Xerox 530 User manual

Xerox Extended Symbol
Xerox530 and Sigma 2/3 Computers
.Language and Operations
. Reference Manual
90 10 52F

© 1972, Xerox Corporation
Xerox Extended Symbol
Xlerox 530 and Sigma 2/3 Computers
language and Operations
Reference Manual
90 10 52F
Apri I 1976
XEROX
File No.: 1 X23
XH36, Rev. 0
Printed in U.S.A

REVISION
This edition merely incorporates the 90 10 52E-l and 90 10 52E-2 revision packages into the manual. Changes in
the text from that of the previous manual are indicated by a vertical line in the margin of the page.
RELATED PUBLICATIONS
Title
Publ ication No.
Xerox 530 Computer/Reference Manual
90 1785
Xerox Sigma 2 Computer/Reference Manual
9009 64
Xerox Sigma 3 Computer/Reference Manual 90 15 92
Xerox Real-Time Batch Monitor (RBM)/RT, BP Reference Manual
90 10 37
Xerox Real-Time Batch Monitor (RBM)/OPS Reference Manual
90 1555
Xerox Real-Time Batch Monitor (RBM)/Userls Guide
90 1960
Xerox Symbol/LN, OPS Reference Manual
90 10 51
Manual Content Codes: BP - batch processing, LN - language, OPS - operations, RBP - remote batch processing,
RT - real-time, SM -system management, TS - time-sharing, UT - utilities.
The specifications of the software system described in this publication are subject to change without notice. The availability or performance af some features
may depend on a specific configuration of equipment such as additional tape units or larger memory. Customers should consult their Xerox sales representative
fOI details.
ii

CONTENTS
PREFACE
vi
Address LiteraIs
22
ADRL
23
1.
INTRODUCTIO N
5.
LOCATION COUNTERS AND PROGRAM
Programming Features__ SECTIONS
24
Error Detection ____
Extended Symbol Language Locati
on Counters
24
Extended Symbol Processor Setting the Location Counters
24
( ORG
24
LOC
25
2.
EXTENDED SYMBOL L/\NGUAGE BOUND
25
ELEMENTS A ND SYNTAX 3
RES
26
COMMON
26
Language Elements __
3 Program Sections
26
Characters ____
3 ASECT/CSECL
26
Symbols 3
Constants ____
4
Expressions 5
6.
EXTENDED SYMBOL DIRECTIVES
28
Litera Is _______
7
Syntax 8
DATA
28
Statements ____
8
DEF
29
Statement Continuatiion 9 DISP
29
Comment Li nes ___
9 DO/ELSE/FIN 30
Processing of Symbols __
9 END
32
Defining Symbols __ 10
EQU
33
Redefinable SymbolL
10
GEN
33
Symbol References __ 10 GOTO
34
Classification of Symbols
10
IDNT 35
Symbol Tables ___
10
LBL
35
Absolute and Relocatable Values
11
LIST
35
Symbols
11
LOCAL
36
Expres'ions
11 PAGE
37
PCC
37
REF
37
3.
XEROX 530 AND SIGMA 2/3 MACHINE
S:STEP 37
INSTRUCTIONS
13
SET
38
SOCW
38
Class 1:
Memory Refere:nce Instructions 13 SPACE
38
Single Precision Clas;s 1 Instructions
13
SREF
38
Multiple Precision Class 1 Instructions
15
TEXT
39
Field Addressing Class 1 Instructions
15
TEXTC 39
Class 2:
Conditional Br<elnch Instructions 16
TITLE
39
Class 3:
Shift Instructions
16
Class 4:
Copy Instructions
16
7.
PROCEDURES
41
Class 5:
Input/Output Control Instructions ___
17
Procedure Format
41
CNAME
41
4.
ADDRESSING
18
PROC
41
PEND
42
Argument Addressi ng Format
18
Procedure References
42
Direct Addressing--
19
Procedure-LocaI Symbol Reg ions
42
Indirect Addressing __
19
Intrinsic Functions
43
BASE ____
19
ABS
43
Symbolic-Relative Addressing 19
AF
44
Automatic Addressing
__
20 AFA
44
Address Generation Dia9nostics 20 AFNUM
45
Literal Pools ____
21 AFR
45
LPOOL
21
AT
45
iii

CF
46 D.
CONCORDANCE PROGRAM
74
CFNUM
46
CFR
47
Introduction
74
UFV
47
Concordance Listing
74
Sample Procedures 48
Local Section
74
Nonlocal Section
74
Proc Section
74
8.
OPERATIONS
50 Opcode Section
74
Concordance Control Command
75
RBM Control Commands
50
Section Control Commands
75
JOB Control Command
50
Error Alarms
75
ASSIGN Control Command 50 Compatibility
76
DEFINE and TEMP Control
Cards
(Temporary Fi Ie Definition) 51
XSYMBOL Control Command 51
E.
EXPANSION OF SIGMA 3 SIMULATED
BA 51 INSTRUCTIONS
77
BO
52
CR
52 No Indirect Addressing 77
DW
52 Indirect Addressing
77
GO
52
LO
52
LU
52
NP
52
PP
52
FIGURES
SL
52
SO
52
l.
Extended Symbol Character Set
3
SS
52
UI
53
2 •.
Xerox Sigma Symbolic Coding Form 8
Updating a Source Program 53
Standard Object Program Format 57 3. Flowchart of DO/ELSE/FIN Loop 31
Object Module Records 57
Load Items 57
4. Deck Setup for Assembling Multiple
Assembler Diagnostics
57 Programs Using the BA Option
54
F1ags
57
Error Messages
58 5. Deck Setup for Assembl ing Multiple
Assembly Listing
63 Programs Without BA Option
54
Summary Tables 63
6. Deck Setup for Using the UI Option With
INDEX
79
the BA Option
55
7.
Deck Setup for Using the UI Option Without
the BA Option
56
APPENDIXES
8. Sample Update Listing With Errors
58
A.
SUMMARY OF XEROX 530 AND SIGMA 2/3
9.
Assembly Listing Format
64
INSTRUCTIONS
67
10.
Assembly Listing
64
Memory Reference Instructions (C lass 1)
67
Basic Set
67
D-l.
Sample Concordance Deck Setup
76
General Register Set
67
Floating-Point Set
67
D-2.
Concordance From Xl RAD
File
Multiple Precision Set
67
Following an Assembly
76
Field Addressing Set
68
Conditional Branch Instructions (Class 2)
68
Shift Instructions (Class 3)
68
Copy Instructions (C lass 4)
68
Input/Output Instructions (Class 5)
68
TABLES
B. EXTENDED SYMBOL DIRECTIVES
69
l. Extended Symbol Operators
6
C. INCOMPATIBILITIES BETWEEN EXTEN DED
SYMBOL AND SYMBOL
73
2. Error Messages
59
iv

