On 9/7/2017 9:35 PM, Chuck Guzis via cctalk wrote: > On 09/07/2017 07:18 PM, Jon Elson wrote: > >> Ah, well, I can see why a 7 track tape won't read well on a 9-track drive! > > I was a bit puzzled at why a tapemark would read as 135 (hex). Sigh--at > least the parity is correct. <groan> > > --Chuck > >
On an IBM 1410 (at least - but I suspect this was likely widely true), tape marks were *always* EVEN parity, 0x0F (Bits 8421), even on an odd parity tape. Learned about this one day when all of a sudden all of our FORTRAN compiles caused the machine to error stop, but the machine seemed OK otherwise. WTH?? So we called IBM, explained what was going on. The CE calmly opened the console panel, and turned on the Asterisk Insert switch. He explained about the tape mark, and the fact that the FORTRAN compiler happened to write its intermediate files in odd parity. With that switch off, the even parity tape mark read under odd parity was placed into core that way - with even parity, which was an invalid core character, which stopped the machine. With that switch on, the tape mark came into storage as an asterisk, and the machine was happy. (In either case, the end-of-file latch was set, of course. See IBM 1410 Principles of Operation, "Read or Write Tape with Word Marks" - in the -3 version, it is pages 87 and 88.) JRJ
