SPL on MPE

SPL is the Original HP 3000 Systems Language

SPL stands for "Systems Programming Language." It is an Algol-based language that was introduced with the original HP 3000. All of the systems code for MPE was written in SPL, rather than in Assembly language. HP did not convert SPL to Native mode when they migrated MPE to the PA-RISC, but some third-party programmers did. Their native-mode SPL compiler is called SPLash!, a compiler that can even generate code for HP-UX.

Here are some SPL tips from the 1985 SMUG IV book:

Efficiency

Avoid ASSEMBLE and TOS because they make the programs hard to understand.
INTEGER is the most efficient data-type.
Use LOGICAL only for items that are flags or that must be unsigned, such as dates from the CALENDAR intrinsic.
Don't use BYTE arrays for small integers unless you check for overflow manually -- SPL uses INTEGER arithmetic.
Use a ripple move to initialize arrays to zero or blank, instead of using a FOR statement or moving a full literal constant. BUF := ""; MOVE BUF(1) := BUF, (65).
Use word moves instead of byte moves where possible.
Use SCAN and MOVE-WHILE, but don't warp the problem to fit them. To get the length, use variable := MOVE|SCAN.
Use local variables and parameters before global variables.
Data stack size equals global variables plus the deepest nesting of local variables.
Remember that your stack is expanded dynamically by MPE, but must be contracted manually by calling ZSIZE.
Use a VALUE parameter instead of a reference when you don't intend to pass back a result; VALUE is more efficient and better. You can pass in an expression, you can use it as a local variable, and you have less chance of disrupting the caller.
Use =PB constant arrays locally, or literals such as "ABC", before using global arrays.

Recommended Subset of SPL

Statements to use freely:

Assignment, MOVE, Procedure call, Subroutine call, IF, CASE, WHILE, DO-UNTIL, and BEGIN-END.

Statements requiring caution:

SCAN and MOVE WHILE.

Statements to avoid:

GO TO, RETURN, ASSEMBLE, FOR, PUSH, and SET.

Data structures to use freely:

Constants and EQUATEs, simple variables, arrays (first element = 0), DEFINEs, and types BYTE, INTEGER, LOGICAL and DOUBLE.

Data structures requiring caution:

REAL and LONG in files (they are not compatible with COBOL), POINTERs and bit fields.

Data structures to avoid:

TOS except after MOVE/SCAN, =register except =PB, and bit fields in permanent files (other languages can't handle them).

Suggested SPL Style

Follow the rules of structured programming and aim for a clear programming style. Use EQUATEs for integer constants, except 0 and 1. Use DEFINEs for other numeric constants. Use equivalenced arrays and length-EQUATEs to construct records. Use SUBROUTINEs to decompose PROCEDUREs into modules. Insert a $PAGE command before each module declaration. For local SUBROUTINE variables, declare dummy VALUE parameters. Use $INCLUDE files for any data or code that should be the same in several programs.