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Hi!
I'm developing my own OS. Successfully started a task in user mode (CPL 3) but when I called a dummy system call it always restarted the CPU. The facts:

architecture: x86, simple 80386 instruction set I use. Test and debug in qemu.
the problem happens in protected mode, fully set GDT and IDT, no paging. Right before the syscall CPU is in a totally consistent state.
dummy syscall means an interrupt gate with DPL=3, resides in kernel (0x8 CS selector), type=0xee, executes cli hlt. Worked fine if I called it from kernel.
User space code is written in C++,with inline asm. Works fine without syscalls.
Eflags' IF is set.
Stacks are OK (both SS and ESP)
IDT vectors look like this: 0x0008090c 0x0010ee00 (DPL=3, S=0, P=1, type=e, selector=8, offset=0x10090c. All offsets are correct.
The restart happens at the 'int 0x30' instruction.
Do you have any idea, what I've missed?

Thanks in advance,
Robert

Did you set up exception handlers? They can help you troubleshoot issues like this.

Did you set up a TSS?

No sure what happened, but CPU restarts in case of a Triple_Fault.
Briefly, it means that a fault occurred while trying to execute an interrupt handler, a software exception triggered by your user code in this case, and that there was no double-fault handler or if a fault occurred while trying to execute the double fault handler itself.

One of the reasons for a fault to occur when the CPU is trying to execute an exception handler is being unable to write to the stack.
That's why Octocontrabass asked if you did setup a TSS. The CPU needs to know which ESP and SS to use in case an
interrupt occurs.

TSS was the solution. Although I use sw task switching,kernel needed a TSS in GDT. Loaded by
LTR then everything worked fine.
Thanks for the help.

Problem solved, this topic became pointless.
Should I close it somehow?