BITCHANGE toggles the state of a specified bit in the local bit table, and return the original value of the bit.
table_no specifies the position in the table of the bit to be changed.
A unique table of 128 bits (numbered 1 to 128) is maintained for each process. Each bit in the table is treated as a two-state flag - the value returned will always be 0 (zero) or 1.
BITCHANGE returns the value of the bit before it was changed. You can therefore check and set (or reset) a flag in one step.
BITCHANGE also provides some special functions if you use one of the following table_no values:
If bit 100 in the table is 0 (zero), it will be set to 1 and 0 will be displayed. If bit 100 is set to 1 (one), the reverse will apply.