EJCAMPLES
21.
GEN2 Directive
34
22.
GOTO Directive
35
l.
Statement Continuaf'ion 9
23. LOCAL Directive
36
2.
Expressions Using +and - Operators 12
24. LOCAL Directive 36
3.
Expressions Using Miscellaneous Operators __
12 25. LOCAL Directive 36
4.
Automatic Addressing 21 26. REF Directive 37
5.
LPOOL Directive __
21
27.
SET Directive
38
6.
ADRL Directive __
23
28. TEXT Directive 39
7. ADRL Directive 23
29.
TEXTC Directive 39
8.
ORG Directive __
25
30.
TITLE Directive
40
9.
LOC Directive ___
25
31. Procedure Definition/Procedure Reference 42
lO.
BOUND Directive __
25 32. Procedure-Loca I Symbol Regions 43
1l.
RES Directive ___
26 33.
ABS Function
44
12.
ASECT and CSECT Directive
27 34. AF/AFA Function
44
13.
DATA Directive __
29 35. AFNUM Function 45
14.
DEF Directive ___
29 36. AT Function
46
15.
DO/FIN Directives.
30
37. CFNUM Function
46
16.
DO/ELSE/FIN Directives
32
38. CFR Function
47
17. DO/FIN Directives _ 32
39.
UFV Function
47
18. END
Directive ___
32 40.
AT and UFV Functions
48
19.
GEN Directive ___
34
41. Conditional Code Generation
48
20.
GEN1 Directive __
34
42. Procedure that References a Procedure 48
v

vi
PREFACE
This manual describes the Xerox Extended Symbol assembly system for the Xerox 530 and Sigma 2/3 computers. It
defines a symbolic programming language and the general operations of the processor under control of the Real-Time
Batch Monitor.
It is intended for use as a reference document by experienced programmers and does not aim to be a programming
primer. It is assumed that the reader is familiar with the basic elements of digital computer programming and with
the description of the Xerox 530 or Sigma 2/3 computers as given in the appropriate computer reference manual.

1. INTRODUCTION
Extended Symbol, the extended assembly system for
Xerox 530 and Sigma 2/3 computers, is both a programming
language and a language processor. The Extended Symbo I
processor accepts as input a source program coded in either
Symbol or Extended Symbol, processes it, and outputs an
object module, diagnostic messages, and an assembly list-,
ing. The objed language format is described in the RBM/
RT, BP Reference Manual, 90 10 37; the diagnostic mes-
sages and the format of the assembly listing are described
in Chapter 8 of this manual.
PROGRAMMING FEATURES
Extended Symbol provides the programmer with a number of
convenient features:
• Forward references, literals, and external definitions
and references simplify the task of referring to other
program elements.
• Local and nonlocal symbols can be specified and used,
• Self-defining constants focilitate use of hexadecimal
and decimal values and character strings.
• Expressions consisting of terms and arithmetic and log-
ical operators may be used as arguments in machine in··
structions and directives.
• Automatic oddressing is invoked by the assembler when
an address value is encountered that is outside the
range allowed for the stat'ement.
• ASECT and CSECT directives allow the partitioning of
a program into absolute and relocatable sections.
• The GOTO directive allows the assembler to condition-
a Ily a I ter the sequence in Vlh i ch statements a re assernbI ed.
• TEXT and TEXTC directivl~s simplify the coding of out-
put messages.
• User,-defined procedures allow the programmer to gen-
erate different sequences of code as determ ined by
conditions existing at ossembly time.
• The GEN directives provide the facility for generating
Class 1 and Class 2 machine instructions.
•
"Common" space may be shared with FORTRAN or other
Extended Symbol subprogrclms.
ERROR DETECTION
During assembly a source progrclm is checked for errors in usage
and syntax. If an error is found, appropriate noti fi cation is
given and the asse mbl yoperation conti nues so that all errors
may be 10c:Jted at one time. An assembly is terminated pre-
maturely (aborted) only if an irrecoverable I/O failure oc-
curs, or one of the assembler tables is exceeded.
EXTENDED SYMBOL LANGUAGE
The Extended Symbol language is comprised of a set of com-
mands and the qualifying rules for constructing program
statements in symbolic terms. There are two classes of com-
mands: mnemonic representations of the machine instruc-
tions and assembler (processor) directives.
A directive is a command to "he assernbler that allows the
programmer to describe or select assembly options at cssem-
bly time and, also, allows him to specify such elements in
his program as groups of data, choracter strings, and storage
areos, The format for coding program statements and the
rules of statement structure are described in thn following
chapters.
EXTENDED SYMBOL PROCESSOR
The Extended Symbol processor is a Xerox 530and Sigma 2/3
machine language program that operates as a three-pass pro-
gram assembler under control of the Real- Time Batch Moni-
tor. These posses are called the encoder, definition, and
generation passes. Throughout this manual the processor is
referred to as Extended Symbol or "the assembler".
During the encoder pass, the assembler checks the syntax of
each source statement, generates the assembler IS symbol
lable and converts constants to binnry. No semantic pro-
cessing nor symbol definition occurs during the encoder pass.
During the definition pass, the assembler a Ilocates space,
defines syrnbols, sets up symbol and literal tables as required,
and in general satisfies the many interconnection conditions
prescribed by the source program.
In the generation poss, Extended Symbol satisfies forward
and literal references and produces an object program, diag-
nostic messages, and on as~E:mbly listing.
External references (references to locations in ot!-.er programs)
and hrword referenc:es to procedure local srmbolscannot be
completely processed by the assembler; however, during the
generotion pas" information is generated in the object pro-
grom so thot the program loader may satisfy these references
prior to program execution.
In operation, the 'J"sernbler maintoins a series of temporary
storage areas:
1. Buffer areas for input of program statements and output
of object code and an assembl>.: listing.
2. Local and nonlocol symbol tables in which statements
and data identifiers {along with their storage assignments
Introduction

and perti nent characteristics) are placed as they are
defined or referenced. Local and nonlocal symbols
are
explained in Chapter 2.
3. Literal tables in which literal references are accumu-
lated until the end of assembly.
4. Three location counters: a load location counter, an
execution location counter, and a common location
counter thot provide information for the object pro-
gram -and consequently for the loader. The execution
location counter is used by the assembler in defining
symbols. The load location counter is used for linking
externa I symbol references.
The common
location counter is affected only by the
COMMON directive. Common symbols may be
2 Extended Symbol Processor
referenced as reI ocotabl e operands; however, the as-
sembler will not generate any instructions or data to
be stored in the common area.
5. Work areas used during assembly.
6. Assembly
variables and flogs in accordance with
directives.
7. Procedure definitions that are processed only when
they are referenced.
Supplied with the assembler are a set of standard procedures
which define the Xerox 530 and Sigma 2/3 machine opera-
tion codes.

