7.5 Task Switching 任务切换

The 80386 switches execution to another task in any of four cases:

80386在以下四种情况下会切换另一个任务去执行：

1. The current task executes a JMP or CALL that refers to a TSS descriptor.

当前任务执行了一个引用了TSS描述符的JMP或CALL。

1. The current task executes a JMP or CALL that refers to a task gate.

当前任务执行了一个引用任务门的JMP或CALL。

1. An interrupt or exception vectors to a task gate in the IDT.

中断或异常向量指向一个IDT中的任务门。

1. The current task executes an IRET when the NT flag is set.

当NT标志被置位时，当前任务执行了IRET指令。

JMP, CALL, IRET, interrupts, and exceptions are all ordinary mechanisms of the 80386 that can be used in circumstances that do not require a task switch. Either the type of descriptor referenced or the NT (nested task) bit in the flag word distinguishes between the standard mechanism and the variant that causes a task switch.

JMP、CALL、IRET、中断、异常都是80386的普通机制，用来在不能要求任务切换的情况下使用。引用的描述符类型或是标志字中的NT（嵌套任务）位区分标准机制和非标准机制引起任务切换。

To cause a task switch, a JMP or CALL instruction can refer either to a TSS descriptor or to a task gate. The effect is the same in either case: the 80386 switches to the indicated task.

为了引起任务切换，JMP或CALL指令可以引用TSS描述符或一个任务门。二者效果相同：80386切换到指明的任务。

An exception or interrupt causes a task switch when it vectors to a task gate in the IDT. If it vectors to an interrupt or trap gate in the IDT, a task switch does not occur . Refer to Chapter 9 for more information on the interrupt mechanism.

当中断或异常指向IDT中一个任务门时，引发任务切换。如果它指向一个IDT中的中断或陷阱门，任务切换不发生。参见第9章关于中断机制的更多描述。

Whether invoked as a task or as a procedure of the interrupted task, an interrupt handler always returns control to the interrupted procedure in the interrupted task. If the NT flag is set, however, the handler is an interrupt task, and the IRET switches back to the interrupted task.

无论是作为任务还是中断任务的程序，一个中断处理例程总会将控制交回到被中断任务里的被中断程序中。然而，如果NT标志置位，处理例程是一个中断任务，并且IRET切换回被中断的任务。

A task switching operation involves these steps:

任务切换操作按如下步骤进行：

1. Checking that the current task is allowed to switch to the designated task. Data-access privilege rules apply in the case of JMP or CALL instructions. The DPL of the TSS descriptor or task gate must be numerically greater (e.g., lower privilege level) than or equal to the maximum of CPL and the RPL of the gate selector. Exceptions, interrupts, and IRET are permitted to switch tasks regardless of the DPL of the target task gate or TSS descriptor.

检查当前任务是否可以切换到指定的任务。在这种情况下，JMP或CALL指令要实施数据访问特权规则。TSS描述符或者任务门的DPL必须在数值上大于（低特权级别）或等于CPL和门选择子之间的最大者。允许异常、中断和IRET切换任务，而不管目标任务门或TSS描述符的DPL值如何。

1. Checking that the TSS descriptor of the new task is marked present and has a valid limit. Any errors up to this point occur in the context of the outgoing task. Errors are restartable and can be handled in a way that is transparent to applications procedures.

检查新任务的TSS描述符是否标记为存在以及限长是否有效。任何错误都是发生在被换出任务的上下文环境中。错误是可重现的，可以被处理，并且对于应用程序是透明的。

1. Saving the state of the current task. The processor finds the base address of the current TSS cached in the task register. It copies the registers into the current TSS (EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI, ES, CS, SS, DS, FS, GS, and the flag register). The EIP field of the TSS points to the instruction after the one that caused the task switch.

保存当前任务的状态。处理器找到绑在在任务寄存器中的当前TSS基址。拷贝寄存器到当前TSS（EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI, ES, CS, SS, DS, FS, GS, and 标志寄存器 EFLAGS）。TSS中的EIP字段指向引发任务切换的下一条指令。

1. Loading the task register with the selector of the incoming task‘s TSS descriptor, marking the incoming task‘s TSS descriptor as busy, and setting the TS (task switched) bit of the MSW. The selector is either the operand of a control transfer instruction or is taken from a task gate.

将随后任务TSS描述符的选择子装入任务寄存器，标志该任务TSS描述符为忙，设置MSW（机器状态字）中的TS（任务切换）位。从任务门中取得的选择子即也是控制转移指令的操作数。

1. Loading the incoming task‘s state from its TSS and resuming execution. The registers loaded are the LDT register; the flag register; the general registers EIP, EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI; the segment registers ES, CS, SS, DS, FS, and GS; and PDBR. Any errors detected in this step occur in the context of the incoming task. To an exception handler, it appears that the first instruction of the new task has not yet executed.

