80-Bus News

change. Since this bug may also be present in other versions (with different addresses) the code which should be changed is just after the prompt for the tape number and the three SCALs to process the answer and get it into HL. The code should read:

Another undesirable feature (alright then, a bug) in TOS occurs if any errors occur when using TOS under program control. Since errors can occur at several ‘depths’ in TOS (e.g. an error in the number of arguments, no tape present, a read/​write error, etc.) there could be problems in knowing how many return addresses should be ‘thrown away’ so that TOS can do a quick exit. TOS gets round this the easy way by warm-starting itself whenever errors occur. No problem when using TOS normally (i.e. from the keyboard) but when using TOS under program control, it means that control remains in TOS. No way to get back to your program. You can’t just type “W” or “R” (Write or Read) because the arguments are obligatory. You can’t return to NAS­SYS to see what they are because ‘N’ cold-starts NAS­SYS, and this resets the workspace. Unless you have designed your program carefully, you have lost all the data your program just spent a long time building. It would be far better to return, say, to the outer level of the Read/​Write routines or whatever, and then return to the caller if an error is detected. The latest versions of TOS have a ‘Vectored Error handling function’ which I presume means that control is passed through a location in RAM to a (user) error routine.

The Write command (and B & P) automatically verifies each block after it has been written by rewinding to the start of the block and then ‘reading’ the block (but not loading it into RAM) and checking the CRC character at the end. If an error occurs, the block is rewritten, and if the error persists, the command is aborted & an error message is displayed. A drawback is that the catalogue will still contain an ‘entry’ for the erroneous file, which will have to be deleted.

The I command is used to initialise a tape, and all it does is write a blank directory to the start of the tape. TOS automatically checks to see whether the tape has already been initialised, and displays an error message if it has. No command has been provided to delete the catalogue (e.g. so that it may be reinitialised with another tape number). This is deliberate, as it avoids the accidental ‘erasure’ of a whole side. It can be done, however, by outputting the relevant commands to the ports, and this method is given in the manual. TOS also remembers where it is on a tape, so when reading more that one file off a tape, there is no need to read in the catalogue both times (or even the once). TOS cannot, however detect if a tape has been removed & replaced with another one. Thus the ‘R’ command (no arguments) is best used immediately after inserting a cassette.

TOS is written for Nascom systems using NAS­SYS monitors (sorry, all those Nasbug fans) and automatically detects whether NAS­SYS 1 or 3 is in use. Much use is made of NAS­SYS routines but not every call is made in the Approved Manner. The offenders are the call to Parse and to Errm. I don’t see why Errm is called (when an invalid command is entered) instead of the address of TOS’s own error routine (which calls ERRM anyway in the Approved Manner). These routines are called by reflections in RAM, so should your NAS­SYS be different (e.g. NAS­SYS 4 if & when,