Intel IA-32 - Manual (Page 134)

4-4 Vol. 3A
PROTECTION
Many different styles of protection schemes can be implemented with these fields and flags.
When the operating system creates a descriptor, it places values in these fields and flags in
keeping with the particular protection style chosen for an operating system or executive. Appli-
cation program do not generally access or modify these fields and flags.
The following sections describe how the processor uses these fields and flags to perform the
various categories of checks described in the introduction to this chapter.
4.2.1 Code Segment Descriptor in 64-bit Mode
Code segments continue to exist in 64-bit mode even though, for address calculations, the
segment base is treated as zero. Some code-segment (CS) descriptor content (the base address
and limit fields) is ignored; the remaining fields function normally (except for the readable bit
in the type field).
Code segment descriptors and selectors are needed in IA-32e mode to establish the processor’s
operating mode and execution privilege-level. The usage is as follows:
•
IA-32e mode uses a previously unused bit in the CS descriptor. Bit 53 is defined as the
64-bit (L) flag and is used to select between 64-bit mode and compatibility mode when
IA-32e mode is active (IA32_EFER.LMA = 1). See Figure 4-2.
— If CS.L = 0 and IA-32e mode is active, the processor is running in compatibility mode.
In this case, CS.D selects the default size for data and addresses. If CS.D = 0, the
default data and address size is 16 bits. If CS.D = 1, the default data and address size is
32 bits.
— If CS.L = 1 and IA-32e mode is active, the only valid setting is CS.D = 0. This setting
indicates a default operand size of 32 bits and a default address size of 64 bits. The
CS.L = 1 and CS.D = 1 bit combination is reserved for future use and a #GP fault will
be generated on an attempt to use a code segment with these bits set in IA-32e mode.
•
In IA-32e mode, the CS descriptor’s DPL is used for execution privilege checks (as in
legacy 32-bit mode).