Copyright © 1973 R.D. Eager
Permission is granted to copy and/or modify this document for private use only. Machine readable versions must not be placed on public web sites or FTP sites, or otherwise made generally accessible in an electronic form. Instead, please provide a link to the original document on the official ML/I web site.
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 4 as described in the supplements to that manual (startlines, stop
markers, controlled line numbers and optional warning markers). In
addition it contains a facility for suppressing the context print-out
following a message produced by a call of the operation macro
MCNOTE.
To run ML/I, the user types the following:
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ML1 | Loads ML/I and enters it
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AC I2 PR O2 TT O3 LP | Assigns required I/O channels
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| Null command starts the process
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MCSKIP MT,<> |
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MCDEF PIG AS DOG |
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PIG |
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^C | Control-C terminates input
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| Returns to command status
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ML/I is available as a self-loading paper tape, also as part of the ADMOS object program library on disc and/or magnetic tape.
ML/I occupies about 5.8K words of store. The entire area between
A$TL and A$FC (the free store) is used as workspace. At
least 2K of workspace is normally required. Adding the size of the
operating system to this, it will be seen that ML/I will just run in 12K
but will easily run in 16K.
The actual amount of workspace available (in words) is placed in
S29 at the start of a process for the user's information. However
it is not subsequently updated unless ML/I is rerun, and cannot
therefore be used during a run to monitor how much workspace remains.
ML/I has quite comprehensive I/O facilities. These are controlled by the
settings of the S-variables S10-S30. The meanings of the
S-variables are described in the sections which follow. In particular,
the input medium is controlled by S10.
Input/output options may be changed dynamically during a process by changing the values of S-variables. Note, however, that if there exists a construction of form:
NL WITHS . . .
or
NL WITH . . .
ML/I needs to look ahead a line in its source text. Hence in this case a
switch of input device will not come into effect immediately, and it is
necessary to place a blank line after the MCSET that effects the
change of input device.
The following sections give a comprehensive description of all the I/O options and the S-variables that control them. A summary of the usage of S-variables appears in Section I.7.
In the sections which follow "8-bit code" means the ISO 8-bit forced-bit-8 character code. "Newline" means the character (value octal 212, in fact) that ML/I uses internally to indicate the end of a line. On output newline is automatically translated back to carriage return, line feed as necessary. On input all characters have bit 8 forced in them before processing.
ML/I can use all four ADMOS input channels. The value of
S10 gives the channel number to be used. S10 is
initialised to 1, so input is normally from the teletype at the start of
a process.
If a change of input channel is made, the original channel is not "forgotten". Any attempt to input from this channel again will cause ML/I to carry on where it left off. When the end of an input channel is reached, ML/I checks to see if it is the revert channel. If it is, the process is terminated; otherwise input is switched to the revert channel.
The revert channel is given by the value of S30, and is
initialised to 1, so that input normally reverts to the teletype after,
say, reading some input from a disc file. It may, however, be changed at
will.
The input terminator is the 8-bit character given by the least
significant 8 bits of S13. It is initialised to decimal 131
(octal 203, Ctrl-C) but may be changed, for instance, to read a
non-standard paper tape.
The source text may be listed, if desired, on channel O3. This is
done by setting S14 to 1. If S14 is zero, as it is
initially, no source listing is produced. Lines of source listing are
preceded by twenty asterisks and a line number. The last line is listed
even if it consists only of a terminator, but is not included in the
final count of source lines.
It is possible to designate that one character be translated to another on input. This makes it possible to input a character that a device does not support, for example a backarrow from the teletype. 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 8-bit ISO code of the character to be translated, and
S17 to the 8-bit ISO code of the
character that is to replace it. For example, if %
(internal code decimal 155) was to represent a @
(internal code decimal 192), S16 and S17 should be set
in the following way:
MCSET S16 = 155 MCSET S17 = 192
Initially, S16 is set to zero, which is the code for runout which
is ignored by the input routine anyway. Thus no translations are made.
The ordering of input operations is as follows:
S16 and S17.
No input line should exceed 72 characters in length, including the carriage return at the end. If 71 characters are input and no carriage return is found, then a newline is artificially inserted.
All four ADMOS output channels may be used by ML/I:
O1
O2
O3
O4
The message output channel is given by the value of S28. It is
initialised to 1 (defaulting to the teletype) but may be changed
dynamically if required.
