80-Bus News


March–April 1984, Volume 3, Issue 2

Page 40 of 51

POP – Removes one address off the stack of GOSUB addresses so the next RETURN ill branch one statement beyond the second most recently executed GOSUB.

PTR – Followed by two values allows the setting of selected scratch pad locations without using POKE or DOKE. The advantage of this is that the XBASIC workspace area is different for various implementations. For example the worksp-ace locations in the Nas-Dos version starts at 1006H but in the CP/M implementation, it is at 0100H. Thus some compatability is achieved. There are 24 selected locations. e.g. A=PTR(14) places the address (in decimal!) of the end of the XBASIC program currently in memory into variable A.

SCRN$ – Followed by a number which must be less than the number of rows on the screen (16 on a standard Nascom), returns the string of characters from this row number, the length of which will always be equal to the number of columns on the screen (48 on a standard Nascom).

SEP – Followed by an ASCII value alters the seperator found in input statements – normally a comma. e.g. SEP 47 makes / the seperator. SEP 0 allows any characters to be input in INPUT statements including commas – although in this case, only one input at a time would be allowed.

SIZE – Returns the current space available for program use. As for FRE(0) in ROM BASIC.

SPEED – Followed by a number from 0 to 255 sets a delay in the character output to the current output device. 0 is very slow whilst 255 is normal speed.

SWAP – Followed by two variables swaps the contents of the variables which may be numeric or string types. Saves a lot of space in sort routines.

ZONE – Followed by two numbers sets the print zone (tab) width and the largest column for which printing to the next zone will stay on the same line. Default values for a standard Nascom are 14 and 36 respectively.

ON ERR – Errors can be trapped and dealt with as you desire. It is possible to deal with certain errors only and then continue in the program whilst unexpected errors will still break out of the program. The function ERL returns the line number where the error occurred, ERR returns the number of the type of error –there is a list in the manual, ERRS$ returns the error string message without the word “error”.

OFF ERR – Turns off the ON ERR command. An ON ERR command will turn off anyway after an error has occurred in case the error routine has an error in it(!) but the error dealing routine can always turn it back on again at the end of the routine.

ON EOF – As for ON ERR except that this deals specifically with the encounter of an end of file when reading disk or tape data files and which would normally cause an End of Text Error. If ON ERR and ON EOF are both in force then the latter takes precedence if an end of file condition is sensed. The main difference between the two error handling statements is that the ON EOF routine does not turn off after an EOF error.

Page 40 of 51