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…00……00……00……00……00……00……00……00……00…8…02…8 8…05…7…08…7…0b…7…0e…7…01…7 7…07…6…0a…6…0d…6…00……86…1 …02… …02… …02… …02…
Instructors Manual for
RST Course, Week 15
CPS/TMA/014
Line Item 8.2.4.2
Birthe Irgens
Kurt Nybroe-Nielsen
SHAPE (3), NCS (1), ORP, PBP, CL, JJD,
BSP, KJA, Conf.Mgmt.
ILS Train.MGT. 821102
Preliminary
821102
…02… CPS/TMA/014
…02… JJD/821102…02…
INSTRUCTORS MANUAL FOR RST COURSE,
WEEK 14 …02… CAMPS
821102 All Prelim. Issue of Document
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BI/821102
Introduction to Progr. 15:1:1 45 CAMPS…0f…
Describe the main purpose of the language design
"Situation questions" during the lesson
CAMPS Classroom
Lecture
OH, Whiteboard
SWELL Reference Manual
…0e… 2975A/ktd
BI/821102
SWELL Structure 15:1:2 45
CAMPS…0f…
Describe the SWELL structure
"Situation questions" during the lesson
CAMPS Classroom
Lecture
OH, Whiteboard
SWELL Reference Manual
…0e… 2975A/ktd
BI/821102
Program Module 15:1:3 45
CAMPS…0f…
Describe the program module and the subblock elements
"Situation questions" during the lesson
CAMPS Classroom
Lecture
OH, Whiteboard
SWELL Reference Manual
…0e… 2975A/ktd
BI/821102
Constant & Type Defin. 15:1:4 45
CAMPS…0f…
Define constants and types
"Situation questions" during the lesson
CAMPS Classroom
Lecture
OH, Whiteboard
SWELL Reference Manual
…0e… 2975A/ktd
BI/821102
Decl. & Denot. of Var. 15:1:5 45
CAMPS…0f…
Declare variables
Use the MON-procedures
"Situation questions" during the lesson
CAMPS Classroom
Lecture
OH, Whiteboard
SWELL Reference Manual
AMOS I/O System Manual
AMOS Kernel Manual
…0e… 2975A/ktd
BI/821102
SWELL Compiler & Ex. 1 15:1:6 180
CAMPS…0f…
Use the SWELL compiler
Write a simple SWELL program
Exercise
CAMPS Classroom
Computerroom
Group Work and Lecture
OH, Whiteboard
HO
SWELL Compiler Manual
SWELL Reference Manual
…0e… 2975A/ktd
BI/821102
Linker 15:2:4 45
CAMPS…0f…
Use the LINKER
Exercise
CAMPS Classroom
Computerroom
Lecture
HO
SWELL Reference Manual
…0e… 2975A/ktd
BI/821102
EXPRESSION 15:2:5 45
CAMPS…0f…
Use the expressions
"Situation questions" during the lesson
CAMPS Classroom
Lecture
OH
SWELL Reference Manual
…0e… 2975A/ktd
BI/821102
STATEMENTS 15:2:6 90
CAMPS…0f…
Use the SWELL Statements
"Situation questions" during the lesson
CAMPS Classroom
Lecture
OH, Whiteboard
SWELL Reference Manual
…0e… 2975A/ktd
BI/821102
SWITCH MON 15:3:2 180
CAMPS…0f…
Use the SWITCH MON statement
"Situation questions" and exercise 2
CAMPS Classroom
Computerroom
OH, HO
Whiteboard, OH
SWELL Reference Manual
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BI/821102
VARIABLE INIT 15:3:6 180
CAMPS…0f…
Use the initialization of variables and
multiply and divide
"Situation questions" and exercise 3
CAMPS Classroom
Computerroom
OH, HO
Whiteboard, OH
SWELL Reference Manual
…0e… 2975A/ktd
BI/821102
Procedures 15:4:4 135
CAMPS…0f…
Write procedures
Exercise 4
CAMPS Classroom
Computerroom
Lecture
Groupwork
OH, HO
SWELL Reference Manual
…0e… 2975A/ktd
BI/821102
Dedicated Statement 15:5:1 225
CAMPS…0f…
Use the dedicated statement
Situation question
Exercise 5
CAMPS Classroom
Computerroom
Lecture and Group work
OH, HO
SWELL Reference Manual
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BI/821102
Introduction to Programming L
SWELL OH 1 S̲oftware E̲ngineering L̲ow-Level L̲anguage
SWELL includes a large number of the program
and data facilities of Pascal.
