In a false-flag all bits are clear (0 when interpreted as integer). In
a canonical true-flag all bits are set (-1 as a twos-complement signed
integer); in many contexts (e.g.,
if) any non-zero value is
treated as true flag.
false . true . true hex u. decimal
Comparison words produce canonical flags:
1 1 = . 1 0= . 0 1 < . 0 0 < . -1 1 u< . \ type error, u< interprets -1 as large unsigned number -1 1 < .
Gforth supports all combinations of the prefixes
0 u d d0 du f f0
(or none) and the comparisons
= <> < > <= >=. Only a part of
these combinations are standard (for details see the standard,
Numeric comparison, Floating Point or Word Index).
You can use
and or xor invert as operations on canonical flags.
Actually they are bitwise operations:
1 2 and . 1 2 or . 1 3 xor . 1 invert .
You can convert a zero/non-zero flag into a canonical flag with
0<> (and complement it on the way with
1 0= . 1 0<> .
You can use the all-bits-set feature of canonical flags and the bitwise
operation of the Boolean operations to avoid
: foo ( n1 -- n2 ) 0= if 14 else 0 endif ; 0 foo . 1 foo . : foo ( n1 -- n2 ) 0= 14 and ; 0 foo . 1 foo .
For reference, see Boolean Flags, Numeric comparison, and Bitwise operations.