2. EXTENDED SYMBOL LANGUAGE ELEMENTS AND SYNTAX
LANGUAGE ELEMENTS
Input to the assembler consists of a sequence of characters
that are combined to form assembly language elements. These
language elements, which include symbols, constants, ex-
pressions, and literals, comprise pmgram statements which
in turn comprise a source program.
CHARACTERS
The Extended Symbol character set is shown in Figure 1.
Alphabetic: A through Z, and $f!-Z" #, L-J (break char-
acter pri nts as 01 underscore")
Numeric:
Special
Characters:
(: is reserved alphClbetic character)
o through 9
Blank
+ Add (or positi ve va I ue)
Subtract (or negative value)
*
/
&
I
II
Multiply, indirect addressing prefix,
source regish::r inversion designator,
or comments line indi cator
Divide
Decimal point
Comma
Left parenthesi s
Right
parenthesis
Constant del imiter (single quotation
mark)
Logical AND
Logical OR (vertical slash)
Logical exclllsiveOR (vertical slashes)
-, Logical NOT or complement
<
> Less
than
Greater than
EquaI or introduces a litera I
<= Less than or equal
>:=; Greater than or equal
Continuation code
**
Binary shift
TAB Equivalent to blank; used to tabulate
keyboard printer output
Figure 1. Extended Symbol Character Set
The colon (:) is an a Iphabetic character reserved for use by
standard Xerox software. It is included in the names of
Moni tor routines (M :POP) and various mathematica I sub-
routines (L:ATAN) to avoid any potential conflict with user
symbols.
SYMBOLS
Characters are combi ned to form symbols. Symbols provide
programmers wi th a conveni ent method of i denti fyi ng pro-
gram elements so they can be referred to by other elements.
Symbols must conform to the followi ng rules:
1. Symbols should consist of 1 to 8 alphanumeric charac-
ters: A-Z, $, Jl~, #, :, L-J, 0-9, of which at least one
must be alphabetic. No special characters or blanks
may appear in a symbol. Only the first eight charac-
ters will be used by the assembler to identify the pro-
gram element represented by the symbol. Anyremaining
characters are ignored in processing the symbol and (if
requested) a warning error is output on the listing.
2. The characters $ and $$ may be used in the argument
field of a statement to represent the current value of
the execution and load location counters, respectively
(see Chapter 5); these characters must not be used as
symbols in the label field (see "Syntax" later in this
chapter).
The following symbols are valid:
ARRAY
R1
INTRATE
BASE
7TEMP
#CHAR
$PAYROLL
$ (execution location counter)
The following symbols are also valid, but only the llnder-
I ined portion is considered by the assembler and (if requested)
a warning error is noted.
~A TE01)NCREASE
Q!JJE~IV ~LUE
ABINVERSE
-----
The following symbols are invalid:
BASE PAY Blanks
may not appear in symbols.
Special characters (~-::) are not permitted
in symbols.
Extended Symbol Language Elements and Syntax 3

CONSTANTS
Acomtan*. isa self-defining language element. Its value
is inherent in the constant itself, and it is assemhled as
part oJ the statement in which it appears.
Six' types of constants are permitted in Extended Symbol
statements: decimal integer constants, character string
cons'tontsi. hexadecimal; constants, fixed-point decimal
cons.tants, floating-point short constants, and floati ng-point
long constants~
Deoimal Integ~r Constants
A decimal' integer consttmt consists of a string of decimal
digits. The value represented by the decimal digits must be
in the rangeO'tb 32767. The decimal integer is converted
to its.internal binary representation and retained in olle full
word of memory.
Examples:
326
32767
5
0,
Character String Constants
-------------
A chara.cter. string constant consists,of 1: through 64 EBCDIC
characters enclosed by single quotation marks (see II Extended
Bin'ary-Coded-Decimal Interchange Code" in the Sigma 2
and Sigma 3 Computer' Reference Manuals, 90 09 64 and
90 15 92, or Xerox 530 Computer/Reference Manual;
90 19 60, as appropriate).
Example:
'ANY CHARACTER INCLUDING BLANKS'
Any EBCDIC character is permitted in a character string
constant. Each character is allocated eight bits of storage.
Because single quotation marks are used as character string
delimiters by the asseinbll'!r, a single quotation mark (or
apostrophe) within a character string must be indicated in a
special' manner. An apostrophe in the string is represented
by two'consecutive apostrophes; for example,
'AB"C"'
represents the stri ng
AB'C'
Char.acter strings are stored two characters per computer
word. The descriptions of IDNT, LBL, DATA, TEXT, TEXTC,
and TITLE directives in Chapter 6 include positioning infor-
mation pertinent to character strings used with these direc-
tives. In all other usages character strings must not contain
Ii
Language Elements
more
than two characters. If the string contains two char-
acters, they occupy the left and right bytes of a single word.
If the string contains one character, it occupies the right
byte of a word and the left byte is filled with a zero (i. e.,
a null EBCDIC character).
Hexadecimal Constants
A hexadecimal constant consists of a string of 1 through 16
hexadecimal digits enclosed by single quotation marks and
preceded by the letter X.
Example:
X'9COl F'
The
assembler generates four bits of, storage for. e.ach hexa-
decimal digitin.the string. Thus, four hexadecimal digits
fill one word of storage. Hexadecimal constants are right-
justified in their storage area; if the number of digits is not
a
multiple of 4, the assembler generates one, two, or three
leading hexadecimal zeros in the leading positions of the
storage area.
The hexadecimal constant in the example above would
be' stored as
word 1
word 2
~
~
Hexadeoimal digits and their binary and decimal equivalents
are:
Hex. Binary Decimal Hex. Binary
Decimal
0 0000 0 8 1000 8
1
0001 1 9
1001
9
2 0010 2 A 1010 10
3 0011 3 B
1011 11
4 0100 4 C 1100 12
5
0101
5 D 1101 13
6 0110 6 E 1110 14
7
0111
7 F
1'111
15
The Xerox 530 and Sigma 2/3 Computer Reference Manuals
contain further information about hexadecimal arithmetic ane!
conversion of numbers from hexadecimal to decimal and deci-
ma I to hexadec ima I.
Fixed-Point Decimal Constants
A fixed-point decimal constant consists of the following
components in order, enclosed by single quotation marks,
and preceded by the letters FX:
1. An option:!!
algebraic sign.
2. d, d., d.d, or .d, where d is a decimal digit string.

