1.0 Introduction

The General Purpose Loader forms the load and go pass of compiling
processes and is automatically included in programs produced by the compilers.
The program's request slip and the loader program are read by EXECUTIVE
which then transfers control to the loader to read in and assemble the program.
When a program has been successfully read in, the loader will clear itself
out of store and enter the program at location 20.

The output of compilers is, in general, in the form of blocks which
are identified by their leading character. A block may contain up to 128 characters
although 80 is regarded as a normal maximum.

Blocks of semi-compiled program contain sequences of five-characters,
of which the first is a label and the other four together constitute a word. The
label informs the loader what it is supposed to do with the word.

The Loading Process

While it is loading a program, the loader keeps a "Transfer Address"
(T), together with 15 relativizers corresponding to the various program and data
areas. The loader provides for any or none of these relativizers to be added to
the word, and for loading it at location T. After loading, a number DT is added
to T. Normally DT is set at 1 but it may be reset at any value by the compiler.
In addition T itself may be reset relative to any of the program or data areas.

1.2 The Relativizers

The initial values of these relativizers are set by a block of the
consolidated-ieader, Relativizers 1-10 are incremented by an end of section
block. Relativizers 12 - 15 are set from the cue table.

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1.3 Nomenclature

Symbols used in this report (other than relativizer names given above)
are:-

For further clarification, values to be interpreted as octal will be
preceded by *, otherwise numbers may be assumed expressed in decimal.

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2.0 Blocks of Information

A block of information consists of a group of up to 80 characters.
This will be contained on one punched card, a strip of paper tape terminated by
'newline' or one 20 word block on magnetic tape. A block is identified by its
leading character and a block format is peculiar to its type.

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2.1 Object Program Block (Semi-compiled)

A block of object program, when placed in an input/output buffer
appears as follows:

The form of the semi-compiled program is given in section 3.

The end of block marker (*04) may occur anywhere in the word
immediately before the check sum.
{end of block was changed from *21 to *04 after November 1963}

The first W words of the buffer are check-summed using function
127 before the block is scanned.

Semi-compiled program is always loaded temporarily with a
parameter S added to the transfer address.

2.2 Object program block (binary program)

This block contains program which is absolute form (i. e. no
relativizers have to be added to it, and it may be loaded under control of
EXECUTIVE without recourse to the loader which itself is in this form).

Binary program is always loaded at the address specified and is
not stored temporarily with a displacement of S as is the case with semi-
compiled program.

The layout of a binary block is as follows:-

The check sum is performed as for semi-compiled program.

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2.3 Entry Point block

An entry point block consists of an entry and possibly a message
to be printed by EXECUTIVE on the monitor typewriter.
EXECUTIVE will type the message;

It is laid out as follows:

If no message is to be typed, the program is entered as indicated
without suspension. If a message is typed the program will be automatically
suspended.

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2.4 Title of a program segment

A title of a program segment must follow either a consolidated
leader or an end of segment block. If it occurs immediately after a block of
semi-compiled or binary program then the loader will be suspended and the
EXECUTIVE will type the message:

LOADER ERROR #05

In both cases the fields are terminated by spaces.

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When the loader detects such a block it will scan the cue-list
and if either the name does not appear or else a segment of this name has
already been read, the loader will continue reading ignoring all blocks until
the next "title of segment" block is found.

If the name is present and a previous segment has not been read
in, the value of relativizer #11 (RPR) is set to the value indicated by the cue
name and the cue list marked that the segment has been accepted.

2.5 End of segment block

An end of segment block terminates a sequence of semi-compiled
program. It contains a sequence of numbers which are treated as increments
for relativizers 1 - 10.

these are:

The increments are added to the various relativizers and the
cue-list examined. If all of the preset type (i, e. constants, literals, program
etc.) have been accepted then the wind-up phase is entered, this is described
in section 4.

If any preset segment is yet to come, the loader will return to
searching for a title.

