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(nasm.info)Section B.4.313

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B.4.313. `SYSCALL': Call Operating System
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     SYSCALL                       ; 0F 05                [P6,AMD]

   `SYSCALL' provides a fast method of transferring control to a fixed
entry point in an operating system.

   * The `EIP' register is copied into the `ECX' register.

   * Bits [31-0] of the 64-bit SYSCALL/SYSRET Target Address Register
     (`STAR') are copied into the `EIP' register.

   * Bits [47-32] of the `STAR' register specify the selector that is
     copied into the `CS' register.

   * Bits [47-32]+1000b of the `STAR' register specify the selector
     that is copied into the SS register.

   The `CS' and `SS' registers should not be modified by the operating
system between the execution of the `SYSCALL' instruction and its
corresponding `SYSRET' instruction.

   For more information, see the `SYSCALL and SYSRET Instruction
Specification' (AMD document number 21086.pdf).