3. An optional exponent: the letter E followed optionally
by an algebraic sign, followed by one or two decimal
digits.
4. A binary scale specification: the letter B followed op-
tionally by an algebraic sign, followed by one or two
decimal digits that designate the terminal bit of the
integer portion of the constant (i. e., the position of
the binary point in the number). Bit position number-
ing begins at zero and refers to the leftmost bit of the
word or the doubleword in which the constant is to be
generated.
Items 3 and 4 may occur in any relative order.
When FX constants are used as explicit arguments in a
DA TA,n directive and 1 < n 5 4, t'hey are treated as a 32-
bit integerquantity(alignment on thebinarypointis relative
to this 32-bit field) and are generoted as such. In all other
uses, FX constants are treated as 16-bit integers, with binary
point alignment relative to this 16-bit field.
No checking is ever made for truncation from the right-
hand side of an FX constant. Loss of significance on the
left-hand side or change of sign is flagged as an error.
Example:
Statement
DATA
FX
I3.75B41
DATA,2
FX
I3.75B41
DATA,3
FX ':3.75B41
SUB
=FX'-.OS32B1SE+41
DATA
FX''l BO'
GEN,S,S
FX
'3.75B41,1
Yloating-Point Short Constants
Generated Hexa ....
decimal Value
lEOO
1 EOO 0000
0000 1 EOO 0000
FF9S (literal value)
(error)
(error)
A
floating-point short constant consists of the following
components in order, enclosed by single quotation marks,
and preceded by the letters FS:
1. An optional algebraic sign.
2. d, d., d.d, or .d where d is a decimal digit string.
3. An optional exponent: the letter E followed optionally
by an algebraic sign followed by one or two decimal
digits.
A floati ng-poi nt short constant requires two memory words
for
storage. For this reason, a floating-point short constant
may appear only in the argument field of a DATA directive.
Example:
Constant Hexadecimal Value
FS'l. I
41100000
Floating-point short constants have a magnitude in the
range 5.398xlO-79 to 7.237xl0
75
(i.e., 16-
65
to 16
63
- 16
49
)
with the associated precision of 6 + significant digits. That
is,
the sixth most significant digit is accurate, while the
seventh will sometimes be accurate, depending on the value
of the constant.
Floati ng-Poi nt Long Constants
A floating-point long constant consists of the followi ng com-
ponents in order, enclosed by single quotation marks and
preceded by the letters F L:
1. An optional algebraic sign.
2. d, d., d.d, or .d where d is a decimal digit stri ng.
3. An optional exponent: the letter E followed optionally
by an algebraic sign followed by one or two decimal
digits.
A floating-point long constant requires three memory words
for
storage. For this reason a floating-point long constant
may appear only in the argument field of a DATA directive.
Example:
Constant
Hexadecimal
FL'-.9SEll
B19999990004
The magnitude of floating-point long constants is the same
as for floating-point short constants; however, floating-
point long constants have an associated precision of 10+
significant digits.
EXPRESSIONS
An expression is an assembly language element that repre-
sents a value. It consists of a single term or a combination
of terms (multitermed) separated by arithmetic, logical, or
relational operators.
A si ngle-termed expression may be any valid symbol refer-
ence (previously defined, forward, common, or external),
a constant, or a literal. (Symbol references and literals are
described later in this chapter.)
A multitermed expression may contain any valid symbol
reference (previously defined or forward) or a constant. It
must
not contain literals, forward procedure local refer-
ences, or external references. Appropriate error messages
are printed if any of these conditions is violated.
Operators and Expression Evaluation
A single-termed expression, such as 52 or $ or AB, takes on
the value of the term involved. A multitermed expression,
such as INDX+4 or ZD*S+XYZ, is reduced to a single value
by the assembler.
Language Elements 5

The value represented by a multitermed expression must not
exceed the 16-bit capacity of one computer word.
The
operators that may appear in an expression are shown
in Table 1.
Multitermed expressions are evaluated as follows:
1. Each term is evaluated and replaced by its internal
value.
2. Arithmetic operations are performed from left to right.
Those with the highest"binding strength" are performed
fi rst. For examp Ie:
A+B<C*D+E
is evaluated as if it were
(A + B) <((C * D) + E)
3. Division always yields an integer result; any fractional
portion is truncated.
An expression preceded by an asterisk (*) usually denotes
indirect addressing. Used as a prefix in this way, the
asterisk does not affect the evaluation of the expression. If
an asterisk precedes a subexpression, it is interpreted as a
multiplication operator.
Table 1. Extended Symbol Operators
Binding
Operators
Strength
Function
--,
7
Unary not
-7 Unary minus
+ 7
Unary plus
** 6 Logical
binary shift
(I eft sh ift if secand operand
is positive, right shift if
second operand is negative)
* 5
Integer multiply
/ 5
Integer di vi de
+ 4 Integer add
-4
Integer subtract
< 3
Less than
> 3
Greater than
<=
3 Less than or equa I
>= 3 Greater than or equa I
-- 3 Equal
--,=
3
Not equal
& 2 Logical
AND
II
1
Exclusive logical
OR
I 1
Inclusive logical OR
6 Language Elements
Logical Operators
The logical NOT (-,), or complement operator, causes a
l's complement of its operand:
Value
3
10
Hexadecimal
Equiv_~L~
00 ... 0011
00 ... 1010
l's Complement
------
11 1100
11 ... 0101
The binary logical shift operator C';*) determines the direc-
tion of shift from the sign of the second operand: a negative
operand denotes a right shift and a positive operand denotes
a left shift. For example:
5** -3
results in a logical right shift of three bit positions for the
value 5, producing a result of zero.
The result of any of the comparisons produced by the com-
parison operators is
a if "false"
1 if "true"
so that
Expre:sion Result
3 >4 a 3 is not greater than 4.
-,3=-4
a the 16-bit value -,3 is equal to
11 ..• 1100 and is not equal to 4;
(i.e., 00 ... 0100).
3-,:=: 4 3 is
not equal to 4.
-, (3:'-4)
11 ... 11 3 is not equal to 4, so the resu~t
of the comparison is a which,
when complemented, becomes
a 16-bit value (all l's).
The logical operators & (AND), I (OR), and II (Exclusive
OR) perform as follows:
AND
First Operond: 0011
Second Operand: 0101
Result of & Operation: 0001
OR
First Operand: 0011
Second Operand: 0101
Result of I Operation: 0111
Exclusive OR
First Operanc~; 0011
Second Operand: 0101
Result of II Operation: 0110