2.6 Pause block

On encountering "pause" the loader switches the peripheral off
line to give the operator an opportunity to replace the tape or make some other
manual adjustment. On pressing "engage" the reading continues as before. The
remainder of such a block is always ignored.

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2.7 Consolidated Leaders

Block identifiers *14 to *17 indicate leader information - this
is more fully described in section 5. Only the first leader is recognized -
any subsequent leaders will be ignored.

2.8 Other block identifiers

Codes *05 - *12 will cause EXECUTIVE to type

LOADER ERROR #01

Codes *20 - *77 will be ignored whenever they occur.

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3.0 Semi-compiled program

Semi-compiled program consists essentially of a field of
characters of which the first one is a label (L), normally a field consists
of five characters, the last four of which form a 24 bit word (N). The
labels and the corresponding interpretation of the rest of the field are:-

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3.1 Normal loading

Usually a word to be loaded needs to have one of the relativizers
added to it and for it to be stored in one of the data areas. For example the
instruction:-

ADX 6 TIME

would require a transfer address setting (suppose it is instruction #203)

e. g. L = *73 N = 203

TIME would be identified by the compiler as Lower Variable #14, and ADX as
numeric function code 001.

Thus the semi-compiled version of this instruction would be:

L = *42 N = 001 6 0 14

And if RPR had been set at 316 and RLV at 89, the instruction would be
loaded at 519 as (001 6 0 103)

Note that the addition of the relativizer is to the least significant 15 bits only
and no further carry is propagated.

3.2 Masked Loading

Occasionally a compiler is not able to determine an address at the
time of compilation, as is the case of forward references to instructions. The
compiler then will compile the function part of the instruction as usual and will
later arrange to insert the address when it is known.

For example consider the instruction:

(200) BNZ 6 END

When the compiler reaches this instruction it has not yet located
the label END. It would thus compile

L = *73 N = 200
L = *40 N = (052 6 0 0)

Later in the compilation it will identify END as say 563 and will output

L = *73 N = 200
L = *00 N = *77777
L = *53 N = 563

The masking operation replaces those bits of the original stored
word with corresponding bits from the new relativized word. The masking is
performed on the next load operation only and this operation must occur in
the same block as the mask setting.

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3.3 Ignoring characters

The label L = *03 is designed to permit compilers to reproduce
source program and comments for listing purposes and for the loader to ignore
such information.

3.4 Transfer Address Increments

The transfer address is normally incremented by unity after each
load operation. The transfer address increment DT may be reset at any time
to any value (in particular to -1 for loading backwards). However, such a
setting will not carry from one program segment to another and DT is reset
to 1 automatically when a title block is read. In additional label L = *01 can
cause T to be incremented by N ( note that any increment at the previous load
is not ignored).

3.5 End of block marker

All blocks of semi-compiled program must be terminated by an
end of block marker (L = *04). This causes all subsequent information in the
block to be ignored.

3.6 Special Relativizers

Four special relativizers, labelled RR1 - RR4 are provided to
permit use of general purpose routines with multiple entry points. By convention
the first two are used for the I/O Package and an appropriate arithmetic
package.

The values of these relativizers are set by labels L = *10 - *13.
Following the label is a sequence of characters terminated by space (*20),
giving the name of the cue. The cue list is searched and the relativizer set
to the corresponding value. If the name is not in the cue list, the loader will
be suspended after typing

LOADER ERROR #02

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3.7 Other labels

The label L = *20 (corresponding to space) is always ignored and
the next character in sequence taken as the label.
Any labels not allocated a meaning will cause, the loader to be
suspended after typing

LOADER ERROR #03

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4.0 Entering the program

Whenever the loader comes across an end of segment block,
it examines the cue list to determine whether all the preset segments that
are required, have in fact been read. If some are still to come the loader
will switch to searching for the next title block. If all segments have been
read the following action is taken: -

(a) Program is moved down S locations (N. B. S is a
parameter supplied by the compiler as part of the consolidated
leader).

(b) The area immediately above the program area is
cleared for S locations. (N.B. The address at which clearing
will commence is taken as the original setting of RUW in the
consolidated leader).

