Chilton::ACL::PDP15 User Notes

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Applications Associates Literature Society Technology Software revisited

Early history □ Minutes of committee meetings □ Progress reports □ Other reports □ News cuttings □ In-house publications □ Committee papers □ Personal recollections □ ACL publications □ Books □ Early Atlas publications □ FR80 reports □ Atlas manuals □ ICL1906A manuals □ Other manuals and papers

General □ Applications Software Group □ Basic Software Group □ FR80 □ FR80 operations □ FR80 project notes □ FR80 technical notes □ FR80 user notes □ Graphics □ Graphics Section □ Graphics Notes □ Graphics User Notes □ PDP15 □ PDP15 Notes □ SPROGS □ SPROGS Technical Notes □ SPROGS Papers □ SPROGS Notes

PDP15 User Note No 6: Bug concerning FORTRAN EXTERNAL statement

W D Shaw

25 February 1975

PROBLEM

The FORTRAN statement:

      EXTERNAL A,B,C,D

declares procedures A,B,C, and D as external global symbols to be passed as formal arguments in a subsequent subroutine call such as:

      CALL FANCY(32,C)

The F4V44 FORTRAN compiler generates a word for each external global symbol called a transfer vector. Transfer vectors should be modified by the loader to contain the run-time addresses of the external procedures to which they refer. Thus, external globals may be addressed using a single indirect reference. If A,B,C and D are all FORTRAN subroutines, the transfer vectors are properly updated to point to the entry points of the routines. It appears, however, that transfer vectors corresponding to MACRO procedures are not set at all, but left zero! No diagnostic is issued by F4V44 or LOADER.

The effect of this bug is that references to an external MACRO procedure are, in effect, references to location 0. Usually, the reference is a JMS 0, causing an IOPS error.

The bug may be avoided by writing a trivial FORTRAN procedure which calls the MACRO procedure and returns. The FORTRAN procedure must be declared external.

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