Nothing can pass a serializing instruction and

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Unformatted text preview: wn algorithms may be developed; however, care must be taken by the developers to ensure that either: • • The differing TLB mappings are not actually used on different processors during the update process. OR The operating system is prepared to deal with the case where processor(s) is/are using the stale mapping during the update process. 7.4. SERIALIZING INSTRUCTIONS The Intel Architecture defines several serializing instructions. These instructions force the processor to complete all modifications to flags, registers, and memory by previous instructions and to drain all buffered writes to memory before the next instruction is fetched and executed. For example, when a MOV to control register instruction is used to load a new value into control register CR0 to enable protected mode, the processor must perform a serializing operation before it enters protected mode. This serializing operation insures that all operations that were started while the processor was in real-address mode are completed before the switch to protected mode is made. The concept of serializing instructions was introduced into the Intel Architecture with the Pentium® processor to support parallel instruction execution. Serializing instructions have no meaning for the Intel486™ and earlier processors that do not implement parallel instruction execution. 7-11 MULTIPLE-PROCESSOR MANAGEMENT It is important to note that executing of serializing instructions on P6 family processors constrain speculative execution, because the results of speculatively executed instructions are discarded. The following instructions are serializing instructions: • • Privileged serializing instructions—MOV (to control register), MOV (to debug register), WRMSR, INVD, INVLPG, WBINVD, LGDT, LLDT, LIDT, and LTR. Nonprivileged serializing instructions—CPUID, IRET, and RSM. The CPUID instruction can be executed at any privilege level to serialize instruction execution with no effect on program flow, except that the EAX, EBX, ECX, and EDX registers are modified. Nothing can pass a serializing instruction, and serializing instructions cannot pass any other instruction (read, write, instruction fetch, or I/O). When the processor serializes instruction execution, it ensures that all pending memory transactions are completed, including writes stored in its store buffer, before it executes the next instruction. The following additional information is worth noting regarding serializing instructions: • The processor does not writeback the contents of modified data in its data cache to external memory when it serializes instruction execution. Software can force modified data to be written back by executing the WBINVD instruction, which is a serializing instruction. It should be noted that frequent use of the WBINVD instruction will seriously reduce system performance. When an instruction is executed that enables or disables paging (that is, changes the PG flag in control register CR0), the instruction should be followed by a jump instruction. The target instruction of the jump instruction is fetched with the new setting of the PG flag (that is, paging is enabled or disabled), but the jump instruction itself is fetched with the previous setting. The P6 family processors do not require the jump operation following the move to register CR0 (because any use of the MOV instruction in a P6 family processor to write to CR0 is completely serializing). However, to maintain backwards and forward compatibility with code written to run on other Intel Architecture processors, it is recommended that the jump operation be performed. Whenever an instruction is executed to change the contents of CR3 while paging is enabled, the next instruction is fetched using the translation tables that correspond to the new value of CR3. Therefore the next instruction and the sequentially following instructions should have a mapping based upon the new value of CR3. (Global entries in the TLBs are not invalidated, refer to Section 9.10., “Invalidating the Translation Lookaside Buffers (TLBs)”, Chapter 9, Memory Cache Control.) The Pentium® and P6 family processors use branch-prediction techniques to improve performance by prefetching the destination of a branch instruction before the branch instruction is executed. Consequently, instruction execution is not deterministically serialized when a branch instruction is executed. • • • 7-12 MULTIPLE-PROCESSOR MANAGEMENT 7.5. ADVANCED PROGRAMMABLE INTERRUPT CONTROLLER (APIC) The Advanced Programmable Interrupt Controller (APIC), referred to in the following sections as the local APIC, was introduced into the Intel Architecture with the Pentium® processor (beginning with the 735/90 and 815/100 models) and is included in all P6 family processors. The local APIC performs two main functions for the processor: • • It processes local external interrupts that the processor receives at its interrupt pins and local internal interrupts that software generates. In multiple-process...
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This note was uploaded on 06/07/2013 for the course ECE 1234 taught by Professor Kwhon during the Spring '10 term at University of California, Berkeley.

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