DECUS 82 FORTRAN Vers3 Nov65

This document describes FORTRAN for the PDP-1, Version 3, released in November 1965 by DECUS. This version replaces Version 002 and is designed for machines with "mul div" hardware.

Key aspects summarized:

1. Improvements & Bug Fixes (vs. Version 002):

   • Includes a check for and disallows mixed-mode arithmetic (mixing fixed and floating-point variables in one statement).
   • Fixes bugs related to the "log10f" function appearing as the first symbol in a statement and incorrect interpretation of blanks in "format" statements.
   • Integrates the FORTRAN "print" statement for use with DEC type 64 line printers. Instructions are provided to modify the subroutine package for systems without this printer.

2. FORTRAN Language Specifications:

   • Constants: Defines rules for integer (magnitude < 131071) and floating-point (up to 10 digits, 2-word format with 8-bit exponent and 28-bit mantissa, max 1x10^38) constants.
   • Variables: Integer variables start with I,J,K,L,M,N; floating-point variables start with any other letter. Names are up to 6 characters and cannot end with 'f'.
   • Statements: Covers arithmetic statements, supported functions (sinf, cosf, etc.), GO TO (unconditional, assigned, computed), SENSE LIGHTS and SENSE SWITCHES, IF (arithmetic), PAUSE, STOP (with optional messages), DO loops (including negative increments), CONTINUE, and END statements.
   • DIMENSION: Requires dimension statements to be in the executed flow, supporting up to 32 subscripts starting from 1. No recursive or floating-point subscripts are allowed.
   • Input/Output: Describes TYPE, PUNCH, PRINT, ACCEPT, READ statements using FORMAT specifications. Details various format codes: 'i' (integer), 'f' (floating-point, with overflow handling), 'x' (skip characters), 'h' and 'a' (alphanumeric, FIO-DEC code), and 'c' (printer carriage control).

3. System Operation & Compiling:

   • The FORTRAN system comprises seven paper tapes: the FORTRAN compiler, a modified DECAL compiler, a linking loader, symbol tapes, a subroutine package, a libetape, and a punch-off routine.
   • Detailed, multi-step procedures are provided for compiling, loading, and executing FORTRAN programs, as well as for compiling and loading the subroutine package.

4. Error Handling & Debugging:

   • Lists a comprehensive set of compiler errors (e.g., illegal paths, variable naming rules, mixed mode, format errors, DO loop issues) and runtime errors (e.g., divide halts, I/O errors, array bounds, exponent under/overflow).
   • Provides guidance on common DECAL compiler errors ("aps" for undefined variables) and specific error codes (140000 series).

5. Special Features & DECAL Inserts:

   • Explains special functionalities like sense switch control (address 7777) and escape characters (carriage return, tab).
   • Outlines how to use DECAL inserts within FORTRAN programs, including restrictions and referencing FORTRAN statements.

The document includes listings of DECAL modifications, symbol tables, and a sample FORTRAN program to illustrate concepts.