Main purpose of
the language
design The main purpose was to enable the programmer
to create very effective object programs
in a well structured manner.
The performance and resource requirements
of SWELL programs are comparable to the corresponding
figures for assembly language programs.
Key words: Low level language
Program performance
Structured programming
System programming language
SWELL/Pascal SWELL differs from Pascal in some essential
points:
1) SWELL does not hide the actual computer
from the programmer.
The virtual computer is identical to
the actual computer, and the programmer
controls register usage, addressing techniques,
etc.
2) SWELL does not include any kind of run
time system.
There are no run time facilities for
maintaining a valiable stack, for parameter
transfer, etc.
No general purpose registers or variables
are allocated or used behind the programmer's
back.
3) SWELL does not define a closed, self
contained world.
Instead, SWELL allows direct access to
system software modules.
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Introduction to Programming L
The CR 80
Computer 16 bit minicomputer manufactured by CR.
The code and data parts of a program are
separated, each part being referenced relative
to its own base register.
Addresses are primarity word addresses (access
to 64K words of code and 64K of data).
Registers 8 general purposes, 16 bits registers used
for word operations or concatenated for double
word operations.
6 primary
addressing modes 1 register direct addressing
2 immediate addressing
3 code base relative addressing
4 data base relative addressing
5 program counter relative addressing
6 register indirect addressing (indexing)
INSTRUCTIONS Single word instructions using the primary
addressing modes.
May be extended by adding the contents of
a "modifier" register.
Data types Directly handled by the CR80 instructions:
1 bits (single bit or a sequence of bits)
2 bytes (8 bits)
3 words (16 bits)
4 double words (32 bits)
The instruction
set includes 1 arithmetic and logical operations on words
2 move operations on bits, bytes, words,
and double words
3 skip and jump instructions, performing
program sequence control
Operands A CR80 instruction may address zero, one
or two operands.
In case of logic and arithmetic operations,
one of the source operands is used as destination
operand.
A number of instructions support memory to
memory operations.
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SWELL Structure L
SWELL Manual CSS/415/
RFM/0002 Modified Backus-Naur Form
and According to traditional BNF, syntactic constructs
are denoted by English words enclosed between
the angular brackets.
These words also describe the meaning of
the construct and are used in the accompanying
description of semantics.
and Possible repetition (or omission) of a construct
is indicated by enclosing the construct within
metabrackets.
empty Denotes the null sequence of symbols.
identifier Is often replaced by more descriptive variants
like type identifier, label identifier
.
Special symbols OH 1 Delimiter symbols:
Her inds`ttes symboler
Reserved words OH 2
Identifiers OH 3 Identifiers are build up by letters, digits
and the underline symbol.
In identifiers a small letter and the corresponding
capital letter are not descriminated.
Examples:
ALFA, SAVERO, STING ̲POINTER, EXIT ̲4711
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SWELL Structure L
Numbers OH 4 Two different notations are used for numbers
1) decimal and 2 hexadecimal notation.
The value of an unsigned number must belong
to the range:
0..65535 (decimal)
0..# FFFF (hexadecimal)
Examples: Legal numbers:
Decimal: 0001, 4711, 14, 00
Hexadecimal: #0001, #1267, #E, #00
Strings 1) Consecutive string
2) Composed string
ad 1)
A sequence or characters prefixed and terminated
by single quote marks. A consecutive string
must not contain line terminating characters
(characters with ordinal values in the range
1 ot 31). Any character may be included in
a string by writing its ASCII value enclosed
in the character brackets (: and :).
