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In Real-Address Mode, the IRET instruction preforms a far return to the interrupted program or procedure. During this operation, the processor pops the return instruction pointer, return code segment selector, and EFLAGS image from the stack to the EIP, CS, and EFLAGS registers, respectively, and then resumes execution
Getting to ring 3 can be done using iret because the way it works has been documented. When you receive an interrupt, the processor pushes: The stack segment and pointer (ss:esp), as 4 words; EFLAGS; The return code segment and instruction pointer (cs:eip), as 4 words; An error code, if required.
If the return is to virtual-8086 mode, the processor also pops the data segment registers from the stack. If the NT flag is set, the IRET instruction performs a task switch (return) from a nested task (a task called with a CALL instruction, an interrupt, or an exception) back to the calling or interrupted task.
See the section titled "Task Linking" in Chapter 6 of the IA-32 Intel Architecture Software Developer's Manual, Volume 3. IRET and IRETD are mnemonics for the same opcode. The IRETD mnemonic (interrupt return double) is intended for use when returning from an interrupt when using the 32-bit operand size; however,
Code is expected to suspend this nested task by executing an IRET instruction, which, because the NT flag is set, will automatically use the previous task link to return to the calling task. (See "Task Linking" in Chapter 6 of the IA-32 Intel Architecture Software Developer's Manual, Volume 3, for more information on nested
1 Mar 2017 CF, iret, pm="82", Interrupt return to lesser privilege, iret. The operating system may load the TSS with the values of the registers that the new task needs and after executing a hardware task switch (such as with an IRET instruction) the x86 CPU will load the saved values from the TSS into x86 assembly
7 May 2016 When an interrupt(be it fault, IRQ, or software interrupt) happens while the CPU is in user mode the CPU needs to know where the kernel stack is . OS to a different CPU, it won't have the TSS, so you'll have to implement software task switching anyway; x86 64bit mode does not allow you to use the TSS
In the original 8086 processor (and all x86 processors in Real Mode), the Interrupt Vector Table controlled the flow into an ISR. The IVT In newer x86 models, the IVT was replaced with the Interrupt Descriptor Table. The iret instruction restores CS:IP and FLAGS, allowing the interrupted program to continue unaffected.
The operating system may load the TSS with the values of the registers that the new task needs and after executing a hardware task switch (such as with an IRET instruction) the x86 CPU will load the saved values from the TSS into the appropriate registers.
CPU: i8086+. Type of Instruction: System Instruction: IRET IRETD Affected Flags: AF, CF, DF, IF, PF, SF, IF, ZF Description: Returns control to point of interruption by popping IP, CS and then Flags from the stack and continues execution at this location. CPU exception interrupts will return to the instruction that couse the

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March 2018