Elements of the language
KEY IMP80 is a high-level block-structured language. Relative to Algol 60, it adds program structuring, data structuring, event signalling and text handling facilities, but removes (or retains in a modified form) intrinsically inefficient features such as the Algol 60 %name-type parameter.
Character set
KEY An IMP80 program is a sequence of statements constructed using the ISO seven bit character set extended with a special alphabet. The special alphabet is used to construct keywords. The ISO seven bit character set is as follows:
_______________________________________________________________________ | | | | | | 0 NUL | 32 SPACE | 64 @ | 96 ` | | 1 SOH | 33 ! | 65 A | 97 a | | 2 STX | 34 " | 66 B | 98 b | | 3 ETX | 35 # | 67 C | 99 c | | 4 EOT | 36 $ | 68 D | 100 d | | 5 ENQ | 37 % | 69 E | 101 e | | 6 ACK | 38 & | 70 F | 102 f | | 7 BEL | 39 ' | 71 G | 103 g | | 8 BS | 40 ( | 72 H | 104 h | | 9 HT | 41 ) | 73 I | 105 i | | 10 LF(NL) | 42 * | 74 J | 106 j | | 11 VT | 43 + | 75 K | 107 k | | 12 FF | 44 , | 76 L | 108 l | | 13 CR | 45 - | 77 M | 109 m | | 14 SO | 46 . | 78 N | 110 n | | 15 SI | 47 / | 79 O | 111 o | | 16 DLE | 48 0 | 80 P | 112 p | | 17 DC1 | 49 1 | 81 Q | 113 q | | 18 DC2 | 50 2 | 82 R | 114 r | | 19 DC3 | 51 3 | 83 S | 115 s | | 20 DC4 | 52 4 | 84 T | 116 t | | 21 NAK | 53 5 | 85 U | 117 u | | 22 SYN | 54 6 | 86 V | 118 v | | 23 ETB | 55 7 | 87 W | 119 w | | 24 CAN | 56 8 | 88 X | 120 x | | 25 EM | 57 9 | 89 Y | 121 y | | 26 SUB | 58 : | 90 Z | 122 z | | 27 ESC | 59 ; | 91 [ | 123 { | | 28 FS | 60 < | 92 \ | 124 | | | 29 GS | 61 = | 93 ] | 125 } | | 30 RS | 62 > | 94 ^ | 126 ~ | | 31 US | 63 ? | 95 _ | 127 DEL| |_______________|____________________|_________________|______________| The use of the character set is summarised in the sections below and described in detail in the rest of the manual. However, some general points about certain characters should be noted at the outset:
a) Quotes Several language constructions call for one or more characters to be enclosed in quotes. Within quotes all characters are significant and stand for themselves; thus, for example, space, newline, and percent characters may appear between quotes and stand for space, newline, and percent. Two quote characters are used: ' - character quote " - string quote Examples: 'A', ';', '"', '!' "String", "sealing wax", "!" If it is required to include the delimiting quote within the text it must be represented by two consecutive quotes.
Examples: '''' - the character quote "A ""big"" dog" - a string of eleven characters However, note '"' and "it's mine". b) Spaces Except when used to terminate keywords or when between quotes, space characters are ignored and may be used to improve the legibility of the program. c) Lower case letters Except when enclosed in quotes, lower case letters are equivalent to the corresponding upper case letters.
d) Non-graphic characters The ISO characters with no graphic representation have code values less than 32 or greater than 126. Any non-graphic characters can be included within quotes in an IMP80 program, but only one, the newline character (NL), is included in the syntactic definition of the language (see Appendix A). This has the ISO code value 10. {EMAS IMP80: all characters with code values less than 32 are treated as spaces, apart from NL (ISO code 10) EM (ISO code 25) - terminates program SUB(ISO code 26) - causes a syntax fault It ignores DEL (ISO code 127) and maps all codes greater than 127 according to the extended Regional Network Code (which is detailed in Regional Communications Memo JID/78/M20.4). }
{IMP77 Except for NL (see above) all characters not enclosed in quotes and with ISO codes outwith the range 32 to 126 inclusive are treated as spaces, but will be sent to the listing unaltered. The character FF (form feed) may be used to cause a new page to be taken in program listing files.}
Statements
KEY An IMP80 program typically consists of an ordered set of statements, as follows: %begin <declaration statements and procedure descriptions> %on %event .... %start <executable statements> %finish <executable statements> <procedure descriptions> %end %of %program
Only the delimiting keywords %begin and %end %of %program must be present. However, assuming that all the types of statement above are represented in an IMP80 program, the effect of running the program on a computer is as follows: First, storage space for the declarations is obtained. Conceptually, all the space is obtained at once - there is no defined order of execution of declaration statements. The procedure descriptions are skipped, as are the statements in the %on %event block, and the first of the executable statements is obeyed. Thereafter, the statements themselves define the sequence of execution ("flow of control"). Control passes to the %on %event block if one of the events defined in the %on %event statement occurs whilst executing a statement. The %on %event block is optional and often omitted.
