Hi Michael, You are correct that interrupts are disable automatically when an interrupt handler is accessed thru interrupt gate, but kernel re-enables the interrupts in handle_IRQ_event() function. > Is this to protect the code before the > handler from being interrupted? If so, why is this neccesary? Actually when interrupt occures the offset field in the interrup gate points to the function (MACRO written in assembly language) whose pointer is saved in interrupt array (read the code in i8259.c). This function pushes the negetive number (IRQ number - 256) (read the code at http://lxr.linux.no/source/include/asm-i386/hw_irq.h?v=2.4.21#L175) on the process specific kernel stack, which is the current stack at that time, as current stack is switched from user space stack of process to kernel space stack. After pushing this number on stack this calls the do_IRQ() function which actually first acquires the IRQ specific lock then acknowledges the interrupt by setting the EOI bit on PIC and also make some changes to the interrupt status (check the code of do_IRQ function at http://lxr.linux.no/source/arch/i386/kernel/irq.c?v=2.4.21#L563). Once it is done it calls the handle_IRQ_event() function which actually first disables the IRQ line by masking it on PIC and then re-enable the interrupts by calling __sti() function. After doing this all the handlers are called one by one and then while returning from this function we again disables the interrupts as the calling functions expects that. Cheers !! Gaurav On Fri, 2004-10-01 at 17:23, mbrennan@xxxxxxxx wrote: > Hello! > > I'm reading Understanding the Linux Kernel, and I'm trying to understand > interrupts and exceptions properly. I wonder if I have understanded this > correctly, this is how I think it works, and I would appreciate if someone > could confirm that its right. > > So, is this correct? > > Linux's interrupts are interleaved, ie. they can be nested. That must mean > that interrupts are enabled when handling interrupts. Now the thing that > confuses me a little is that Linux uses interrupt gates, which clears the > IF flag, and disables interrupts. But it seems like Linux sets the IF flag > again before the actual handler. Is this to protect the code before the > handler from being interrupted? If so, why is this neccesary? > > When interrupts are disabled by masking the maskable interrupts, does that > mean that the CPU ignores the signal on the INTR pin. And when a IRQ is > masked on the PIC, does that mean that the PIC doesn't send any interrupt > to the CPU when that IRQ is raised? > In other words, the IF flag makes the CPU ignore all maskable interrupts, > while masking a particular IRQ, it prevents the interrupt from reaching > the CPU. > > When executing an interrupt handler, maskable interrupts are enabled but > the IRQ of the interrupt is masked out. Other interrupts may interrupt the > execution, but not the same as the one currently executed? > > Thank you! > > /Michael > > -- > Kernelnewbies: Help each other learn about the Linux kernel. > Archive: http://mail.nl.linux.org/kernelnewbies/ > FAQ: http://kernelnewbies.org/faq/ > -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/ -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
