2015-10-04 19:59:15 +02:00
|
|
|
/*
|
|
|
|
|
# Protected mode
|
|
|
|
|
|
2015-10-06 12:27:24 +02:00
|
|
|
Major changes:
|
|
|
|
|
|
|
|
|
|
- BIOS cannot be used anymore
|
|
|
|
|
|
|
|
|
|
- GDT takes effect immediately so we *have* to deal with it now,
|
|
|
|
|
and we will later turn on paging
|
|
|
|
|
|
|
|
|
|
- we have to encode instructions differently.
|
|
|
|
|
Note that in 16-bit 32-bit instructions were encodable, but with a prefix.
|
|
|
|
|
|
2015-10-04 19:59:15 +02:00
|
|
|
TODO get working.
|
|
|
|
|
|
|
|
|
|
- http://wiki.osdev.org/Journey_To_The_Protected_Land
|
|
|
|
|
- http://wiki.osdev.org/Protected_Mode
|
|
|
|
|
- https://github.com/chrisdew/xv6/blob/master/bootasm.S
|
|
|
|
|
- https://thiscouldbebetter.wordpress.com/2011/03/17/entering-protected-mode-from-assembly/ FASM based. Did not word on first try, but looks real clean.
|
2015-10-05 16:55:48 +02:00
|
|
|
- http://stackoverflow.com/questions/28645439/how-do-i-enter-32-bit-protected-mode-in-nasm-assembly
|
2015-10-04 19:59:15 +02:00
|
|
|
|
|
|
|
|
## GDT
|
|
|
|
|
|
|
|
|
|
Table in memory that gives properties of segment registers.
|
|
|
|
|
|
|
|
|
|
Segment registers in protected mode point to entries of that table.
|
|
|
|
|
|
|
|
|
|
The GDT modifies every memory access of a given segment by adding an offset to it.
|
|
|
|
|
|
|
|
|
|
GDT is used as soon as we enter protected mode, so that's why we have to deal with it, but the preferred way of managing program memory spaces is paging.
|
|
|
|
|
|
|
|
|
|
## GDTR
|
|
|
|
|
|
|
|
|
|
## GDT register
|
|
|
|
|
|
|
|
|
|
In 32-bit, a 6 byte register that holds:
|
|
|
|
|
|
|
|
|
|
- 2 byte length of the GDT (TODO in bytes or number of entries?)
|
|
|
|
|
- 4 byte address of the GDT in memory
|
|
|
|
|
|
|
|
|
|
In 64 bit, makes 10 bytes, with the address having 8 bytes
|
|
|
|
|
|
|
|
|
|
GRUB seems to setup one for you: http://www.jamesmolloy.co.uk/tutorial_html/4.-The%20GDT%20and%20IDT.html
|
|
|
|
|
*/
|
|
|
|
|
|
|
|
|
|
#include "common.h"
|
|
|
|
|
BEGIN
|
|
|
|
|
cli
|
|
|
|
|
|
|
|
|
|
/* Set the GDT register with start address of Global Descriptor Table */
|
|
|
|
|
lgdt gdt
|
|
|
|
|
mov %cr0, %eax
|
|
|
|
|
/* Set PE (Protection Enable) bit in CR0 (Control Register 0) */
|
|
|
|
|
or $1, %al
|
|
|
|
|
mov %eax, %cr0
|
|
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
TODO why 8?
|
|
|
|
|
|
|
|
|
|
Perform far jump to selector 08h (offset into GDT,
|
|
|
|
|
pointing at a 32bit PM code segment descriptor)
|
|
|
|
|
to load CS with proper PM32 descriptor).
|
|
|
|
|
*/
|
|
|
|
|
ljmp $0x08, $PModeMain
|
|
|
|
|
|
|
|
|
|
PModeMain:
|
|
|
|
|
/* TODO load DS, ES, FS, GS, SS, ESP. */
|
|
|
|
|
|
|
|
|
|
hlt
|
|
|
|
|
|
|
|
|
|
gdt:
|
|
|
|
|
.word 0x1234
|
|
|
|
|
.long 0x12345678
|
