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Reader's Clinic - Volume Pointer Record

A reader writes "I've been using the tips from the December issue of REVMEDIA on creating Volume file variables and I keep getting a W559 error message"

                           Cannot continue                   
          An attempt was made to execute a null ("") program 
                       Press any key to contine              

This problem occurs when the Volume pointer in Voc is modified to establish Volume MFSs. (It is possible to have an MFS apply to every file on a volume by inserting the MFS name before the BFS name (@VM delimited) in the Volume pointer.) When this happens the structure of the volume file variable changes slightly. In this case the @VMs in the Filing System list must be converted to @SVMs. Thus the code given on page 11 of issue 7 would be modified to be

0001   FS = VOL_REC<4>
0002   DOS = VOL_REC<5>
0004   FILE_VAR = FS : @VM : DOS

Alternatively, if the Volume has not been attached, another way of getting all of the information required to create a volume file variable is to make a direct call to RTP49 (the routine used by the V18 ATTACH processor). This is a subroutine taking four parameters, (A,B,C,D) where A is the volume name, B is a flag (set to 0), C is the Volume Record returned and D is the Volume Pointer Record returned. C has five fields, the fifth of which is the Map file variable. D has three fields, the second of which is the FS list. Thus

0001   CALL RTP49(A,B,C,D)
0002   DOS = C<5>
0003   FS = D<2>
0005   FILE_VAR = FS : @VM : DOS

(Volume 2, Issue 8, Page 3)
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