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
To run ML/I, the user types the following:
|Loads ML/I and enters it
|Assigns required I/O channels
|Null command starts the process
|Control-C terminates input
|Returns to command status
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$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
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 . . .
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.
initialised to 1, so input is normally from the teletype at the start of
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
The input terminator is the 8-bit character given by the least
significant 8 bits of
S13. It is initialised to decimal 131
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),
S17 should be set
in the following way:
MCSET S16 = 155 MCSET S17 = 192
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:
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:
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
S21 is one then output is sent to channel
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
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
S24 for channels
O4 respectively, being the
character whose 8-bit code is given by the least significant 8 bits of
the appropriate S-variable. Both
initialised to decimal 131 (octal 203,
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
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
|Assign declarations channel
|Assign code channel
|... process to generate output ...||
|Assign declarations to input channel
|Assign code to input channel
|Input from channel |
|Input from channel |
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
S20 is zero, no listing is produced; if
is one, a listing without line numbers is produced; if
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
To suppress the context print-out (but not the message itself),
S4 should be set to 1. To allow the print-out,
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.
ILLEGAL INPUT STREAM
S10has been set to a value not in the range 1 to 4.
The current process is aborted.
ILLEGAL DEBUGGING CHANNEL
S28has been set to a value not in the range 1 to 4.
S28is set to 1, the above message is output using this value, and processing continues.
Occurs after a non-continuable error such as a break or an attempt to use a non-existent input channel.
The process is aborted,
A$EPis 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:
|meaning a space.
|meaning a newline.
|meaning the character backslash (octal 334).
|meaning the imaginary startline character.
|meaning a sequence of one or more spaces.
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 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.
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.
S1is one, the imaginary startline character is inserted on input. If
S1is zero, no startlines are inserted; this is the initial setting.
S2; it may be changed at any time.
S3is one, the error message normally generated if a warning marker is not followed by a macro name is suppressed. If
S3is zero (the initial value), the message is produced.
S4is one, the context print-out normally given after a call of
MCNOTEis suppressed. If
S4is zero, the context print-out is given; this is the initial setting.
S14is zero, the source text is not listed. If
S14is one, the source text is listed. The initial value is 0.
S16are translated to characters with the ISO 8-bit code given by
S17, on input. Initially
S16is -1, so no translations are performed.
S18is one at the end of a process, a list is given of all currently defined constructions. This is output to the message stream. If
S18is zero, the list is not produced. The initial value of
S20controls output to the listing file. See Section I.2.4.2 for details.
S21controls output to channel
O2; see Section I.2.4.2 for details.
S22controls output to channel
O4; see Section I.2.4.2 for details.
S23contains the ISO 8-bit code for the terminator character to be written to channel
O2. It is initially set to decimal 131 (octal 203).
S24contains 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. It may be changed if desired.
S26is zero, write the terminating character to channel
O2at the end of the process. If
S26is one, do not write the terminating character to the channel. The initial value is 0.
S27is zero, write the terminating character to channel
O4at the end of the process. If
S27is one, do not write the terminating character to the channel. The initial value is 0.
S30contains the current revert channel. See Section I.2.3 for details.