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in a specified column (with the exception of the label field).
The ru
I
es for wri ti ng free-form symbol i c lines are:
1.
The assembler interpret's the fields from left to right:
label; command; argument; comments.
2.
A blank column terminates any field (except the com-
ments field, which is terminated at column 72 on card
input or by a new line character on paper tape input).
3.
One or more blanks at the beginning of a line specifies
there is no label field.
4.
The label field, when present, must begin in column
1.
5.
The command field begins with the first nonblank col-
umn following the label field or in the first nonblank
columnfollowingcolumn
1
if th'e label field is omitted.
6.
The argument field begins with the first nonblank col-
umn following the command field.
An argument field
is designated as blank in either of two ways:
a.
Eleven or more blonk columns follow the command
field.
b.
The end of the active line (column 72) is encoun-
tered, less than
10
blank columns appear after the
command field, and the active
I
ine is notcontinued.
7.
The comments field beSJins in the first nonblank column
following the argument field or after at least
11
blank columns following the command field when the
argument field is empty.
A source statement may consist of one to four entries written
on a coding sheet in the appropriate fields: a label field
entry (optional), a command field entry (required), an argu-
ment field entry (usually required), and a comments field
entry (optional).
A label entry is a symbol that identifi,;!s the statement in which
itappears. The label enables a programmer to refer to a spe-
cific statement from other statements within his program.
The command entry is a mnemonic code representing a machi ne
i nstructi on or assembler directive specifyi ng, respectivel y, the
machine operation or assemblerfunction to be performed. A
commond entry is required in every ()ctive line. Thus, if a
statement
I
ine is entirely blank following the label field or if
the command entry is invalid (i. e., notan acceptable instruc-
tion ordirective), the assemblerdecklres the statement in error,
generates a word of all zeros in the object program, and flags
the statement in the assembly listing. The mnemonic codes for
machi ne i nstructi ons and assembler directives recognized by
the assembler are listed in Appendixes A and B, respectively.
An argument entry consists of one or more symbols, con-
stants,
I
iterals, or expressions separa'red by commas. The argu-
ment entries for machine instructions usually represent such
things as storage locations or const<:mt values.
Arguments
for assembler directives provide the information needed by
Extended Symbol to perform the designated operation.
A comments entry may be any information the user wishes to
record.
It is read by the assembler and is output as part of
the source image on the assembly listing.
Comments have
no effect on the assembly.
STATEMENT CONTINUATION
The semicolon (;) may be used in a statement to signal the
continuation of the statement on the subsequent lines.
This
continuation code may be placed following a label entry,
following a command entry, or within an argument entry.
It must not follow the last character of the label orcommand
entry.
If
it is within a character string enclosed by single
quotation marks, or is a character in the comments field,'the
semicolon does not cause continuation. A maximum of two
continuation lines may be used for each statement.
Example
1.
Statement Continuation
BEGIN
LDA
A·
,
Continuation
NEW
-tB
TEXT
'A;B '
LOCAL A, START, R 1, ;
; is not a continua-
tion character
D,RATIO,B12,
i
Continuation
C,MAP
Leading blanks on continuation lines are ignored by the
assembler.
Thus, significant blanks that must follow
label or command entries must precede the semicolon
indicating continuation.
ANS
LDA
The blank that ter-
SUM"
1
minates the command
field precedes the
semicolon.
COMMENT LINES
An entire line may be used for a comment by writing an as-
terisk in column 1. All valid characters may be used in
comments.
Extensive comments may be written by using a
series of lines, each with an asterisk in column 1.
The assembler reproduces the comment lines on the assembly
listing and counts comment lines in making line number
assignments.
PROCESSING OF SYMBOLS
Symbols are used in the label field of a machine instruction
to represent its
I
ocati on in the program.
In the argument
field of an instruction, a symbol identifies the location of
an instruction or a data value.
The treatment of symbols that appear in the label orargument
field of an assembler directive varies. The description in the
following chapters define the use of symbols in directives.
Processing of Symbols
9
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