RTL/2

RTL/2

Paradigms	Multi-paradigm: imperative, structured, real-time
Family	ALGOL
Designed by	J.G.P. Barnes
Developer	Imperial Chemical Industries
First appeared	1972;50 years ago (1972)
Typing discipline	static, strong, safe, structural
Scope	Lexical
Platform	PDP-11, VAX
OS	Cross- (multi-) platformRSX-11M, VMS
Dialects
none
Influenced by
ALGOL 68

RTL/2 (Real-Time Language) is a discontinued high-level programming language for use in real-time computing, developed at Imperial Chemical Industries, Ltd. (ICI), by J.G.P. Barnes. It was originally used internally in ICI but was distributed by SPL International in 1974. ^[1] It was based on concepts from ALGOL 68, and intended to be small and simple. ^{[2] [3] [4]} RTL/2 was standardised in 1980 by the British Standards Institution. ^[5]

Language overview

The data types in RTL/2 were strongly typed, with separate compiling. The compilation units contained one or more items named bricks, i.e.:

• Procedure bricks
• Data bricks
• Stack bricks

A procedure brick was a procedure, which may or may not return a (scalar) value, have (scalar) parameters, or have local (scalar) variables. The entry mechanism and implementation of local variables was reentrant. Non-scalar data could only be accessed via reference (so-called REF variables were considered scalar).

A data brick was a named static collection of scalars, arrays and records. There was no heap or garbage collection, so programmers had to implement memory management manually.

A stack brick was an area of storage reserved for running all the procedures of a single process and contained the call stack, local variables and other housekeeping items. The extent to which stack bricks were used varied depending on the host environment in which RTL/2 programs ran.

Access to the host environment of an RTL/2 program was provided via special procedure and data bricks called SVC procedures and SVC data. These were accessible in RTL/2 but implemented in some other language in the host environment.

Hello World

TITLE Hello World;  LET NL=10;  EXT PROC(REF ARRAY BYTE) TWRT;  ENT PROC RRJOB() INT; TWRT("Hello World#NL#"); RETURN(1); ENDPROC; 

Embedded assembly

RTL/2 compiles to assembly language and provides the CODE statement to allow including assembly language in RTL/2 source code. This is only available when compiled with a systems programming option (CN:F)

The CODE statement takes two operands: the number of bytes used by the code insert and the number of bytes of stack used.

Within code statements two trip characters are used to access RTL/2 variables. These vary between different operating systems. On a Digital Equipment Corporation (DEC) PDP-11 running RSX-11M, and a VAX running VMS, the trip characters are * and /.

While the specifics varied by operating system the following is an example of a code insert on VAX/VMS:

CODE6,0;JMPCODE_ENT; This code insert can be set to a fixed length as it jumps to a new psect.; this method is especially useful on systems such as VMS where the length of; instructions is variable.SAVE_PSECT; Save current program section.PSECTASMB_CODE,EXE,NOWRT,LONGCODE_ENT:MOVL*PARAM1(AP),*COUNTER/MYDATAJMPCODE_EX.RESTORE_PSECTCODE_EX:*RTL

This code insert moves the value of a variable passed into the RTL/2 procedure into a variable named COUNTER in a data brick named MYDATA.

Reserved words

• ABS
• AND
• ARRAY
• BIN
• BLOCK
• BY
• BYTE
• CODE
• DATA
• DO
• ELSE
• ELSEIF
• END
• ENDBLOCK
• ENDDATA
• ENDPROC
• ENT
• EXT
• FOR
• FRAC
• GOTO
• HEX
• IF
• INT
• LABEL
• LAND
• LENGTH
• LET
• LOR
• MOD
• NEV
• NOT
• OCT
• OF
• OPTION
• OR
• PROC
• REAL
• REF
• REP
• RETURN
• RTL
• SHA
• SHL
• SLA
• SLL
• SRA
• SRL
• STACK
• SVC
• SWITCH
• THEN
• TITLE
• TO
• VAL
• WHILE

References

1. Barnes, J.G.P. (September 1980). "The Standardisation of RTL/2". Software: Practice and Experience. Wyley. 10 (9): 707–719. doi:10.1002/spe.4380100904. S2CID   5050804.
2. RTL/2 Language Specification
3. "The Encyclopedia of Computer Languages". Archived from the original on 10 March 2011.
4. Barnes, J.G.P. (1 January 1976). RTL/2 design and philosophy. Heyden. ISBN   978-0855012243.
5. BS 5904:1980: Specification for computer programming language RTL/2. British Standards Institution. 30 September 1980. ISBN   978-0580114410.

SPL documentation

SPL published a range of documentation for RTL/2. Each such document was assigned a reference number. The following is an incomplete list.

RTL/2 Ref 1  – RTL/2 Language Specification RTL/2 Ref 2  – Introduction to RTL/2 RTL/2 Ref 3  – RTL/2 Training Manual RTL/2 Ref 4  – System Standards RTL/2 Ref 5  – Stream I/O RTL/2 Ref 18 – Hints on writing RTL/2 Programs RTL/2 Ref 26 – Language Reference Card RTL/2 Ref 39 – Run time environment on the PDP-11 RTL/2 Ref 63 – User Manual for the PDP-11 under RSX-11M RTL/2 Ref 107 – VAX/VMS RTL/2 User Manual RTL/2 REF 130 – The RTL/2 32-bit run time environment on the VAX