面向程序设计的硬件基础ComputerSystems

most system calls
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Linux/IA32 System Calls
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Trap Example: Opening File
User calls: open(filename, options) Function open executes system call instruction int
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Today
Exceptional Control Flow Processes
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Processes
Definition: A process is an instance of a running program.
▪ One of the most profound ideas in computer science ▪ Not the same as “program” or “processor”
▪ Aborts
▪ Unintentional and unrecoverable ▪ Examples: parity error, machine check ▪ Aborts current program
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Exception Table IA32 (Excerpt)
256 different exception types
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Exception table
Exception numbers
Exception Table
0 1
2 ...
n-1
code for exception handler 0
code for exception handler 1
code for excepfor exception handler n-1
▪ This sequence is the CPU’s control flow (or flow of control)
Physical control flow
Time
<startup>
inst1 inst2 inst3 …
instn <shutdown>
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Altering the Control Flow
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Exceptional Control Flow
Exists at all levels of a computer system Low level mechanisms
▪ Exceptions
▪ change in control flow in response to a system event (i.e., change in system state)
Exception Class Fault Fault Fault Abort Interrupt or trap Trap Interrupt or trap
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Linux/IA32 System Calls
Examples of popular system calls
System calls are provided via a trapping instruction int $0x80 All parameters are passed through registers The standard C library provides a set of convenient wrapper functions for
a[500] = 13; }
80483b7:
c7 05 10 9d 04 08 0d movl $0xd,0x8049d10
User Process
OS
exception: page fault
movl
Create page and
returns
load into memory
Page handler must load page into physical memory Returns to faulting instruction Successful on second try
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Asynchronous Exceptions (Interrupts)
Caused by events external to the processor
▪ Indicated by setting the processor’s interrupt pin ▪ Handler returns to “next” instruction
movl
detect invalid address signal process
Page handler detects invalid address Sends SIGSEGV signal to user process User process exits with “segmentation fault”
Each type of event has a unique exception number k
k = index into exception table (a.k.a. interrupt vector)
Handler k is called each time exception k occurs
▪ Soft reset interrupt
▪ hitting Ctrl-Alt-Delete on a PC
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Interrupt handling
After the current instruction finishes executing Reads the exception number from the system bus Returns control to “next” instruction
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Synchronous Exceptions
Caused by events that occur as a result of executing an instruction:
▪ Traps
▪ Intentional ▪ Examples: system calls, breakpoint traps, special instructions ▪ Returns control to “next” instruction
▪ Combination of hardware and OS software
Higher level mechanisms
▪ Process context switch ▪ Signals ▪ Nonlocal jumps: setjmp() / longjmp() ▪ Implemented by either:
0804d070 <__libc_open>:
...
804d082:
cd 80
804d084:
5b
...
int $0x80 pop %ebx
User Process
OS
int
exception
pop
open file
returns
OS must find or create file, get it ready for reading or writing
▪ Faults
▪ Unintentional but possibly recoverable ▪ Examples: page faults (recoverable), protection faults
(unrecoverable), floating point exceptions ▪ Either re-executes faulting (“current”) instruction or aborts
0~31 Intel
32~255 OS
Exception Number 0 13 14 18 32-127 128 (0x80) 129-255
Description Divide error General protection fault Page fault Machine check OS-defined System call OS-defined
Process provides each program with two key abstractions:
▪ Logical control flow
▪ Each program seems to have exclusive use of the CPU
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Abort handling
Result from unrecoverable fatal errors Typically hardware errors Such as parity errors that occur when DRAM or SRAM bits are corrupted
Returns integer file descriptor
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Fault Example: Page Fault
User writes to memory location That portion (page) of user’s memory
is currently on disk
int a[1000]; main () {
▪ Data arrives from a disk or a network adapter ▪ Instruction divides by zero ▪ User hits Ctrl-C at the keyboard ▪ System timer expires
System needs mechanisms for “exceptional control flow” (ECF)
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Fault Example: Invalid Memory Reference