Contatenations Strings are concatenated by using the
catenation operator &.
Syntactically concatenated strings are regarded
as one single string.
Quote mark The single quote mark may me used within
a string by writing it twice.
Short strings Containing one or two characters
Long strings containing more than two characters
Examples OH 5 'ABC''XYZ
is equivalent to
'ABC(:39:)XYZ'
and equivalent to
…0e…AB' & 'C(:39:)X' & 'YZ'
Comments example "THE REST OF THIS LINE IS A COMMENT
"MORE CODE MAY BE WRITTEN AFTER THIS COMMENT"
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Program Module L
Program Module OH 1 1) Main module
OH 2 2) Sub module
A program consists of one main module and
an optional number of submodules.
A program can only have one statement part.
This statement part is placed in the main
module of the program.
Subblock OH 3 1) label declaration part
elements 2) constant definition part
3) type definition part
4) variable declaration part
5) variable initialization part
6) procedure declaration
Label OH 4 The label declaration part specifies label
declaration identifiers which mark a statement in the
part statement part.
Constant OH 5 A constant definition part contains constant
definition synonym definitions which are local to the
part block.
Type OH 6 A type definition part contains type
definition definitions which are local to the block.
part
Variable OH 7 A variable declaration part contains
declaration variable declarations local to the block.
part Variable declarations may be of the following
types:
1. allocation of permanent space for variables
within the data part (base relative part)
of the program. These variables may at
the same time be exported for external
use.
2. allocation of temporary space for variables
within the data part (base relative part)
of the program. All blocks share a common
pool of locations for temporary variables
(the same location may be used by many
procedures for different purposes and
it is up to the programmer to avoid conflicts).
3. import of variables, declared and allocated
in a foreign module.
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Program Module L
Variable OH 8 A variable initialization part assigns
initialization initial values to permanent variables.
part
Statement part OH 9 The statement part
specifies the algorithmic
actions to be execured
upon an activation
of a program or a procedure.
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Constant and Type Definition L
Constant and
type definition Constant and type definitions are abstract
ways of describing the data structures
on which a program is to operate. They
result in no code generation and no allocation
of data space in the object program. Therefore,
SWELL has almost directly adopted these
program parts from PASCAL.
A difference from PASCAL is that an expression,
involving constant operands only, is considered
a constant and may be used to define other
constants.
Constant OH 1 A constant definition introduces an
definition identifier as a synonym to a constant.
Examples CONST
LASTINDEX = N-1;
AREA = LENGTH*WIDTH;
NL = '(:10:)';
VERSION = 'VERSION 1';
Data type OH 2 Every variable is associated to a data type.
definitions The data type of a variable determines the
structure of the variable and the set of
values which the variable may assume.
Standard types
Byte An unsigned integer, which can be held in
one byte (8 bits) and therefore lies in the
range (0:255).
Char = Byte
Enteger Can be held in one word (16 bits). There
is no standard interpretation of this data
type. The interpretation depends on the operations
performed on variables of this type. But
normally INTEGERs are used as unsigned integers
or as signed integers using 2s-complement
arithmetic.
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Constant and Type Definition L
Long Is an integer which can be held in a double
word (32 bits).
As only a few operations on double word operands
are supported by the CR80 directly, the type
is 'predeclared' as:
LONG=
RECORD
LEAST, MOST: INTEGER
END;
String Is an array, which can hold a number of chars.
The type is 'declared' as:
STRING = ARRAY 0..131 OF CHAR;
Scalar types OH 3 A scalar type defines an ordered set of values
by enumeration of identifiers which denote
these values.
The base of a scalar type is integer, and
the values of the elements are equal to the
constants 0,1,2,...
