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Further reading □ FORTRAN graphics on the PDP15 □ Simple complete FORTRAN program using FOG □ FOG subroutines □ FOG errors □ Input routines for use with graphics programs □ Loading and overlaying FOG □ Appendices □ A: Summary of FOG subroutines □ B: VT15 instruction set □ C: 7-bit ASCII character codes □ D: Display orders generated by FOG routines □ E: The structure of FOG display files □ F: The VTA handler
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Further reading ⇓

□ FORTRAN graphics on the PDP15
□ Simple complete FORTRAN program using FOG
□ FOG subroutines
□ FOG errors
□ Input routines for use with graphics programs
□ Loading and overlaying FOG
Appendices
□ A: Summary of FOG subroutines
□ B: VT15 instruction set
□ C: 7-bit ASCII character codes
□ D: Display orders generated by FOG routines
□ E: The structure of FOG display files
□ F: The VTA handler

6. LOADING AND OVERLAYING FOG

6.1 Loading

All FOG subroutines are contained in the system library (.LIBR) which is automatically searched by the loader (or XCHAIN). The sources for the various subroutines are contained in the following files:

FOG     SRC  Most subroutines + FOG standard display file
FOG2    SRC  BLANK and UNBLNK
FOG3    SRC  DELETE
FOG4    SRC  Save/restore routines
VTERR   SRC  VTERR
VTARGL  SRC  VTARGL

[The routines for using the writing tablet are contained in TABLET and the routines for the lightpen in either LTPEN or LTPEN1 depending on the version required.]

Display files cannot cross bank boundaries; the following ways can be used to stop this happening:

  1. The display file can be set up in a subroutine - subroutines are loaded so that they do not cross page boundaries. This limits the size of the display file to 4K. Also all explicit references to the display file (DCHOOS, DINIT, DRAW and IDRAW) must be contained in the subroutine.
  2. The display file can be put in COMMON and the VTC option used in either CHAIN or XCHAIN. This allows one to use the maximum size display file of 8K. However, problems arise if the program is large and the display file is large as CHAIN or XCHAIN does not search for COMMON until it has loaded all of the program and consequently, the chances of finding a 8K bank empty are small.
  3. The display file can be put into BLOCK DATA. This allows one to specify the order of loading of the display file. It is thus possible by referring to the Load map to specify the order of loading and the display file size so that the display file does not cross a bank boundary. It is usual to specify, the BLOCK DATA segment as the second module to be loaded after the main routine. (It is not possible to load a BLOCK DATA segment first.)

6.2 Overlaying

If one wants to call FOG routines in an overlay environment it is only possible if either FOG is made resident, or all of FOG is put in one overlay. In this latter case the display must be initialised on loading of the overlay (DCHOOS'd and DINIT'd) and switched off on exit from the overlay (DCLOSE). It is not possible to overlay the FOG routines and preserve the display files as the standard FOG display file which calls the user display files is buried in the FOG routines.

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