A trap flag permits operation of a processor

Processor

- Computing :* Central processing unit , an electronic circuit which executes computer programs, containing a processing unit and a control unit...

 in single-step mode. If such a flag is available, debugger

Debugger

A debugger or debugging tool is a computer program that is used to test and debug other programs . The code to be examined might alternatively be running on an instruction set simulator , a technique that allows great power in its ability to halt when specific conditions are encountered but which...

s can use it to step through the execution of a computer program

Computer program

A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...

.

Single-step interrupt

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

Intel 8086

The 8086 is a 16-bit microprocessor chip designed by Intel between early 1976 and mid-1978, when it was released. The 8086 gave rise to the x86 architecture of Intel's future processors...

 trap flag and type-1 interrupt

Interrupt

In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....

 response make it quite easy to implement a single-step feature in a 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

Stack (data structure)

In computer science, a stack is a last in, first out abstract data type and linear data structure. A stack can have any abstract data type as an element, but is characterized by only three fundamental operations: push, pop and stack top. The push operation adds a new item to the top of the stack,...

.

Setting

The 8086

Intel 8086

The 8086 is a 16-bit microprocessor chip designed by Intel between early 1976 and mid-1978, when it was released. The 8086 gave rise to the x86 architecture of Intel's future processors...

 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:

1. PUSHF ; Push flags on stack
2. MOV BP,SP ; Copy SP to BP for use as index
3. OR WORD PTR[BP+0],0100H ; Set TF flag
4. POPF ; Restore flag Register

Resetting

To reset the trap flag, simply replace the OR instruction in the preceding sequence with the instruction:

AND WORD PTR[BP+0],0FEFFH

The trap flag is reset when the 8086

Intel 8086

The 8086 is a 16-bit microprocessor chip designed by Intel between early 1976 and mid-1978, when it was released. The 8086 gave rise to the x86 architecture of Intel's future processors...

does a type-1 interrupt, so the single-step mode will be disabled during the interrupt-service procedure.