A function returning the value zero.
10 flag=FALSE 20 ... 150 IF flag ...
BBC BASIC (Z80) does not have true Boolean variables. Instead, numeric variables are used and their value is interpreted in a 'logical' manner.
A value of zero is interpreted as FALSE and NOT FALSE (in other words, NOT 0) is interpreted as TRUE. In practice, any value other than zero is considered TRUE.
You can use FALSE in a REPEAT...UNTIL loop to make the loop repeat for ever. Consider the following example.
10 terminator=10 20 REPEAT 30 PRINT "An endless loop" 40 UNTIL terminator=0
Since 'terminator' will never be zero, the result of the test 'terminator=0' will always be FALSE. Thus, the following example has the same effect as the previous one.
10 REPEAT 20 PRINT "An endless loop" 30 UNTIL FALSE
Similarly, since FALSE=0, the following example will also have the same effect, but its meaning is less clear.
10 REPEAT 20 PRINT "An endless loop" 30 UNTIL 0
See the keyword AND for logical tests and their results.