Output may be sent to either, both or neither of channels O2 and
O4. If S21 is one then output is sent to channel O2
and if S21 is zero then output is not sent to this channel.
S22 controls channel O4 in the same way. The maximum
length of an output line on either channel is 72 characters plus
carriage return, line feed. If an attempt is made to output a longer
line on either channel, then a carriage return followed by a line feed
is artificially inserted on that channel. Such a case is not apparent
from the listing on channel O3, since the line overflow may only
occur on one output channel.
If a channel is used for output during a process, then an end-of-file
character is written to that channel at the end of the process. This may
be suppressed by setting S26 to 1 in the case of channel
O2, or S27 to 1 in the case of channel O4. If the
channel is not used at all during a process (e.g. because output is
suppressed) then no end-of-file is written.
The end-of-file character is determined by the values of S23 and
S24 for channels O2 and O4 respectively, being the
character whose 8-bit code is given by the least significant 8 bits of
the appropriate S-variable. Both S23 and S24 are
initialised to decimal 131 (octal 203, Ctrl-C).
Both the facilities should be used with extreme caution, especially if the output channels are assigned to disc or magnetic tape files, as a corrupt unterminated file could result. They are primarily intended for producing non-standard paper tapes for use on other machines.
The line number of the output text (even if such text is sent to null or
suppressed) is maintained in S25, so that it is accessible if
required. It may be changed at will.
It can be seen that it is possible, by dynamically switching the values
of S21 and S22, to generate two separate sets of output
from ML/I; for example when generating assembly code, declarations could
be sent to channel O2 and code to channel O4. These two
channels could subsequently be merged, if need be, by a second ML/I
process, e.g.:
AC O2 DC T 1 | Assign declarations channel
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AC O4 DC T 2 | Assign code channel
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| Start process
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| ... process to generate output ... |
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Ctrl-C | Input terminator
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AC I2 DC T 1 | Assign declarations to input channel
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AC I3 DC T 2 | Assign code to input channel
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| Start process
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MCSET S10 = 2 | Input from channel I2
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MCSET S10 = 4 | Input from channel I4
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Ctrl-C | Input terminator
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Messages are omitted from the above for clarity.
Output channel 3 may be used for source and output listings, also for
error messages if S28 is set to 3 during processing. These are
all interspersed with each other. If both source and output are listed
simultaneously an extra carriage return, line feed is sent to channel
O3 before each line of the source listing in order to separate
source from output.
S20 controls whether the output text is to be listed on channel
O3: 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 to the count
of source lines. Initially S20 has the value two.
Lines of listing are 91 characters long, plus carriage return, line feed. If a longer line needs to be listed, an artificial carriage return, line feed is inserted after 91 characters have been output on a given line. Note that this insertion will not in general correspond with any artificial insertion on an output channel, since the line length is 72 characters on such a channel. However, the artificial newline insertion on input is listed.
ML/I is serially reusable. 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. This allows, for instance, a string of paper tape
processes to be run under some delimiter settings that have been
MCALTERed by typing at the teletype in a previous process.
Usually an error (e.g. a break, whether from the keyboard or forced by the system) causes a return to command status within ML/I. However should control be lost from ML/I it may be restarted from octal 1000, after assigning I/O channels.
ML/I uses forced-bit-8 characters throughout. Bit 8 is forced in all characters input, and all characters output have bit 8 set in them. It accepts the full set of ISO code characters with codes between octal 240 and 337 inclusive. In addition it accepts carriage return (which is taken as "newline"), also linefeed and delete characters which are both, however, ignored.
It will also accept lower case alphabetic characters, which are converted by the input routine into their upper case equivalents. Note that this is in contradiction to the ML/I User's Manual, as they are not thereafter treated as separate characters, and if output, will be output as upper case. The reason for this is to allow input to be taken from the VISTA terminal without having to continually shift out when a letter is required.
Error messages are output on the channel given by the value of
S28, as they occur.
With reference to Chapter 6 (of the ML/I User's Manual),
the number 2N (the maximum number of characters in a piece of
text inserted into an error message without being truncated) is 60.
Illegal input characters give rise to the appropriate message given in
Section 6 of the ML/I User's Manual. The input routine then
replaces them by the error character, which is an exclamation mark
(!).
One addition not mentioned in the ML/I User's Manual is the
facility for suppressing the context print-out on the message stream
after a message produced by a call of the operation macro MCNOTE.