Note: E1 <E2<E3 cannot be used to determine whether E2
is within the limits E1 and E:3. Instead it is evalu-
ated as if it had been written as (E
1< E2)< E3.
That
is, the triad El < E2 results in a value, b, of 0 or 1.
Then
this value is \Jsed for the triad b< E3 to
yield another binary result. The correct form is
El
<E
2&E2
<E
3·
Parentheses Within Expressions
Mull"itermed expressions frequently require the use of paren-
theses to control the order of evaluation. Terms inside
parentheses are reduced to a single value before being com-
bined with the other terms in the expression. For example,
in the expression
ALPHA* (BETA+5)
the term BETA+5 is evaluated first, and that result is mul-
tiplied by ALPHA.
Expressions may contoin parenthesized terms within paren-
thesized terms:
DATA + (HRS/8~(TIME*'2*(AG+FG))+5)
The innermost term (in this example, AG + FG) is evaluated
first. Parenthesized terms may be n(~sted to any depth.
Expressions must not contain two consecutive binary oper-
ators. The assembler distinguishes between the unary op-
erators (-), (+), and(--,), and the binary operators as follows:
1. An operator precedi ng on expression may only be a
unary operator, as in -27.
2. The first operator following a tElrm in a multitermed
expression must be a binary operator:
-l27~L:ary
binary
---unary
At~S tL-unar
y
binary
In general, Extended Symbol wi II accept any combination
of operators that is algebrait::ally 10~Jical; that is,
5*-BETA
5-*BETA is
permissible.
is not permissible.
Summary of Operator and Expression Syntax
1. Labels (symbols) and constants are single-termed
expressions.
2. If El is an expression, then (El) is an expression.
3. IfEl isanexpression, then-Elp+E1and--.El are
expressions.
4. If El and E2 are expressions, then El **E
2,
E1 *E
2,
E1
/E2' El + E2, El -E2' El <E2' El >E2' El <=E2'
El >=E2, El-,=E2, El &E2, El =E2, E1 II E2, and
Ell E2 are all expressions.
5. Externa I and forward procedure local references may
occur only as single-termed expressions.
UTERALS
Constants provide one means of incorporating data directly
into a program at the time it is being written; literals pro-
vide another means. A literal is written as a constant
(decimal, hexadecimal, or character string) or symbol refer-
ence preceded by an equal sign. The literal, in contrast to a
constant, is not processed as part of the program statement
in which it appears. Instead, the literal is evaluated and
assigned to a storage location in a literal pool, and the
address of that location is assembled into the instruction.
Literals are useful in statements that require the address of
a data value rather than the dota value itself. Without I it-
erals it would be necessary in such situations not only to
enter the address (or symbolic location) of the data value
into the statement, but also to establish the value in the lo-
cation referred to by using a DATA directive, for example.
By using a literal, the value can be written directly in the
statement; the storing of the value in a memory location
and the substitution (in the statement) of the value's address
are tasks performed automatically by Extended Symbol.
A literal consists of an equal sign followed by a single-
termed expression (other than a I iteral) or an equal sign
followed by a multi-termed expression.
The value represented by a literal must not exceed the
16-bit capacity of one computer word.
Examples of valid and invalid literals:
Literal Notation
=-185
-=e'K'
::;::'ABC'
=X'5DF'
Description
Valid. Decimal value -185
Valid. Alphanumeric constant in
storage as 10000100001110110010 I
o K
Inval id. Exceeds 1-word capacity
Valid. Hexadecimal constant in
storage as 10000101011110111111 I
o 5 D F
Language Elements 7