Exampels TYPE
BOOLEAN=(FALSE, TRUE);
WEEKDAYS=(MONDAY, TUESDAY, WEDNESDAY,
THURSDAY, FRIDAY, SATURDAY,
SUNDAY);
Subrange type OH 4 A type may be defined as a subrange of another
scalar type by indication of the least and
the largest value in the subrange. The first
constant specifies the lower bound, and must
not be greater than the upper bound.
Example TYPE
WORKDAY=MONDAY..FRIDAY;
TYPE DEF./
CONSTANT The rules for type compatibility are much
more liberal in SWELL than in PASCAL. This
is due to the explicit use of registers.
Very few operations may be carried out without
involving registers, so it would be very
inconvenient, if the type of a register did
not match all other scalar and subrange types.
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Constant and Type Definition L
The following definitions are therefore equivalent:
1) TYPE
WORKDAY= (MONDAY, TUESDAY, WEDNESDAY,
THURSDAY, FRIDAY);
2) CONST
MONDAY = 0;
TUESDAY = 1;
WEDNESDAY= 2;
THURSDAY = 3;
FRIDAY = 4;
TYPE
WORKDAY= MONDAY..FRIDAY;
Array types and record types may also be
defined in SWELL.
Structured OH 5 A structured type is characterized by the
types types of its components and by its structuring
method.
Array Types OH 6 An array type is a structure consisting of
a fixed number of components, which are all
of the same type, called the component type.
The elements of the array are designed by
indices, values belonging to the so-called
index type.
The base of the index type must be INTEGER,
while the components may be of any type.
Arrays having component base type different
from BYTE, INTEGER or LONG, can only be indexed
by using constant indices.
An array type has the same base as another
array type, if the index types are equivalent,
and the components have the same base type.
Examples ARRAY 1..10 OF = 0000.. OFFF;
ARRAY 'A'..'Z' OF INTEGER;
ARRAY WEEKDAY OF WORKHOURS;
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Constant and Type Definition L
Record types OH 7 A record type is a structure consisting of
a fixed number of components, possibly of
different types.
The record type definition specifies for
each component called a f̲i̲e̲l̲d̲, its type and
an i̲n̲d̲e̲n̲t̲i̲f̲i̲e̲r̲ which denotes it.
The scope of these field i̲d̲e̲n̲t̲i̲f̲i̲e̲r̲s̲ is the
record definition itself, and they are also
accessible within a field designator referring
to a record of this type.
A record may contain a fixed part and/or
a varible part.
A variable part is conceptually equivalent
to a set of overlayed subrecords, each of
which in turn may consist of a fixed part
and/or a variable part.
The size of a record including a variable
part is the sum of the fixed part size and
the size og the largest of the overlayed
subrecords in the variable part.
It is up to the user to handle the overlay
mechanism (keep track of visible variants
etc.)
The base of the component type must n̲o̲t̲ be
BYTE.
A record type has the same base as another
record type, if they are equivalent.
Examples DAYTYPE=
RECORD
DAY: 1..31;
MONTH: 1..12;
YEAR: INTEGER
END;
PERSON=
RECORD
NAME: ARRAY 1..NAMELENGTH OF CHAR;
BIRTHDAY: DAYTYPE
END;
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Constant and Type Definition L
VARREC=
RECORD
VARIANTS:
(SCH: SCHOOLTYPE;
VARIANTS:
(CLASS: INTEGER) !
(FACULTY:INTEGER)
END;) !
(KINDERGARDEN: STRING)
END;
END;
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Decl. & Denot. of var. L
Declaration Variables may be declared and initialized
in
and denotation SWELL.
of variables But as no runtime system exists, allocation
of varibales is not performed dynamically.
Space for all declared variables is allocated
at compile time.
If a programmer wants another allocation
method, he has to implement it himself.
Declaration OH 1 Variable declarations consist of a list of
identifiers denoting the variables, followed
by their type.
Examples VAR
I, J: INTEGER;
L: LONG;
OPERATOR: (PLUS, MINUS, TIMES);
HASHTABLE: ARRAY O..HASHSIZE-1 OF INTEGER;
CLASSTABLE: ARRAY 0..127 OF CLASSTYPE;
The base of the variable type must not be
byte.
