问题
I'm trying to understand the workings of the D flag in the code segment descriptor when used in the x86-64 code. It's set in the D/B bit 22 of the code segment descriptor as shown on this diagram:
The Intel documentation (from section 3.4.5 Segment Descriptors) states the following:
D/B (default operation size/default stack pointer size and/or upper bound) flag
Performs different functions depending on whether the segment descriptor is an executable code segment, an expand-down data segment, or a stack segment. (This flag should always be set to 1 for 32-bit code and data segments and to 0 for 16-bit code and data segments.)
• Executable code segment. The flag is called the D flag and it indicates the default length for effective addresses and operands referenced by instructions in the segment. If the flag is set, 32-bit addresses and 32-bit or 8-bit operands are assumed; if it is clear, 16-bit addresses and 16-bit or 8-bit operands are assumed. The instruction prefix 66H can be used to select an operand size other than the default, and the prefix 67H can be used select an address size other than the default.
So I'm trying to understand which x86-64 instructions does it affect and how?
PS. When I try to run some tests (in Windows kernel) by setting that bit on, the OS immediately triple faults.
回答1:
If L (long mode) is set for a code segment descriptor, D must be clear. The L=1 / D=1 combination is currently meaningless / reserved. Intel documents this nearby in the same document you were looking at.
If L is clear, then D selects between 16 and 32-bit mode. (i.e. the default operand / address size). And yes, 16-bit protected mode exists, but no, nobody uses it.
There are only 3 possibilities for default address/operand-size:
- 16-bit modes (real, vm86, protected): default address and operand-size = 16-bit
- 32-bit protected mode: default address and operand-size = 32-bit
- 64-bit mode: default address size = 64-bit, default operand-size = 32-bit
There's no option to have 16x 64-bit registers but a default operand size of 16-bit or 64-bit. Or a default address size of 32-bit overrideable to 64.
来源：https://stackoverflow.com/questions/51322139/what-does-the-d-flag-in-the-code-segment-descriptor-do-for-x86-64-instructions