Literal Notation Description
---~~- ~---.-- -----
"'-XIAF6BE' Invalid. Exceeds l-word capacity.
cALPHA Val id. Address value of symbol ALPHA.
ALPHA +3
-ALPHA
=*ALPHA
Valid.
Valid (provided ALPHA is absolute).
Invalid. Multiple level indirect
addressing is not allowed.
When the assembler encounters a literal, it checks the lit-
eral for validity, generates error notations if necessary,
determines the literal value, allocates storage for the value
in a future literal pool (see LPOOL directive, Chapter 4),
and generates an address poi nting to the I iteral. This ref-
erence address is used in the generation pass for assembling
the stotement in which the literal occurred.
SYNTAX
The assembly language elements may be combined with
machine instructions and assembler directives to form state-
ments which comprise the source program.
STATEMENTS
A statement is the basic component of on assembly language
source program. A statement is also called a source
statement, a program statement, or a symbolic line.
Source stlltements are written on the standard coding sheet
shown in Figure 2.
Fields
The body of the coding sheet is divided into four fields:
lobel, command, argument, and comments. The coding sheet
is also divided into 80 individual columns. Columns 1
through 72 constitute the active I ine; columns 73 through 80
are ignored by the assembler except for I isting purposes and
may be used for identification and a sequence number.
The col umns on the codi ng sheet correspond to those on a
standard 80-column card; one line of coding on the form
can be punched into one card.
Extended Symbol provides for free-form symbolic lines; that
is, it does not require thot each field in a statement begin
XEROX
PROBLEM ____________ _
SIGMA
SYMBOLIC CODING FORM
Identification
PAGE ______ OF ____ __
73
I I I I I I
PROGRAMMER ________ _
DATE ____________ __
80
LABEL
5 COMMAND
ARGUMENT
COMMENTS
10 15
20 25 30 35 40
45 50 55 60 65 70 72
I
IT I I I I I , , I I
I I T I I I 1 I I I I I
ii'
I I
I I I 1 I I
I' ~TI·l-r~l~l~l-r~l-l~I,-~l·l-rl~~I~,·~
~~rTIT~~~"-r~-OTI4-~"-r~~"-~TO-r~I-r~-.+,,-r._,TI,-rTl.l-ri~l-···rr~I,-~,-~I-r~-T~~~~I~~~~'~
~~-rTI~r+~,~~-rTI~~~I~~-r'T,-~~'~~~~'-r~~I~~~--T~'l-r~-r~-rrT~~~r~'-~~I~
~~~I'~~~~~~~~,~~-,rT~~',~~~-~I-r~-~,"~-r,·,-r~I~I-rTl,-rTl'I-r1 ~-rl~I_r~,_~_r~l_rl- ~-r~,~~
·1 '" 1 I I I I I I f I I
~~~"~~~~I-~~"~I~~,-~~-~·rl"rTlll-ri '~~~,~.-r~,-r~-IT,.-~I,rr~~I,-r~-rT~-r~-~I.-~-I~
........ -..- ........ -,-.,...-,-4-+--.-
,
",,---.-.,.,. ,+-f---.--'T~-.-'---'----rT---'--'--'-'-T-·-r-r-r~,-+-~~"T,-r"'T"'"'T--r--r-r-r-~'-r-""""""-T"""'-~""T-"'-""""-"'--r,--r-~-r~ 1--1
~-~'~T~rrTl--r-~~,~rT--r-'~~~~'~~II--r-~~I~~~ITI--r-~T"TI~~,,-r~~I~~-r~I~~~I'---'-~~I~
~~~'-""'-'-~~-r -r,-~,-rt'-I'~""'-'--rr.-rTf-r~-rT1-r~-r~~"rr~~~~~r('-~-r,.....~~-r~-r .. ~
I I , I I I I , I I I ,
T 1 , ,1, I I I , I I 1 I
T I I , r , , 1 I I I 1 I I il' I I I
T , I I T I IT' -"--ri-I"~rT.-,,......-rlTI,I_r""""'rTl'~rT'-rTI4
~~..-r'l--r--~~" 1~"-OTI4-~--'I-"'-· .,......,r-r-.,..I.-~,-.,...-,- ,,,,,,-,,-,--'-+-,-.-r-rl ..... I--.-~r-T'""_I' ,....,... . ., _. ,'·-rl .......... --r- ...... --r.~~ ..... ,---.-r-r-r-r-T ,-.-..,......,.-,.-,-,~
T I I I I I I I I I I I I I I
r.'-rT,-"1-~'~"'-"I4-~--"--r--.,......,r-r-,,,,,---.-~-r~,-r~-"'-+-'-·~'i-r~~I'--~ '-"-'-'I~~rr""l-rTO-'-~I-r.,......,-~,-r~-,~
~.-r-r,-"",-,-~~I""",-r,~~"~""-rr""-~~r~'~~I'~-"-'II,-.~r-r-~-r""--rr,,r-r-~~'-rT""-'---'--'-""'-'-·~~---'--r-Tl~
I I 1
... ·......--,.-.....--r---.---.--~·""T"""1-r-"T,-+--r-~·-r---,----.r-r"""j1r-r--.--r-r-r-,r-r-r--rr.-~-r-rT,-,.-,-·--.-~---.-..,....,. I .....
, I I I I
r-rT--r-TI'-""'-'-"!i'---'-~-I',-r-"-'-'-""r-r'-rT---.-~-r~-r~~,,--r""""~~I~
I I I I , I , I I ,
I I I ,
I"-'---r--r , I I I I .....,- ,
r-""-rT,-~--Hr--.--"-..--.---r-"",+t-.,-,,-.-~,r-r-II---'-"T""T'-,-r ,-r...--r-,rl--.-.---."-,--.,......,r-r-,,.-..--r--'--T"""T" I-'~·'-'---r-rl-'I -.-r-r--r-I -,-I---r-,-,-.-.,.....,...~
1 1 I 1 I I
~~-.-rT-,-,".,......,~II-r"-rTT.,..-rT---.-",-r~-,~~
T 1
~"I""I' 'I' 'I 'I
Figure 2. Xerox Sigma Symbolic Coding Form
8 Syntax

in a specified column (with the exception of the label field).
The ruI es for writi ng free-form symbol ic 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

DEFINING SYMBOLS
Asymbol becomes "defined" by its appearance as a label entry
on machine instructions and certain directives. "Defined"
means that it is assigned a value. The definition, assigned to
the symbol by the assembler, depends on assembly conditions
when the symboI is encountered, the contents of the command
field, and the current contents of the execution location
counter (see Chapter 5').
Any machine instruction may be labeled; the label is assigned
the current value of the execution lacation counter.
Information regarding the use of labels in directives is con-
tained in the description of each directive.
Note: The use of labels isaprogrammeroption, andasmany
or as few labels as desired may be used. However,
since symbol defining requires assembly time and
storage space, extraneous labels should be avoided.
REDEFINABLESYMBOLS
Two directives, DOandSET, establish redefinable symbols.
These symbols
are redefi ned by the assembler during the pro-
cessing of a DO-loop (see DO Directive, Chapter 6) or by
a
subsequent SET di rective (see SET Directive, Chapter 6).
SYMBOL REFERENCES
A symbol used in the argument field of a machine instruc-
tion or directive is called a symbol reference. There are
three types of symbol references.
Previously Defined References
A
reference made to a symbol that has already been defined
is a previously defined reference. All references to such
symbols are completely processed by the assembler during
the definition pass. Previously defi ned references may be
used in
any machine instruction or directive.
Forward References
A
reference made b a symbol that has not yet been defined
is a forward reference. Forward references are defined during
the definition pass, and machine instructions that reference
them are completely assembled during the generation pass.
Forward References (Procedure Locals)
Forward references to symbols declared local within a pro-
cedure are incompletely assembled. The object code gen-
erated for such references allows the forward references
and their associated definitions to be linked at load time.
The load location counter is used for this linking operation.
A forward reference to a procedure local symbol must not
be a term
in a multi termed expression.
Any machine instruction may use a forward reference. Only
the GOTO, LOCAL, REF, SREF, DEF, GEN, GEN1, GEN2,
ADRL, and DATA directives may use forward references.
10 Processing of Symbols
ExternaI References
A reference made to a symbol that is defined in a program
other than the one in which it is referenced is an external
reference.
A program that defines external references must declare
them as external by use of the DEF directive (see Chapter 6).
An
external definition is output by the assembler as part of
