The trap flag (TF) : Setting TF puts the processor into single step mode for debugging. In single stepping, microprocessor executes a instruction and enters into single step ISR.
After that user can check register or memory contents, if found ok, user can proceed further, else necessary action will be taken. This utility is, to debug the program. If TF = 1, the CPU automatically generates an internal executes instruction by instruction, allowing a program to be inspected as it executes instruction by instruction.
Used by debuggers for single step operation.
Tf = '1' - Trop on, TF = '0' - Trap off.