Here is an IMP80 program: %begin %integer A, B, SUM, DIFFERENCE, MAX, MIN MAX = -1; MIN = 5000 %cycle READ(A) %exit %if A=-1; ! End of input. READ(B) PRINTSTRING("Input values:") WRITE(A,3); PRINTSYMBOL(',') WRITE(B,3) SUM = A+B; DIFFERENCE = A-B PRINTSTRING(" ... Sum is") WRITE(SUM,3) PRINTSTRING(", Difference is") WRITE(DIFFERENCE,3); NEWLINE %if SUM>MAX %start MAX = SUM %finish %else %if SUM<MIN %start MIN = SUM %finish %repeat ! Now print out the maximum and minimum of the sums of all ! the pairs of numbers read in. NEWLINE PRINTSTRING("Maximum sum is") WRITE(MAX,3); NEWLINE PRINTSTRING("Minimum sum is") WRITE(MIN,3); NEWLINE %end %of %program
The different types of statement in IMP80 are listed below, with references: %comment 1.2.3.1 %include 1.2.3.2 %list & %end %of %list 1.2.3.3 declaration 2 assignment 2 %begin 3.1 %end %of %program 3.1 %on %event 3.1.1 procedure specification 3.2/5 procedure heading 3.2 procedure call 3.2 %end %of %file 3.3.1 jump & label 4.2.2 %start/%finish 4.3 %cycle/%repeat 4.4
Continuation
KEY Statements are normally terminated by a newline or semi-colon character. However, a statement may extend over several physical lines provided that each line break occurs after a comma, or is preceded by the keyword %c (which is otherwise ignored). Thus: %if X=Y %then P = 1 %c %else P = 0 is exactly equivalent to %if X=Y %then P =1 %else P = 0 and %own %integer %array X(1:10) = 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 is exactly equivalent to %own %integer %array X(1:10) = 10,9,8,7,6,5,4,3,2,1
{EMAS IMP80: Blank lines between the lines of a continued statement are ignored.} {IMP77: the line break can also occur after the keywords %or and %and.}
Statement components
KEY IMP80 statements are made up of atoms. An atom is an identifier, a constant, a special symbol or a keyword.
Identifiers
KEY An identifier is a sequence of any number of letters and digits, starting with a letter. For example: C1900 TO 1970, NUMBER OF BLOCKS, MAX, x, item1, Item 2, item 2b. Spaces within identifiers are permitted but ignored, as is the case of the letters; thus Item 2 and ITEM2 are not distinguished. It is recommended that meaningful identifiers be used whenever possible, to improve the clarity of programs. In this manual, identifiers are always given in upper case. It is stressed that this is merely a convention.
Identifiers are used to name the following entities: Arithmetic, string and record variables and arrays Reference variables Record sub-fields Procedures (i.e. routines, functions, maps) Record and array formats Named constants Simple labels and switch vectors Identifiers used in an IMP80 program must be declared before they may be used. The only exception to this general rule is that simple labels are not declared. Further rules relating to the uniqueness of identifiers are given in the relevant sections.
