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5-20 Vol. 3
PROTECTION
Note that the P flag in a gate descriptor is normally always set to 1. If it is set to 0, a
not present (#NP) exception is generated when a program attempts to access the
descriptor. The operating system can use the P flag for special purposes. For
example, it could be used to track the number of times the gate is used. Here, the P
flag is initially set to 0 causing a trap to the not-present exception handler. The
exception handler then increments a counter and sets the P flag to 1, so that on
returning from the handler, the gate descriptor will be valid.
5.8.3.1 IA-32e Mode Call Gates
Call-gate descriptors in 32-bit mode provide a 32-bit offset for the instruction pointer
(EIP); 64-bit extensions double the size of 32-bit mode call gates in order to store
64-bit instruction pointers (RIP). See
Figure 5-9:
• The first eight bytes (bytes 7:0) of a 64-bit mode call gate are similar but not
identical to legacy 32-bit mode call gates. The parameter-copy-count field has
been removed.
• Bytes 11:8 hold the upper 32 bits of the target-segment offset in canonical form.
A general-protection exception (#GP) is generated if software attempts to use a
call gate with a target offset that is not in canonical form.
• 16-byte descriptors may reside in the same descriptor table with 16-bit and
32-bit descriptors. A type field, used for consistency checking, is defined in bits
12:8 of the 64-bit descriptor’s highest dword (cleared to zero). A general-
protection exception (#GP) results if an attempt is made to access the upper half
of a 64-bit mode descriptor as a 32-bit mode descriptor.