Variable OH 2 Denotations of variables either designate
a
Denotations register variable, an entire variable, a
component variable, a variable referenced
by a pointer, or a constant parameter record.
R0-R7 OH 3 The general purpose registers are predeclared
as variables of type INTEGER.
R01-R70 The same variables may be used as variables
of type LONG.
In this case two registers are concatenated
and used as one variable.
Redefinition The type of a register may be redefined to
another type with the same base type.
Entire OH 5 An entire variable is denoted by its
Variable identifier. An entire variable is addressed
as a base relative operand, where no index
register is used. The address refers to the
first word of the variable.
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Decl. & Denot. of var. L
Component OH 6 A component of a variable is denoted
variable by the variable followed by an selector specifying
the component.
Indexed OH 7 A component of an array variable is denoted
variable by the variable followed by an index expression.
Field OH 8 A component of a record is denoted by the
designator record variable followed by the field identifier
of the component.
Example L. LEAST;
Referenced OH 9 The base type of the pointer operand must
be
variable integer.
The SWELL, the notation of a variable referenced
by a pointer differs from the notation used
in PASSCAL.
This is so because a pointer is not associated
with variables of a specific type.
In SWELL, pointers must therefore be qualified
explicitly when they are used.
Examples R4 ENTRYTYPE;
R4 ENTRYTYPE.NEXT INTEGER;
R4 DAYTYPE.MONTH;
P INTEGER;
If a register is used as a pointer variable,
a referenced variable may be addressed in
a very efficient way on a CR80, as the addressing
may be performed by an index register.
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Decl. & Denot. of var. L
Standard OH 10 A standard procedure MON is predeclared in
procedure the implementation of SWELL. This procedure
MON is intended for calling system procedures
via the MON instruction. The procedure differs
from procedures declared in SWELL programs
because:
- R7 is used as link register (optional)
(usually only R4, R5 and R6 may be used
for this purpose).
- the procedure may be called with
varying numbers and types of parameters
- The first parameter specifies the
system procedure; and must therefore
be
a constant expression.…86…1
…02… …02… …02… …02… …02…
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SWELL Compiler & Ex. 1 L
SWELL The SWELL compiler consists of three main
passes and a service pass.
All passes are executed in the same program
and data areas, using an overlay technique.
The compiler might be characterized as a
single pass compiler for a virtual CR80 (executing
pseudo instructions, using logical references),
followed by a two pass code generator.
The main duties of each individual pass are:
Pass 1 Lexical analysis
Syntax analysis
Name analysis
Semantic analysis
Pass 2 Preliminary code selection
Worst case address allocation
Error reporting
Pass 3 Final code selection
Code optimizing
Final address allocation
Pass 4 Generation of cross references
Generation of debug information
During Pass 1 The program source text is transformed into
an intermediate form.
The intermediate code includes operators
and symbolic operands, leaving it to the
subsequent passes to find CR80 instructuions
performing the operations on operands of
the specified kinds (registers, constants
etc.).
All declarations are totally consumed by
pass 1 and all structures are broken down
into simple context free pseudo instructions.
Pass 2 and 3 Thus, pass 2 and pass 3 are not specifically
SWELL oriented, and they may very well be
used as code generators for future compilers
for the CR80 computer. It should be mentioned,
that the compiler produces link modules,
and the final code assembly is performed
by a linkage editor.
Exercise 1 HO 1
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EXPRESSION L
Expressions OH 1 Constant expressions and variable expressions
OH 2 'J+H' is not allowed.
Constant A constant expression is an expression, for Expressions which
for
values
of
all
operands
are
constants,
and
where
the
expression
therefore
may
be
evaluated
at
compile
time.
As
for
the
variable
expressions
the
arithmetic
operators
have
the
same
precedence,
and
they
are
executed
from
left
to
right.
Address Denotes that the address of an operand is
to be used as operand.
The address is a word address, and it is
not possible to take the address of a variable
with base type BYTE.
