupt version of DRIP, causing any characters read
beyond the previous halt-code to be ignored), and processing continues
with the next paper tape.
H means halt. This causes ML/I to end the current process.
T means switch to teleprinter input. A newline is fed to ML/I and
S10 is set to 2 thus causing subsequent input to be taken from
the control teleprinter.
If during paper tape input the input device is switched by changing
S10, the paper tape reader is not closed and, if input is
subsequently switched back to paper tape, it resumes where it left off.
(This Section does not apply to ASSIGNed input).
Teleprinter input is taken a line at a time. When ML/I wants a line of input it types a newline, the current line number and a colon. The user then types his line of input. Any character acceptable by the device routine EXTYPE (in single character mode) may be typed. The following characters have a special meaning:
ML/I can, if desired, take its input from items in the Algol library (SBL5) on magnetic tape or on disc. In particular, when magnetic tape is used, the source code can be placed in the Algol library and subsequently edited using the program MTEDIT (available from the Applications Group library) before being fed to ML/I.
The value of S10 used to select library item input is one hundred
plus the required handler number.
ML/I uses the standard program LIBRARY to access library items. It loads LIBRARY dynamically if it is needed and it is not already in core. Library items come from magnetic tape or from disc according to which version of the program LIBRARY is supplied.
Library items may be inserted within source coming from another device or the source text may consist exclusively of library item(s).
If ML/I is to take its input from the library at any point during processing (even if input is initially from some other device) the names of the library items to be inserted, in the order in which insertion is to take place, should be supplied as parameters at the end of the parameter list in the call of ML/I, e.g.:
&ML1;C,LIB1,LIB2;
If input is initially from the library ML/I inserts the library items in its parameter list one by one until it reaches the last one and then stops.
If input is initially from some other device and library item input is
selected by resetting S10, ML/I inserts one library item
and then switches input back to the device that was used initially. Thus
library items can easily be interspersed with other text.
At the end of each library item an extra newline is fed to ML/I.
If the call of ML/I is
&ML1;C,LIB1,LIB2;
where LIB1 and LIB2 are both library items on handler 3,
and the text on cards is of form
XXX MCSET S10 = 103 YYY MCSET S10 = 103 ZZZ
then the contents of LIB1 will be inserted between XXX and
YYY and the contents of LIB2 between YYY and
ZZZ.
The previous description should be adequate for most uses of library item input, but in special circumstances the user might like to have a more exact description of the mechanisms involved so that he may modify them to suit his own needs. This Section therefore provides a detailed description.
The library item facilities use the S-variables S30 to
S41.
S30 specifies the revert device. At the end of each library
item ML/I resets S10 to the value of S30. Initially
S30 is set to the initial value of S10 but its value can
be changed dynamically if desired (the same mechanism is used for the
insertion of ACIO streams).
On the first request for input from a handler, S31 and S32
are examined. If S31 is zero the current process is ended.
Otherwise the library item named by S31 and S32 is found
and the list of library items is shifted down one item. Thus S31
becomes S33, ..., S39 becomes S41. Hence the
name of the next library item (if any) will then be in S31 and
S32. At the start of each subsequent library item on the same
handler, or if input is switched to a different handler, the above
process is repeated. Note, however, that if, within a library item,
input is switched to, say, cards and then back again to the same
handler, input will remain where it left off in the original library
item.
It is quite legitimate for a process to end without using all the library items named as parameters.
By suitably modifying S30 to S41, it is possible for the
user to effect such facilities as the dynamic inclusion of library
items, the inclusion of more than five items, various unusual switches
of input device, etc.
The library item routines in ML/I are based on routines developed by W.H. Purvis of the University of North Wales.
When not running under DES there is a fixed designation of output
streams; the channel controlled by S21 goes to paper tape
punch one and the channel controlled by S22 goes to
the control teleprinter.
ML/I is serially re-usable. This means that after it has been used for one process it can be used for another without being reloaded. All S-variables, etc, are re-initialised at the start of each process.
However, there is one proviso to this. If one process executes a
MCALTER it will remain in effect for subsequent processes until
ML/I is reloaded.
Under the DES operating systems, it is possible to dump an ML/I environment to backing store so that it can be restored on later runs. The dump takes the form of core images in a special relocatable form. The main advantage of using dumps is speed. If a set of macros is to be used frequently, it may be best to "compile" them and dump them to backing store so that the resultant core images can quickly be retrieved on every subsequent run.
A dump is made at the end of an ML/I process, and is controlled by bits
2 to 4 of the variable S18 (bit 1 of S18 is already
spoken for -- see Section E.4). It is possible to dump selective parts
of the environment, though in practice, no doubt, it will most often be
required to dump a complete environment. If any of bits 2 to 4 of
S18 is one a dump is made; the individual bits control what is to
go into the dump, as follows:
The dump is always made to channel 200.
A sample deck to create a dump is as follows:
&ASSIGN;200;DC;2;LISTDUMP,BSR001; |
|
&DELETE;200; | (in case it exists already)
|
&ML1;DES; |
|
MCSET . . . |
|
MCDEF . . . |
|
| etc. |
|
MCSET S18 = -1 | (to set all bits in S18)
|
^^^ |
|
The dump is created as an SBL8 C-file, but, not being a character file, it cannot be subjected to the usual SBL8 operations, such as editing.
Note that two constituents of the environment are not dumped. These are
the delimiters of operation macros (which can be MCALTERed) and
the effects of MCNODEF, MCNOINS, MCNOSKIP and
MCNOWARN. MCALTERs can be used in creating the material to
be dumped, e.g.
MCALTER OR TO OC MCDEF OR AS . . .
