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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:
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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
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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
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Actually you do not use the Trap flag in this way, because you are normally monitoring a program from an
Interrupt Service Routine (ISR). You continue execution of the program by an IRET.
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does a type-1 interrupt, so the single-step mode will be disabled during the interrupt-service procedure.
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To reset the trap flag, simply replace the OR instruction in the preceding sequence with the instruction:
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using only integer (otherwise you must also store floating point registers)
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continue execution for ONE instruction, then calling ISR again.
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Stack: Ret, Flags, AX, CX, DX, BX, SP, BP, SI, DI, DS, ES
269:Stack: Ret, Flags, AX, CX, DX, BX, SP, BP, SI, DI
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185:can use it to step through the execution of a
93:. Unsourced material may be challenged and
157:Learn how and when to remove this message
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308:Set TF flag in the stored Flag register
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91:adding citations to reliable sources
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290:Stack: Ret, Flags, AX, CX, DX, BX,
181:mode. If such a flag is available,
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260:Int3ServiceRoutine: ;
42:and remove advice or instruction.
343:The trap flag is reset when the
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239:Copy SP to BP for use as index
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501:Central processing unit
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253:Restore flag Register
202:trap flag and type-1
193:Single-step interrupt
294:, BP, SI, DI, DS, ES
87:improve this article
40:rewrite this article
338:AND WORD PTR,0FEFFH
232:Push flags on stack
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304:OR WORD PTR,0100H
262:Stack: Ret, Flags
242:OR WORD PTR,0100H
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33:a manual or guide
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282:... the ISR code
187:computer program
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358:Status register
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147:September 2011
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102:Find sources:
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72:This article
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48:February 2016
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85:Please help
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38:Please help
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246:Set TF flag
106:"Trap flag"
495:Categories
200:Intel 8086
177:in single-
117:newspapers
506:Debugging
329:Resetting
301:Stored SP
286:MOV BP,SP
235:MOV BP,SP
204:interrupt
183:debuggers
175:processor
171:trap flag
74:does not
322: ;
306: ;
299: ;
288: ;
277: ;
267: ;
251: ;
244: ;
237: ;
230: ;
297:MOV BP,
275:PUSH ES
272:PUSH DS
215:Setting
131:scholar
95:removed
80:sources
481:Flags
314:POP DS
311:POP ES
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265:PUSHA
228:PUSHF
209:stack
138:JSTOR
124:books
345:8086
320:IRET
317:POPA
249:POPF
221:8086
219:The
179:step
110:news
78:any
76:cite
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430:O
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