A trap flag permits operation of a processor in single-step mode. If such a flag is available, debuggers can use it to step through the execution of a computer program.[1]
When a system is instructed to single-step, it will execute one instruction and then stop. The contents of registers and memory locations can be examined; if they are correct, the system can be told to go on and execute the next instruction. The Intel 8086 trap flag and type-1 interrupt response make it quite easy to implement a single-step feature in an 8086-based system. If the trap flag is set, the 8086 will automatically do a type-1 interrupt after each instruction executes. When the 8086 does a type-1 interrupt, it pushes the flag register on the stack.
The 8086 has no instruction to directly set or reset the trap flag. These operations are done by pushing the flag register on the stack, changing the trap flag bit to what the programmer wants it to be, and then popping the flag register back off the stack. The instructions to set the trap flag are:
; ... the ISR code using only integers (otherwise you must also store floating point registers)
mov bp, sp ; Stack: Ret, Flags, AX, CX, DX, BX, SP, BP, SI, DI, DS, ES mov bp, [bp+10] ; Stored SP or word [bp], 100h ; Set TF flag in the stored FLAGS register pop es pop ds popa iret ; continue execution for one instruction, then calling the ISR again.
To reset the trap flag, simply replace the OR instruction in the preceding sequence with the instruction:
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