175 lines
4.8 KiB
HTML
175 lines
4.8 KiB
HTML
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<head><title>Lecture 6: Interrupts & Exceptions</title></head>
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<body>
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<h1>Interrupts & Exceptions</h1>
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<p>
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Required reading: xv6 <code>trapasm.S</code>, <code>trap.c</code>, <code>syscall.c</code>, <code>usys.S</code>.
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<br>
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You will need to consult
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<a href="../readings/ia32/IA32-3.pdf">IA32 System
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Programming Guide</a> chapter 5 (skip 5.7.1, 5.8.2, 5.12.2).
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<h2>Overview</h2>
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<p>
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Big picture: kernel is trusted third-party that runs the machine.
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Only the kernel can execute privileged instructions (e.g.,
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changing MMU state).
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The processor enforces this protection through the ring bits
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in the code segment.
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If a user application needs to carry out a privileged operation
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or other kernel-only service,
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it must ask the kernel nicely.
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How can a user program change to the kernel address space?
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How can the kernel transfer to a user address space?
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What happens when a device attached to the computer
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needs attention?
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These are the topics for today's lecture.
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<p>
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There are three kinds of events that must be handled
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by the kernel, not user programs:
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(1) a system call invoked by a user program,
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(2) an illegal instruction or other kind of bad processor state (memory fault, etc.).
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and
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(3) an interrupt from a hardware device.
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<p>
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Although these three events are different, they all use the same
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mechanism to transfer control to the kernel.
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This mechanism consists of three steps that execute as one atomic unit.
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(a) change the processor to kernel mode;
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(b) save the old processor somewhere (usually the kernel stack);
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and (c) change the processor state to the values set up as
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the “official kernel entry values.”
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The exact implementation of this mechanism differs
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from processor to processor, but the idea is the same.
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<p>
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We'll work through examples of these today in lecture.
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You'll see all three in great detail in the labs as well.
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<p>
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A note on terminology: sometimes we'll
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use interrupt (or trap) to mean both interrupts and exceptions.
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<h2>
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Setting up traps on the x86
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</h2>
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<p>
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See handout Table 5-1, Figure 5-1, Figure 5-2.
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<p>
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xv6 Sheet 07: <code>struct gatedesc</code> and <code>SETGATE</code>.
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<p>
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xv6 Sheet 28: <code>tvinit</code> and <code>idtinit</code>.
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Note setting of gate for <code>T_SYSCALL</code>
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<p>
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xv6 Sheet 29: <code>vectors.pl</code> (also see generated <code>vectors.S</code>).
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<h2>
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System calls
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</h2>
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<p>
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xv6 Sheet 16: <code>init.c</code> calls <code>open("console")</code>.
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How is that implemented?
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<p>
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xv6 <code>usys.S</code> (not in book).
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(No saving of registers. Why?)
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<p>
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Breakpoint <code>0x1b:"open"</code>,
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step past <code>int</code> instruction into kernel.
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<p>
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See handout Figure 9-4 [sic].
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<p>
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xv6 Sheet 28: in <code>vectors.S</code> briefly, then in <code>alltraps</code>.
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Step through to <code>call trap</code>, examine registers and stack.
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How will the kernel find the argument to <code>open</code>?
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<p>
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xv6 Sheet 29: <code>trap</code>, on to <code>syscall</code>.
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<p>
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xv6 Sheet 31: <code>syscall</code> looks at <code>eax</code>,
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calls <code>sys_open</code>.
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<p>
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(Briefly)
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xv6 Sheet 52: <code>sys_open</code> uses <code>argstr</code> and <code>argint</code>
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to get its arguments. How do they work?
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<p>
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xv6 Sheet 30: <code>fetchint</code>, <code>fetcharg</code>, <code>argint</code>,
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<code>argptr</code>, <code>argstr</code>.
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<p>
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What happens if a user program divides by zero
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or accesses unmapped memory?
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Exception. Same path as system call until <code>trap</code>.
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<p>
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What happens if kernel divides by zero or accesses unmapped memory?
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<h2>
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Interrupts
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</h2>
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<p>
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Like system calls, except:
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devices generate them at any time,
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there are no arguments in CPU registers,
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nothing to return to,
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usually can't ignore them.
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<p>
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How do they get generated?
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Device essentially phones up the
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interrupt controller and asks to talk to the CPU.
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Interrupt controller then buzzes the CPU and
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tells it, “keyboard on line 1.”
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Interrupt controller is essentially the CPU's
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<strike>secretary</strike> administrative assistant,
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managing the phone lines on the CPU's behalf.
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<p>
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Have to set up interrupt controller.
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<p>
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(Briefly) xv6 Sheet 63: <code>pic_init</code> sets up the interrupt controller,
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<code>irq_enable</code> tells the interrupt controller to let the given
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interrupt through.
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<p>
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(Briefly) xv6 Sheet 68: <code>pit8253_init</code> sets up the clock chip,
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telling it to interrupt on <code>IRQ_TIMER</code> 100 times/second.
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<code>console_init</code> sets up the keyboard, enabling <code>IRQ_KBD</code>.
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<p>
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In Bochs, set breakpoint at 0x8:"vector0"
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and continue, loading kernel.
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Step through clock interrupt, look at
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stack, registers.
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<p>
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Was the processor executing in kernel or user mode
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at the time of the clock interrupt?
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Why? (Have any user-space instructions executed at all?)
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<p>
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Can the kernel get an interrupt at any time?
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Why or why not? <code>cli</code> and <code>sti</code>,
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<code>irq_enable</code>.
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</body>
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</html>
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