Constants
KEY IMP80 includes the following types of constant: Decimal integer constants e.g. 2243, -16, 1 000 000 Base constants e.g. 2_1001, 8_7720, 16_A06C, B'1011', X'1A69' Real constants e.g. 120.0, 120, 1.2@2, 12@1, 1200@-1 Character constants e.g. 'A', 'a', '+', '"', ' ', '''', '6' Multi-character constants e.g. M'Four', M'MAX', M'1+1=', M'"#%' String constants e.g. "Here is a string", "A", "123", "a ""good"" boy" Named constants (also called e.g. %constant %integer PRICE=23 "%constant variables") : TOTAL COST = PRICE * NUMBER
Special symbols
KEY These are as follows: _________________________________________________________________ | | | ! !! ' " # (or \= or <>)| | | | ## (or \==) & ( ) * | | | | \ \\ + , - | | | | -> . / // : | | | | ; < <- << <= | | | | = == > >= >> | | | | @ \ (or ~) _ { } | | | | newline space | |_________________________________________________________________|
{IMP77: \ and \\ may be replaced by ^ and ^^ respectively.} {EMAS IMP80: \ and \\ may be replaced by ** and **** respectively.} The special symbols are used in IMP80 in various ways: * as operators (arithmetic, string, record, relational, logical) * as separators, e.g. (..) {..} : ; , newline space * in constants, e.g. 3_201, +27@4, -17.6, X'8F', "Constant" * in record subfield references, e.g. TAX(MONTH)_OVERTIME * ! as an alternative to the keyword %comment
Keywords
KEY A keyword is a sequence of letters in a special alphabet. In programs stored in a computer in text form ("source" programs), representation of this alphabet is achieved by using the character percent (%), which is defined as causing the subsequent letters (upper or lower case) to be shifted into the special alphabet. The effect of the % character is terminated by any non-alphabetic character, including space and newline. Hence the following statements are equivalent: %string(7) %array %name P %STRING (7) %ARRAYNAME P In this manual keywords are given in lower case.
The following is a list of all the IMP80 keywords: ____________________________________________________________ | | | %alias %and %array %begin %byte | | %c %comment %const %constant %continue | | %cycle %dynamic %end %event %exit | | %external %file %finish %fn %for | | %format %function %half %if %include | | %integer %list %long %map %monitor | | %name %not %on %of %or | | %own %program %real %record %repeat | | %result %return %routine %short %signal | | %spec %start %stop %string %switch | | %system %then %unless %until %while | |____________________________________________________________|
Miscellaneous statements
KEY Certain statements are described here rather than in subsequent sections, as they are different in kind from the other statements of the language, and do not readily fit into any of the categories used in the rest of the manual.
Comments
KEY Textual comments can be included in an IMP80 program. They have no effect on the program when it runs, but may be used to render it meaningful both to the originator and to others. A comment is a statement, and must be separated from the preceding statement by a newline or semi-colon and from the succeeding statement by a newline - not by a semi-colon. Note that the continuation rules described above apply to comment statements. The keyword %comment is used to introduce the text of the comment, which may contain any character except newline. %comment may be replaced by the symbol '!'. {IMP77: comments cannot be continued onto subsequent lines by any of the methods described.} Example: %begin %comment This program sorts a list of names %comment into alphabetical order : : %end %of %program
The IMP80 language also allows comments to be embedded within other statements. An embedded comment is any sequence of characters, excluding '}' and newline, enclosed in a pair of braces, '{' and '}'. This form of comment may appear between any two atoms, but may not occur within an atom. For convenience the closing brace may be replaced by a newline. Example: ! This is a portion of a test program COUNT = 0; ! Note the semi-colon before the '!'; note thenext one ! next one (in the text of the comment). LIMIT = 100 {Only 100 cases} MINIMUM = 0 {all positive PROCESS(X {cases}, Y {total cost}) This fragment of program is exactly equivalent to
COUNT = 0 LIMIT = 100 MINIMUM = 0 PROCESS(X,Y) Note that this type of comment never counts as a statement in its own right, even when given on a line by itself. Thus it can be interposed between lines of a continued statement.
%include statement
KEY A file of IMP80 statements (terminated by the statement %end %of %file) may be included in a source program by giving a statement of the form %include <file specification> where <file specification> is a string constant representing a system-dependent file name. Refer to the relevant section of Appendix B for details of any implementation-dependent limitations on the use of %include. Example: %begin %include "WXYZ83.EXTSPECS" %integer TIME, DISPLACEMENT, ... : : Note that the %include statement appears in the program listing generated by the compiler, followed by the contents of the file referred to (including the %end %of %file). The %include statement should not be followed on the same line of the source program by other statements, since the complete source line will appear in the program listing before the statements included, while in the actual program the statements following the %include follow the statements included.
%list and %end %of %list
KEY Normally the compiler generates a listing of a program as it compiles it. The precise layout of this listing is implementation-dependent. The production of the listing can normally by controlled by setting a system option prior to the compilation. In addition, however, the programmer can control the generation of the listing by use of the statement %end %of %list, which inhibits the listing until the end of the program or the statement %list is encountered. The default is for the listing to be generated. The statements %end %of %list and %list may appear anywhere within an IMP80 program. Note that when listing is inhibited, incorrect statements and the accompanying fault messages are still listed. {IMP77: %end %of %list and %list are nested, so that two %end %of %list statements require two matching %list statements to switch on the listing again. This means that it is possible within an included file (see above) to control the listing of the contents of the file, without changing the listing status of the rest of the program.}