(c) The area (if any) between location 30 and the start of
the preset information is cleared. (N.B. The address, up to
which the store will be cleared will be given by the original setting
of RCN in the consolidated leader).

(d) The program is entered at location 20.

4.1 Setting of Entry Points

Locations 20-29 are reserved for program entry points. These
locations should be set by the compiler in absolute form using the binary load
facilities (Block type *01) or else it should arrange to set the transfer address
using label *05 to ensure that the word is stored in the area 20-29 which is
not affected by the final shift.

An entry point is normally an unconditional branch to an appropriate
location in the program area.

The normal entry point is assumed to be in location 20. If this
location is not set by the compiler the loader will type

LOADING COMPLETE

on reaching this point and suspend itself. The operator should then type (for
example)

GO 0#NAME AT 75

to activate the program.

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5.0 The Consolidated Leader

Leader information produced whenever a compilation is concluded.
At this stage the program may or may not be complete. If it is not complete
the leaders must be processed by the consolidation phase which produces a
consolidated leader.

If multiple copies of (a), (b) and (d) are included in the program,
the first only is read, all others being completely ignored.

Cues may be read in any order and they are added to the cue list
when read. All cues following the terminator are ignored.

When the terminator block is read the loader checks that at least
one each of (a) and (b) have been read. Otherwise it will type:-

LOADER ERROR #06

and suspend itself.

All incomplete leader information is always ignored.

5.1 Relativizers

A relativizer block consists of the first character (B=* 15)
followed by ten octal fields giving the initial settings of relativizers 1-10.
Fields are separated by at least one space.

When this block is read, Relativizers 4 and 9 are stored for
future use to determine areas to be cleared.

5.2 Parameters

The Parameter block consists of the first character (B=*16)
followed by two octal fields giving the values of the various parameters these
are:

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5.3 Cue Block

The octal and alphanumeric fields are terminated by at least one
space character.

The type and value of a cue is specified by the cue "data word"
which is split into the 9 most significant and the 15 least significant bits. The
15 least significant bits determine the "cue value" and the 9 most significant
define the "cue type".

5.4 Cue Types

The type bits are numbered from B0 to B8 (B0 being the "sign"
bit of the data word. Meaning is allocated as follows:

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5.5 Cue Values

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6.0 Reading and writing modes

The modes of reading by the loader, and writing by the compiler
are defined below to ensure compatibility.

6.1 Paper Tape

The loader reads paper tape always in mode 10; This has the following properties:

A compiler should punch a block of N characters using mode 8
and follow this with newline and blank tape. To produce this the compiler
should output the two words:-

*77762076

*20762000

in mode 0, using a control word count of 7. This gives newline followed by
3 blank characters.

Leader information which may be typed will be accepted by the
loader out it should be typed in strict adherence to the restrictions given in
section 5 and use only characters in the restricted binary set.

6.2 Punched Cards

Punched cards will always be read as alphanumeric characters
(mode 0) and should be punched in the same way.

{RDM: By convention the three compilers only used columns 1-72 for GPL/CNSL data;
columns 73-76 contained the the first four characters of the module name;
columns 77-80 was a sequence number. A twelve overpunch in column 77 denoted leader;
a twelve overpunch in column 80 denoted the last card
of the semi-compiled program or the leader.}

6.3 Magnetic Tape

Magnetic tape is read in mode 0 using a standard block length of
20 words. It is assumed that the information was put on the tape by a card-
tape or paper tape-magnetic tape transcription program which would write on
tape in mode 1.

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7.0 Error messages

When the loader detects an error in the data presented to it
(e, g. an illegal identifier) it will print a message, of the form:-

LOADER ERROR #03

and will be suspended with the input peripheral disengaged. To restart,
the operator should move back two blocks, that is reinput the last two
cards in the output stack or move the paper tape back to the blank section
before last:-

The peripheral should then be re-engaged and a message typed on the console
typewriter

GO 0#NAME

where "NAME" is the name of the program being loaded.

7.1 Error message Summary