the object program for use by the loader.
A program that uses external references must declare them
as such by use
of a REF or SREF directive (see Chapter 6).
A machine instruction containing an external reference is
incompletely assembled. The object code generated for
such
references allows external references and their asso-
ciated external definitions to be linked at load time. The
load location counter is used for the linking operation.
An external reference must not be a term in a multitermed
expression.
Any class 1 machine instruction (see Chapter 3) may contain
an externa I reference. ExternaI references are not allowed
in any directive except GEN, GEN1, DATA, ADRL, REF,
and SREF.
CLASSIFICATION OF SYMBOLS
Symbols may be classified as local, procedure-local, or
nonlocal.
A local symbol is a symbol that is defined and referenced
within a restricted program region. The program region is
designated by the LOCAL directive (see Chapter 6); this
directive also declares which symbols are to be local to
the region.
A procedure-local symbol is a local symbol that is defined
and referenced wi th ina parti cu lar procedure (see Chapter 7).
A symbol
not declared as local or procedure-local by use
of the LOCAL directive is a nonlocal symbol. A nonlocal
symbol may be defined and referenced in any region of a
program
including local and procedure-local symbol regions.
Note that the same symbol may be both nonlocal and local
(or procedure-local) in which case the nonlocal and local
forms identify different program elements.
SYMBOL TABLES
Extended Symbol maintains three internal symbol tables in
which it stores each symbol along with its assigned value
and/or control information pertinent to that symbol. These
tables are the nonlocal symbol table, the local symbol table,
and procedure-local symbol table.
The nonlocal symbol table contains nonlocal symbols and is
active throughout an assembly.
The
local symbol table contai ns symbols that are declared to
be local (see LOCAL directive, Chapter 6) to a region in the

program. This table is temporary and may be erased and re-
establ ished with new symbols by a subsequent LOCAL directive.
The procedure-local symbol t(lble contains symbols that are
declared to be local to a particular plIOcedure (see II Proce-
dures
ll
, Chapter 7). Each symbol in a local directive within
a procedure causes the previous definition of that symbol to
be temporari Iy suspended, and the symbol is set as unde-
fined in the current procedure local sY'mbol table. At the
end of the procedure, the last' previously suspended local or
procedure-local definition of the syst,em is reactivated.
When the assembler encounters a symbol in the label field,
it refers to the last active local or procedure-local symbol
table (if assembl ing a local or procedure-local regi on, re-
spectively); if necessary, it then refers to the nonlocal sym-
bol table. If I'he symbol is not in an active table, the symbol,
its value, and control information are entered in the appro-
priate table. Atthispoint, the symbol is completelydefined.
If the symbol is found in 0 table, one of the following con-
trol conditions applies and is indicated in the symbol's con-
trol information.
1.
2.
3.
4.
Symbol Control
Local or procedure-local
and not previously defined.
Previously defined in the
approprial'e table.
Dec lared external to pro--
gram being assembled by
REF or SREF.
Dec Ia red externa I by DE F;
defi ned wi thin this program.
5. Previously referenced but
not defi ned.
Result
The symbol becomes
defined.
Symbol is taggedasmulti-
defined and retains the
fi rst address vaI ue - an
error condition.
Provides information in
object program for loader
to complete linkages.
Defi nes the symbol and
provides object program
information tor the loader.
Provides control i nforma-
tion for completion of
references in generation
pass. The symbol is de-
fined at this point.
When the assemblerenc:ounters a symbol in an argument field
it refers to the last active locol or procedure-local symbol
table (if assembling a local or procedure-local symbol region,
respectively); if necessalY, it then reFers to the nonlocal
symbol table to determine if the symbol has already been
defined. If it has, the assembler obtcdns information about
the symbol from the table and is then able to assemble the
appropriate object program information. (Actual assembly
occurs during the generation pass; entries into symbol tables
occur during the definition). If the !iymbol is not in any
active symbol t~ble, the assembler enters its name and con-
trol information in the appropriate table but does not assign
an address value until the symbol is defined in the label
field. Symbols are entered in the nonlocal table unless
they have been declared as locol by use of the LOCAL
directive. Symbols dec:lared CIS local ore entered in either
the active local or the active procedL,re-local symbol table.
If any undefined symbols remain in the nonlocal symbol table
at the end of an assembly, their definitions are declared
'unknown
'
and appropriate messages are produced. Error
messages
are not produced for undefined local or procedure-
local symbols that are not referenced within that region.
ABSOLUTE AND RELOCATABLE VALUES
The value of a symbol or expression may be absolute, re-
locatable, or common relocatable. An absolute value,
which is assigned at assembly time, is the same value that
wi II be used by the program at execution time. A relocat-
able or common relocatable value may be altered by the
loader at execution time.
SYMBOLS
A symbol is assigned an absolute value by one of the
following methods:
1. By equating the symbol to an absolute numeric quantity.
2.
3.
SUM EQU 2
SUM is assigned the absolute value 2.
By equating the symbol to an absolut,e expression.
A
B
ANSWER
EQU
RES
EQU
EQU
$
10
$
A - B
ANSWER is assigned the absolute value -10.
By using the symbol as a label entry in absolute pro-
gram or program secti on (see Chapter 5).
The value of an absolute symbol does not change, even if
it is part of a relocatable program (a program that can be
executed anywhere in memory).
A symbol has a relocatable value unless declared absolute
as described above. The value of a relocatable symbol may
be altered by the loader when the symbol is a part of a
relocatable program; i. e. I the loader wi II add the reloca-
tion bias to each symbol used as a label entry in a re-
locatable program or program section (see Chapter 5).
A symbol is common relocatable if it appeared in the label
field of a COMMON directive.
, EXPRESSIONS
The value of a single-term expression has the same attributes
(absolute, relocatable, or common relocatable) as the single
symbol or constant of which it is composed.
The value of a multi-termed expression will be absolute
if only absolute terms are used in the expression. All op-
erators in Table 1 may be used to combine absolute terms.
Absolute and Relocatable Values 11

A m'i..dti-termed expression may be composed of absolute,
