up:
Chapter 10 -- Initialization
prev: 10.3 Switching to Protected Mode
next: 10.5 Initialization Example
10.4 Software Initialization for Protected Mode
Most of the initialization needed for protected mode can be done either
before or after switching to protected mode. If done in protected mode,
however, the initialization procedures must not use protected-mode features
that are not yet initialized.
10.4.1 Interrupt Descriptor Table
The IDTR may be loaded in either real-address or protected mode. However,
the format of the interrupt table for protected mode is different than that
for real-address mode. It is not possible to change to protected mode and
change interrupt table formats at the same time; therefore, it is inevitable
that, if IDTR selects an interrupt table, it will have the wrong format at
some time. An interrupt or exception that occurs at this time will have
unpredictable results. To avoid this unpredictability, interrupts should
remain disabled until interrupt handlers are in place and a valid IDT has
been created in protected mode.
10.4.2 Stack
The SS register may be loaded in either real-address mode or protected
mode. If loaded in real-address mode, SS continues to point to the same
linear base-address after the switch to protected mode.
10.4.3 Global Descriptor Table
Before any segment register is changed in protected mode, the GDT register
must point to a valid GDT. Initialization of the GDT and GDTR may be done in
real-address mode. The GDT (as well as LDTs) should reside in RAM, because
the processor modifies the accessed bit of descriptors.
10.4.4 Page Tables
Page tables and the PDBR in CR3 can be initialized in either real-address
mode or in protected mode; however, the paging enabled (PG) bit of CR0
cannot be set until the processor is in protected mode. PG may be set
simultaneously with PE, or later. When PG is set, the PDBR in CR3 should
already be initialized with a physical address that points to a valid page
directory. The initialization procedure should adopt one of the following
strategies to ensure consistent addressing before and after paging is
enabled:
- The page that is currently being executed should map to the same
physical addresses both before and after PG is set.
- A
JMP instruction should immediately follow the setting of PG.
10.4.5 First Task
The initialization procedure can run awhile in protected mode without
initializing the task register; however, before the first task switch, the
following conditions must prevail:
- There must be a valid task state segment (TSS) for the new task. The
stack pointers in the TSS for privilege levels numerically less than or
equal to the initial CPL must point to valid stack segments.
- The task register must point to an area in which to save the current
task state. After the first task switch, the information dumped in this
area is not needed, and the area can be used for other purposes.
up:
Chapter 10 -- Initialization
prev: 10.3 Switching to Protected Mode
next: 10.5 Initialization Example