To suppress the context print-out (but not the message itself),
S4 should be set to 1. To allow the print-out, S4 should
be set to 0. Initially S4 is zero.
Extra diagnostic information, including a list of the names of all
active constructions, is automatically printed at the end of each
process. It may be suppressed by setting S18 to zero. An
end-of-file (decimal 131, octal 203) character is written to the message
stream at the end of each process. Note that this only applies to the
channel in use at the end of the process, and not any intermediate ones.
The user should thus be wary of using disc or magnetic tape for error
messages unless they are used at the end of the process. However if the
same channel is being used for output text the terminator for the output
stream will suffice to close the channel, and this does not matter.
There are a number of extra error messages peculiar to this inplementation and these are described in the sections which follow.
Message
ILLEGAL INPUT STREAM
Description
S10 has been set to a value not in the range 1 to 4.
System Action
The current process is aborted.
Message
ILLEGAL DEBUGGING CHANNEL
Description
S28 has been set to a value not in the range 1 to 4.
System Action
S28 is set to 1, the above message is output using this value,
and processing continues.
Message
PROCESS ABORTED
Description
Occurs after a non-continuable error such as a break or an attempt to use a non-existent input channel.
System Action
The process is aborted, A$EP is set to point to the command input
routine within ML/I, and command status within ADMOS is
entered without printing the diagnostic information referred to above.
No recovery is possible as the run-time stack within ML/I may be
corrupt.
The initial environment contains ten permanent variables, all set to the value zero. All integers in, or derived from, macro expressions should be less than 32768 in magnitude. Overflow is not always detected, except in the case of division by zero, and its effect is undefined.
The following are the layout keywords for this implementation:
SPACE | meaning a space.
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NL | meaning a newline.
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TAB | meaning the character backslash (octal 334).
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SL | meaning the imaginary startline character.
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SPACES | meaning a sequence of one or more spaces.
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One implication of the treatment of the TAB keyword is that the
backslash character is ignored within structure representations, and
must be specified by means of TAB. In the same way, any
occurrence of a backslash within an error message will be replaced by
the message (TAB).
The reason for this is that the backslash is often used as a field delimiter in the same way as tab (e.g. by the DAP-16 assembler) and a tab is peripheral-dependent.
The following sections summarise the current uses of S-variables, and their initial values.
If S10 or S28 are set to illegal values the appropriate
error message of Section I.4 is produced, but if any of the other
S-variables have illegal values the effect is undefined. It is dangerous
to change the values of S-variables not mentioned below.
S1-S9S1
S1 is one, the imaginary startline
character is inserted on input. If S1 is zero, no startlines are
inserted; this is the initial setting.
S2
S2; it may be changed at any time.
S3
S3 is one, the error message normally
generated if a warning marker is not followed by a macro name is
suppressed. If S3 is zero (the initial value), the message is
produced.
S4
S4 is one, the context print-out
normally given after a call of MCNOTE is suppressed. If
S4 is zero, the context print-out is given; this is the initial
setting.
S5
S6
S7
S8
S9
S10-S30S10
S11
S12
S13
S14
S14 is zero, the source text is not listed. If S14 is
one, the source text is listed. The initial value is 0.
S15
S16
S16 are translated to
characters with the ISO 8-bit code given by S17, on
input. Initially S16 is -1, so no translations are
performed.
S17
S16 above.
S18
S18 is one at the end of a process, a list is given of all
currently defined constructions. This is output to the message stream.
If S18 is zero, the list is not produced. The initial value of
S18 is 1.
S19
S20
S20 controls output to the listing file. See Section
I.2.4.2 for details.
S21
S21 controls output to channel O2;
see Section I.2.4.2 for details.
S22
S22 controls output to channel O4;
see Section I.2.4.2 for details.
S23
S23 contains the ISO 8-bit code for the terminator
character to be written to channel O2. It is initially set to
decimal 131 (octal 203).
S24
S24 contains the ISO 8-bit code for the terminator
character to be written to channel O4. It is initially set to
decimal 131 (octal 203).
S25
S25. It may
be changed if desired.
S26
S26 is zero, write the terminating character to channel
O2 at the end of the process. If S26 is one, do not write
the terminating character to the channel. The initial value is 0.
S27
S27 is zero, write the terminating character to channel
O4 at the end of the process. If S27 is one, do not write
the terminating character to the channel. The initial value is 0.
S28
S29
S30
S30 contains the current revert channel. See Section I.2.3 for
details.