reldcdtdble, and common relocatable terms, subject to the
restrictions itemized below. IIOperand
ll
refers to a single
symbol or constant, or to the value of a subexpression at
the time it is-combined into the expression with one of the
operators shown in Table 1 (see IIOperators and Expression
Evdl uation II edrlier in this chapter) ..
• The relational operators, <, <=, >, >=, =, and -'=,
require that both operands be of the same mode (ab-
solute, relocatable, or common relocatable).
• The operators * and / and the logical operators, ...."
**, &, I, and II may not be used with a relocatable
or common relocatable operand.
• In evaluating an expression, the assembler maintains
a count of the number of terms added or subtracted
that are relocatable or common relocatable. A sep-
arate counter is used for the two relocation types
and each counter is incremented or decremented
by 1 whenever a term of the corresponding relocation
type is added to or subtracted from the expression.
The final value is absolute if both counters are equal
to O. If the final Cdunt in one (and only one) of
the relocation counters is equal to +1, the value
of the expression is relocatable or common relocat-
able, depending on which counter is equal to +1.
Any
other accumulation in the two relocation coun-
ters is an error and results in a diagnostic frag.
Example 2. Expressions Using + and - Operators
A's5ume Rf, R2, and R3 are program relocatable terms;
C1' and C2 are common relocatable terms; and A1 and
-A2 are absolute terms.
Expression: R1±Al I
Legal, program
Common count:
0 0
Relocatable count: 1 1
relocatable
-Expression
Cl±A1
) LegaI, common
' Comnion count:
1 1
Relocatcible count:
0 0
relocatable
12 Absolute and Relocatable Values
Expression: R1+R2-Cl-R3+C2j
Legal, program
Common count:
o 0 -1 -1 0
Relocatable count:
1 2. 2 1 1
relocatable
Expression: -R1+Al+R2 )
Common count: 0 0 0
Legal, absolute
Relocatable count:
-1
-1 0
Expression: Rl+A1+C1 )
Illegal, diag-
Common count:
0 0 1
Relocatable count:
1 1
nostic error
Expression:
R1+A 1+R2
I
Illegal, diag-
Common count:
0 0 0
Relocatable count:
1 1 2 nostic
error
Expression: A1±A2
jlegal, absolute
Common count: 0 a
Relocatable count: 0 0
Example 3. Expressions Using Miscellaneous Operators
R1, R2, R3, C 1, C2, A 1 and A2 have the same mean-
ings as in Example 2, above.
Expression
Al * A2
A1 * (R 1-R2)
(C1-C2)/A 1
R1 * A1
C1-C2/A 1
A1 &A2
A1 ** (A2-R1+R2)
-A1
Result
) legal, absolute
} Illegal, diagnostic error
-,(C l-C2+A 1) Legal, absolute
R1....,=R2
C1 > C2
A1 > (R2 > R3)
A1 & R1
-R1
R1 <= C1
A 1> R2> R3
C1 ** A 1
R1 < R2 < R3
] Illegal, diagnosfic error

3. XEROX 530 AND SIGMA 2/3 MACHINE INSTRUCTIONS
Xerox machine instructions may be written symbolically
and combined with other assembly language elements to
form symbol ic instruction st(ltements.
A symbolic instruction statement consists of four fields.
Fi eld Contents
Label Any valid symbol. Use of the label entry
is optiona I. When present, the symbol may
be referenced by other instructions and
direct i ves.
Command Any mnemonic operation code listed in
Appendix A.
Argument One or more subfields such as elll indirect
address designator, an argument address ex-
pression, a post-index expression, a dis-
placement expressi on, a base address speci-
fication (pre-indexing), or a shift count,
dependi ng on the speci fie instruction.
Comments Any remark explaini ng the specific pur-
pose of an instruction or the overall func-
tion of the program.
The Xerox machine instructions recc,gnized by Extended
Symbol are described below and in Appendix A. The syn-
tactical rules used in the instruction descriptions of Appen-
dix A are as follows:
1. Underscored items are the required parts of a symbolic
instruction statement.
2. Nonunderscored items me optional parts of an instruc-
tion statement.
3. m designates a mnemonic operation code.
4. * designates indirect addressin~~ for Class 1 instructions;
for Class 4 instructions, it indicates that the contents of
the source regisl'er specified by the instruction are tobe
inverted (onels complemented) before the operation is
performed.
5. a designates the (lrgument address used in the instruction.
6. x designates post-inde)<ing (with index register 1); if
x i:- 0, perform i ndexin~~; if x = 0 or blank, no indexing.
7. b designates addressing relative to the base register
(with index register 2). This is (II so called pre-indexing.
If b =I Or base-relative addressing is used, and the argu-
ment address represents the displacement value. If b =0
or is blank, the assembler may automatically impose
base-relative addressing on the instruction, depending
on the value of "a" and on whe,ther or not the BASE
directive option is active.
8. c designates a count used with the Shift instruction.
9. s designates the source register used with the Copy
instruction.
10. d designates the destination register used with the
Copy instruction.
There are five classes of machi ne i nstructi ons for the Xerox 530
and Sigma 2/3 computers: memory reference, conditional
branch, shift, register copy, and input/output control.
CLASS 1: MEMORY REFERENCE INSTRUCTIONS
SINGLE PRECISION ClASS 1 INSTRUCTIONS
Class 1 instructions may reference any location in memory
through use of the various addressing techniques and may
appear in anyone of the following forms:
1. Nonrelative Addressing
2.
o ~~ ,I~I~I~I~I. ~~~:~::",,1
The reference address is the value of the address field.
Base-Relative Addressing
The reference address is equa I to the va lue (0 to +255)
in the displacement field plus the 16-bit base address
value in index register 2.
3. Self-Relative Addressing
The reference address is equal to the value (-256 to +255)
in
the displacement field plus the 16-bit instruction ad-
dress value in the H register. Since the H register con-
tains the address of the instruction being executed, the
reference address produced is relative to the instructionls
own location. The value in the displacement field is
treated as an 8-bit positive integer if bit 7 is a 0, and
as a 9-bit, two's complement negative integer if bit 7
is a 1. Thus the reference address derived during pro-
gram execution will be the current instruction address
plus the sign extended displacement value with the sum
treated modulo 216.
In all three forms of memory reference instructions, the
reference address may be further modified to produce
the final or effective address, depending on bits 5 and 6
of the instruction. If bit 5 is a 1, the reference address
will be treated as an indirect address; that is, a 16-bit
direct address value will be obtained from the location
specified by the reference address. If bit 5 is a 0, the
direct address is the same as the reference address. If
bit 6 is a 1, the direct address will be modified byadd-
ing the 16-bit value in index register 1 with the sum
treated modulo 216. Programmer control of addressing
is explained in Chapter 4.
Xerox 530 and Sigma 2/3 Machine Instructions 13