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into the operating system when the program attempts to use it. The OS then typically stops delivering interrupts to the program until the program executes STI (which would cause another fault). However, the POPF instruction is not privileged and simply fails silently to restore the IF. The result is
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programs that use a protected mode DOS extender and install their own interrupt handlers (usually games) use the CLI instruction in the handlers to disable interrupts and either POPF (after a corresponding PUSHF) or IRET (which restores the flags from the stack as part of its effects) to restore it.
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There are few satisfactory resolutions to this issue. It is usually not possible to modify the program, as source code is typically not available and there is no room in the instruction stream to introduce an STI without massive editing at the assembly level. Removing CLI's from the program or
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In some implementations of the instruction which enables interrupts, interrupts are not enabled until after the next instruction. In this case the sequence of enabling interrupts immediately followed by disabling interrupts results in interrupts not being recognized.
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causing the V86 host to ignore CLI completely might cause other bugs if the guest's interrupt handlers aren't designed to be re-entrant (though when executed on a modern processor, they typically execute fast enough to avoid overlapping of interrupts).
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that the OS stops delivering interrupts to the program, which then hangs. DOS programs that do not use a protected mode extender do not suffer from this problem, as they execute in V86 mode where POPF does trigger a fault.
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This works if the program was started in real mode, but causes problems when such programs are run in a
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In systems that support privileged mode, only privileged applications (usually the OS
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mechanism in uniprocessor systems. For example, a CLI is used in
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Intel 64 and IA-32 Architectures
Software Developer Manuals
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219:being set or cleared based on the bit in the
211:lags) removes a word from the stack into the
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392:Advanced Programmable Interrupt Controller
117:Learn how and when to remove this message
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343:enables interrupts by setting the IF.
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53:Please improve this article by adding
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359:only affects a single processor. In
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351:Multiprocessor Considerations
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518:Comparison of assemblers
153:. If the flag is set to
167:non-maskable interrupts
466:- Retrieved 2017-09-14
403:Non-maskable interrupt
42:relies excessively on
607:Open Watcom Assembler
548:x86 assembly language
204:nterrupt). The POPF (
688:Instruction listings
293:Disabling interrupts
177:Setting and clearing
683:Calling conventions
583:High Level Assembly
500:x86 assembly topics
332:Enabling Interrupts
193:nterrupt) and STI (
678:Processor register
316:code (typically a
181:In a system using
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595:Netwide Assembler
513:Assembly language
398:Interrupt handler
326:interrupt handler
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107:December 2018
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68: –
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62:Find sources:
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40:This article
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18:IF (x86 flag)
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538:Machine code
523:Disassembler
444:. Retrieved
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320:) can avoid
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617:Programming
719:Interrupts
708:Categories
636:Carry flag
626:Call stack
558:Assemblers
446:2007-07-13
421:References
387:Intel 8259
324:within an
151:interrupts
77:newspapers
44:references
693:Registers
656:Zero flag
377:Interrupt
267:Some old
249:Real mode
566:A86/A386
371:See also
239:. In an
297:In the
141:in the
91:scholar
668:Opcode
619:issues
609:(WASM)
603:(TASM)
597:(NASM)
591:(MASM)
573:(FASM)
506:Topics
394:(APIC)
318:driver
314:kernel
247:code (
233:kernel
93:
86:
79:
72:
64:
631:Flags
585:(HLA)
579:(GAS)
441:(PDF)
411:(PIC)
405:(NMI)
365:locks
282:fault
278:NTVDM
98:JSTOR
84:books
355:The
274:DPMI
189:ear
129:The
70:news
415:x86
338:x86
299:x86
269:DOS
241:x86
206:Pop
183:x86
171:INT
145:'s
143:CPU
139:bit
46:to
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429:^
367:.
187:Cl
135:IF
57:.
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486:t
479:v
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209:F
202:I
199:t
197:e
195:S
191:I
159:0
155:1
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105:(
95:·
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