In this case the definition of OR as a macro would be in the
environment when it was restored, but the effect of MCALTERing
OR to OC would not be restored. The effect is exactly as
if all delimiters and keywords were MCALTERed back to their
standard values before the dump was made.
When S-variables are dumped, some of them are omitted from the dump.
These are the S-variables that are set directly by the parameters of the
call of ML/I, i.e. S10 and S30 to S41. Moreover all
the bits of S18 except bit 1 are reset to zero before the dump is
made (otherwise, on restoring a dump, the extra bits in S18
would cause it to be re-dumped at the end of the run).
Dumps can be restored at the start of an ML/I run. It is possible to
restore more than one dump. For example if one dump contained an
environment to add list processing to Algol and another dump
contained an environment to add character manipulation to
Algol, then, provided the two were mutually compatible, both
could be restored for the same run. If the same construction is defined
in more than one restored environment the normal ML/I overriding rules
apply. It does not matter if the same construction is defined in several
environments (e.g. MCINS %.), except that it would
slightly slow down processing.
To cater for the problem of restoring values of the permanent variables for several different dumps, the following rules have been adopted:
As a consequence of this, if two dumps use different permanent variables then both can be restored simultaneously with suitable initial values of their own permanent variables.
To restore some dumps ML/I should be called with the parameter
REST. The dumps are assigned to channels 100, 101, 102, etc., an
unassigned channel indicating the end of the list. If any of the
assigned channels corresponds to anything other than a properly
constructed dump, the message:
REST ERR
is printed and the run abandoned. In detail the action of ML/I when the
REST parameter is given is as follows:
S2 (the line count) to zero and proceed as with a normal
&ML1;DES; run.
A simple example of the deck to restore a single dump is as follows:
&ASSIGN;100;DC;2;LISTDUMP,BSR001; &ASSIGN;31;... &ML1;REST; . . .
In the above case the user should check that channel 101 was not assigned (e.g. as a result of previous activity in the job).
ML/I accepts any character that the appropriate device routine accepts.
Error messages are output on the lineprinter as they occur (with reference to Chapter 6 of the User's Manual, the number 2N, the maximum number of characters inserted into an error message without truncation, is 64).
Illegal input characters ordinarily give rise to messages from the appropriate device routines but ML/I itself detects errors from cards, giving the message:
ILLEGAL INPUT CHARACTER
and substituting a dot for the wrong character.
If the variable S18 is one, then a list of all the construction
names in the environment is printed at the end of a process (this is
also done if a dump of the environment is made).
There is a number of extra error messages peculiar to this implementation and these are described in the Sections which follow.
Message
PARAMERR
(this message is output by OUTNAME both to the printer and to the
control teleprinter.)
Description
The call of ML/I has illegal parameters, or more than five library items have been named in the parameter list.
System Action
No processing takes place.
Message
ILLEGAL INPUT STREAM
Description
S10 has been set to a value not corresponding to any permissible
input stream, or library item input has been demanded when no library
item names were supplied in the call of ML/I.
System Action
The process is aborted.
Message
LIBRARY ITEM name NOT FOUND ON HANDLER number
Description
Self-explanatory.
System Action
The process is aborted.
Message
ACIO OVERFLOW
Description
The ACIO buffer has become too full.
System Action
The process is aborted.
All macro variables and constants used in macro expressions should be less in absolute value than 2^23-1. Overflow is not detected and its effect is undefined.
The initial environment contains ten permanent variables, which are all set to zero.
There are no extra layout keywords, beyond those described in the ML/I User's Manual.
The following table summaries the current uses of S-variables and their initial values. The indication char means "any legal 7-bit internal code character".
If S10 is set to an illegal value the error message of Section
E.4.2 is produced, but if any of the other S-variables have illegal
values the effect is undefined.
| Variable | Permissible values | Meaning | Initial value
|
S10 | 0 | card input | \
|
| 1 | paper tape input | | Set by
| |
| 2 | teleprinter input | | parameters
| |
| 100+n | input from library on handler n | | to ML/I
| |
| -n | input from ACIO channel n | /
| |
S11 | any | starting card column | 1
|
S12 | any | closing card column | 80
|
S13 | any | card terminating column | code for &
|
S14 | 0 | don't list source text | 0
|
| 1 | list source text |
| |
| 2 | list source text if S10 = S30 |
| |
S15 | 0 | delete all spaces at end of card | 0
|
| 1 | delete all but one space at end of card |
| |
| 2 | delete no spaces at end of card |
| |
S16 | any | code to be translated on input | -1
|
S17 | char | character to replace S16 | 0
|
S18 | any | controls environment listing and dumps | 1
|
| Variable | Permissible values | Meaning | Initial value
|
S20 | 0 | don't list output | 2
|
| 1 | list output |
| |
| 2 | list output with line numbers |
| |
S21 | 0 | Direct case: don't punch output | 1
|
| ACIO case: don't output to channel 31 |
| ||
| 1 | Direct case: punch output |
| |
| ACIO case: output to channel 31
| |||
S22 | 0 | Direct case: don't output to CTP | 0
|
| ACIO case: don't output to channel 33 |
| ||
| 1 | Direct case: output to CTP |
| |
| ACIO case: output to channel 33
|
| Variable | Permissible values | Meaning | Initial value
|
S30 | as for S10 | revert device | same as S10
|
S31 | see below | name of first library item | zero or
|
| and | parameter to
| ||
S32 | ML/I
| ||
S33 | ditto | names of subsequent items | zero or
|
| to | parameter to
| ||
S40 | ML/I
| ||
S41 | 0 | terminator of list | 0
|
Permissible values for the pairs of S-variables S31-S32,
S33-S34, ..., are zero, and the name of any permissible library item.