3-4 Vol. 3

PROTECTED-MODE MEMORY MANAGEMENT

FFFF_FFF0H. RAM (DRAM) is placed at the bottom of the address space because the

initial base address for the DS data segment after reset initialization is 0.

3.2.2 Protected Flat Model

The protected flat model is similar to the basic flat model, except the segment limits

are set to include only the range of addresses for which physical memory actually

exists (see Figure 3-3). A general-protection exception (#GP) is then generated on

any attempt to access nonexistent memory. This model provides a minimum level of

hardware protection against some kinds of program bugs.

Figure 3-2. Flat Model

Figure 3-3. Protected Flat Model

Linear Address Space
(or Physical Memory)
Data and
FFFFFFFFH
Segment
LimitAccess
Base Address
Registers
CS
SS
DS
ES
FS
GS
Code
0
Code- and Data-Segment
Descriptors
Stack
Not Present
Linear Address Space
(or Physical Memory)
Data and
FFFFFFFFH
Segment
LimitAccess
Base Address
Registers
CS
ES
SS
DS
FS
GS
Code
0
Segment
Descriptors
LimitAccess
Base Address
Memory I/O
Stack
Not Present