Location Denotes that the location of a procedure
or a label is to be used as operand.
Size Denotes the size of a variable of the specified
type.
The size of a variable is defined to be the
smallest number of words necessary to hold
the variable.
Min/Max Denotes that the minimum/maximum value of
the values defined in the list is to be used
as an operand. When the list element is a
constant, its value is used.
When the list element is a subrange type
identifier, the lower/upper bound is used.
When the list element is an enumeration type
identifier, the value of the first/last enumeration
type element is used.
Examples 15
'a'
-19
SIZE (LONG)
MAX (SIZE (T1), SIZE (T2), SIZE (T3))
ADDRESS ('SWELL')
A + B + (C+D)
A constant expression is associated a base
type corresponding to the concept defined
for variables.
T̲h̲e̲ ̲b̲a̲s̲e̲ ̲t̲y̲p̲e̲ ̲o̲f̲ ̲a̲ ̲c̲o̲n̲s̲t̲a̲n̲t̲ ̲i̲s̲ ̲i̲n̲t̲e̲g̲e̲r̲.
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EXPRESSION L
Variable OH 3 Variable expressions are constructs denoting
Expressions rules for obtaining values or addresses of
variables and generating new values by application
of operators.
In most arithmetic and logic instructions
CR80 uses one of the source operands as destination
operand. As SWELL supplies no runtime system
to support the execution of the program,
and as the language reflects the instruction
set of CR80, the following rules concerning
evaluation of expressions has been defined:
* Operators are applied strictly from
left to right.
* Intermediate results are not stored
in
work locations. Instead one of the
source operands is used as destination
and thus changed.
* If more than one operator have to be
applied, the destination operand of
an
operation is used as source operand
in
the next operation.
Relational OH 4 A relational expression is a rule for
Expressions combining a series of relations. Each relation
indicates a test (comparison) giving the
result true or false, and the result of a
relational expression is true or false.
However, the result is not explicitly accessible
(cannot be used as an operand). It may only
be used as a controlling condition within
a statement (conditional or repetitive statement).
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STATEMENTS L
Statements OH 1 Statements denote algorithmic actions, and
are said to be executable.
A label may be defined by prefixing a statement
by the label. A label must be declared and
defined in the same block.
A label may be referenced from the procedure
in which it is declared, and in all inner
procedures.
Simple OH 2 A simple statement is a statement of which
statements no part constitutes another statement. The
empty statement consists of no symbols and
denotes no action.
GOTO OH 3 A goto statement serves to indicate that
statements processing should continue at a continuation
point, defined by a label.
Structured OH 4 Structured Statements are constructs
Statements composed of other statements, which have
to be executed either in sequence (compound
statement), conditionally (conditional statements),
or repeatedly (repetitive statements).
Compound OH 5 The compound statement specifies that its
Statements component statements are to be executed in
the same sequence as they are written. The
symbols 'BEGIN' and 'END' act as statement
brackets.
Conditional OH 6 A conditional statement selects for
Statements execution a single one of its component statements.
IF
CASE
SWITCH
If Statements OH 7 The if statement specifies that a statement
is to be executed only if a certain condition
(relational expression) is true. If it is
false, then either no statement is to be
executed, or the statement following the
symbol 'else' is to be executed.
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STATEMENTS L
Case OH 8 The case statement consists of an expression
Statements (the selector) and a list of statements,
each being labelled by a constant or a constant
identifier. It specifies that the one statement
is to be executed, whose label is equal to
the value of the selector.
If the case selector is of subrange type,
the case jump table includes one entry for
each value in the subrange, and no limit
checking is performed. Otherwise the case
jump table will include one entry for each
value in the range defined by the largest
and the smallest occurring case labels, and
index checking is performed.
Repetitive OH 9 Repetitive statements specify that certain
Statements statements are to be executed repeatedly.
While OH 10 The while statement specifies that a
Statements statement is to be executed repeatedly until
a condition (a relational expression) becomes
false. If the condition is not true at the
beginning, the statement is not executed
at all.
