User's Manual

Vol. 3 3-15

PROTECTED-MODE MEMORY MANAGEMENT

store its own data, such as information regarding the whereabouts of

the missing segment.

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.

• Stack segment (data segment pointed to by the SS

size of the stack pointer used for implicit stack operations (such as

pushes, pops, and calls). If the flag is set, a 32-bit stack pointer is

used, which is stored in the 32-bit ESP register; if the flag is clear,

a 16-bit stack pointer is used, which is stored in the 16-bit SP

segment (described in the next paragraph), the B flag also

specifies the upper bound of the stack segment.

• Expand-down data segment. The flag is called the B flag and it

specifies the upper bound of the segment. If the flag is set, the

upper bound is FFFFFFFFH (4 GBytes); if the flag is clear, the

upper bound is FFFFH (64

KBytes).

Figure 3-9. Segment Descriptor When Segment-Present Flag Is Clear

14 12

Available

TypeS

Available