从TSS中装入随后的任务状态并恢复执行。被装载的寄存器为LDT寄存器；标志寄存器；通用寄存器EIP, EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI；段寄存器 ES, CS, SS, DS, FS, GS；以及PDBR寄存器。在这一步侦测到的任何错误都发生在随后的任务上下文中。对于一个异常处理例程，很显然新任务中的第一条指令并没有被执行。

Note that the state of the outgoing task is always saved when a task switch occurs. If execution of that task is resumed, it starts after the instruction that caused the task switch. The registers are restored to the values they held when the task stopped executing.

注意，当任务切换发生时，被换出的任务状态总是被保存的。如果这个任务被恢复，它从引发任务切换的指令后面开始。寄存器的值被恢复到任务停止执行时的值。

Every task switch sets the TS (task switched) bit in the MSW (machine status word). The TS flag is useful to systems software when a coprocessor (such as a numerics coprocessor) is present. The TS bit signals that the context of the coprocessor may not correspond to the current 80386 task. Chapter 11 discusses the TS bit and coprocessors in more detail .

每次任务切换都会设置MSW（机器状态字）中的TS（任务切换）位。当协处理器（例如数据协处理器）存在时，TS标志对于系统软件很有用。TS位发送信号，来表明协处理器上下文，不用和当前80386任务一致。第11章讨论TS位和协处理器的更多细节。

Exception handlers that field task-switch exceptions in the incoming task (exceptions due to tests 4 thru 16 of Table 7-1) should be cautious about taking any action that might load the selector that caused the exception. Such an action will probably cause another exception, unless the exception handler first examines the selector and fixes any potential problem.

在随后任务（表7-1的4到16的测试关于异常）中处理任务切换异常的例程应当关注任何会引发异常的装入选择子动作。这样的动作可能会引发另一个异常，除非异常处理例程首先检测选择子并修任何潜在的问题。

The privilege level at which execution resumes in the incoming task is neither restricted nor affected by the privilege level at which the outgoing task was executing. Because the tasks are isolated by their separate address spaces and TSSs and because privilege rules can be used to prevent improper access to a TSS, no privilege rules are needed to constrain the relation between the CPLs of the tasks. The new task begins executing at the privilege level indicated by the RPL of the CS selector value that is loaded from the TSS.

在被恢复执行的随后任务中的特权级别即不受被切出的任务的特权级别的约束，也不受影响。因为任务是通过独立的地址空间和TSS隔离的，同时特权规则可以用来阻止对TSS的不正确访问，没有特权规则用来限制任务的CPL之间的关联。从TSS中装入的CS选择子的RPL表明了新任务的特权级别。

Table 7-1. Checks Made during a Task Switch 任务切换时的检验

NP = Segment-not-present exception

GP = General protection fault

TS = Invalid TSS

SF = Stack fault

Validity tests of a selector check that the selector is in the proper

table (e.g., the LDT selector refers to the GDT), lies within the bounds of

the table, and refers to the proper type of descriptor (e.g., the LDT

selector refers to an LDT descriptor).

Test     Test Description                   Exception    Error Code Selects

  1      Incoming TSS descriptor is

         present                            NP           Incoming TSS

  2      Incoming TSS descriptor is

         marked not-busy                    GP           Incoming TSS

         marked not-busy

  3      Limit of incoming TSS is

         greater than or equal to 103       TS           Incoming TSS

             -- All register and selector values are loaded --

  4      LDT selector of incoming

         task is valid                      TS           Incoming TSS

  5      LDT of incoming task is  

         present                            TS           Incoming TSS

  6      CS selector is valid               TS           Code segment

  7      Code segment is present            NP           Code segment

  8      Code segment DPL matches  

         CS RPL                             TS           Code segment

  9      Stack segment is valid             GP           Stack segment

 10      Stack segment is present           SF           Stack segment

 11      Stack segment DPL = CPL            SF           Stack segment

 12      Stack-selector RPL = CPL           GP           Stack segment

 13      DS, ES, FS, GS selectors are

         valid                              GP           Segment

 14      DS, ES, FS, GS segments

         are readable                       GP           Segment

 15      DS, ES, FS, GS segments

         are present                        NP           Segment

 16      DS, ES, FS, GS segment DPL  

         >= CPL (unless these are

         conforming segments)               GP           Segment

【译】x86程序员手册26-7.5任务切换