Repeat OH 11 The repeat statement specifies that a number
Statement of statements are to be executed repeatedly
until a condition (relational expression)
becomes true. The statements will be executed
at least once.
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SWITCH MON L
Switch OH 1 The switch statement is very much like a
Statement case statement. However, the selector of
a switch statement is a switch call (a call
of a procedure with multiple exits).
The switch statement specifies that the one
statement is to be executed whose label is
equal to the exit number through which the
procedre terminates.
The type identifier used in switch call indicates,
via the type associated to it, a range of
exit numbers.
This facility is mainly used in connection
with standard procedures (like MON) having
different sets of exits depending on actual
parameters.
Exercise 2 HO
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VARIABLE INIT L
Variable OH 1 A variable initialization is a compile time
assignment of values to variables.
Values must not be assigned to imported variables.
Temporary variables are not assignable at
compile time.
Variables are n̲o̲t̲ initialized a runtime (e.g.
at procedure entry).
In the initialization part values may be
assigned to variables of simple types (integer
or char) and to arrays having components
of simple type.
Examples INIT
COUNTER=0;
CLASSTABLE 0..127 = IGNORE;
CLASSTABLE 'A'..'Z' = ALFA;
DAYS ̲PER ̲MONTH= 31, 28, 31, 30, 31, 30,
31, 30, 31, 30, 31;
How to HO
Multiply/
Divide
Exercise 3 HO
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PROCEDURES L
Procedure OH 1 A procedure statement serves to execute the
Statements procedure denoted by the procedure identifier.
The procedure statement contains a list of
actual parameters, which must correspond
to the formal parameters defined in the procedure
declaration (Chapter 13). The correspondance
is established by the positions of the parameters
in the lists of actual and formal parameters
respectively.
The exit type of the procedure may be overruled
by specifying the type in the procedure call,
but the exit type should always include the
exit numbers actually used in the procedure
body.
Procedure OH 2 Procedure declarations serve to define parts
Declarations OH 3 of programs and to associate indentifiers
with them, so that they can be activated
by procedure statements.
The procedure heading specifies the identifier
naming the procedure and the formal parameters.
Four kinds of parameters exist:
- constant parameters
- link register
- exit parameters
- register parameters
Exit OH 4 An exit statement serves to indicate that
Statements processing should terminate within the actual
procedure and continue at a continuation
point in the calling body.
The continuation point is explicitly defined
via an expression (of which a register is
a simple case) defining the link and optionally
an exit number (within the exit range, defined
at procedure declaration).
If no exit number is specified, this is equivalent
to exit no zero.
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PROCEDURES L
No implicit exit statement is generated (e.g.
at the terminating end of a procedure), so
it is the programmers responsibility that
a procedure is left properly.
Examples EXIT (R4) 'return to link'
EXIT (R4, SW-ERROR); 'return to error
exit'
EXIT (R4 + SIZE (PROC)); 'return after
constant
parameters'
Constant OH 5 An element denoted by this construction is
Parameter the UNASSIGNABLE record corresponding to
the
Record constant parameters in the actual call of
the procedure.
The constant parameters are accessed by specifying
the formal parameters as fields within this
record.
Examples Denotation of constant
parameter record : R6 COPY
Denotation of a constant
parameter : R6 COPY.FROM
ENTER OH 6 An enter statement serves to indicate that
statements processing should continue at a continuation
point defined by an expression. The return
point may be saved by defining a link register.
Example ENTER (R7, R4); "active a program
"part addressed by R7
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Dedicated Statement L
Dedicated OH 1 Is a statement dedicated for the
statements OH 2 execution of one specific CR80 machine
OH 3 instruction.
May be used to gain address to special features
of the CR80 Hardware, e.g. a control operation
on an external device.
The syntax is very similar to the procedure
statement, except for the missing link register
(because they do not perform any "jump to
subroutine" but merely executes inline code.
Use of HO
dedicated
Statements
Exercise 5 HO
