171 lines
4.4 KiB
ArmAsm
171 lines
4.4 KiB
ArmAsm
/*
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# Protected mode
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Major changes:
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- BIOS cannot be used anymore
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- GDT takes effect immediately so we *have* to deal with it now,
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and we will later turn on paging
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- we have to encode instructions differently.
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Note that in 16-bit 32-bit instructions were encodable, but with a prefix.
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## Bibliography
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- http://stackoverflow.com/questions/28645439/how-do-i-enter-32-bit-protected-mode-in-nasm-assembly
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- http://wiki.osdev.org/Journey_To_The_Protected_Land
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- http://wiki.osdev.org/Protected_Mode
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- https://github.com/chrisdew/xv6/blob/master/bootasm.S
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- https://thiscouldbebetter.wordpress.com/2011/03/17/entering-protected-mode-from-assembly/ FASM based. Did not word on first try, but looks real clean.
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- http://skelix.net/skelixos/tutorial02_en.html
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## GDT
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Table in memory that gives properties of segment registers.
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Segment registers in protected mode point to entries of that table.
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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.
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### Effect on memory access
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The GDT modifies every memory access of a given segment by:
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- adding an offset to it
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- limiting how big the segment is
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If an access is made at an offset larger than allowed: TODO some exception happens, which is like an interrupt, and gets handled by a previously registered handler.
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The GDT could be used to implement virtual memory by using one segment per program:
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+-----------+--------+--------------------------+
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| Program 1 | Unused | Program 2 |
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+-----------+--------+--------------------------+
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^ ^ ^ ^
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Start1 End1 Start2 End2
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The problem with that is that each program must have one segment, so if we have too many programs, fragmentation will be very large.
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Paging gets around this by allowing discontinuous memory ranges of fixed size for each program.
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The format of the GDT is given at: http://wiki.osdev.org/Global_Descriptor_Table
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### Effect on permissions
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Besides fixing segment sizes, the GDT also specifies permissions to the program that is running:
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- ring level: limits several things that can or not be done, in particular:
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- instructions: e.g. no in / out in ring 3
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- register access: e.g. cannot modify control registers like the GDTR in ring 3. Otherwise user programs could just escape restrictions by changing that!
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- executable, readable and writable bits: which operations can be done
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## GDTR
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## GDT register
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In 32-bit, a 6 byte register that holds:
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- 2 byte length of the GDT (TODO in bytes or number of entries?)
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- 4 byte address of the GDT in memory
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In 64 bit, makes 10 bytes, with the address having 8 bytes
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GRUB seems to setup one for you: http://www.jamesmolloy.co.uk/tutorial_html/4.-The%20GDT%20and%20IDT.html
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*/
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#include "common.h"
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BEGIN
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CLEAR
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cli
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/* Tell the processor where our Global Descriptor Table is in memory. */
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lgdt gdt_descriptor
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/* Set PE (Protection Enable) bit in CR0 (Control Register 0) */
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mov %cr0, %eax
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orl $0x1, %eax
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mov %eax, %cr0
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/* TODO why does equ not work? What is the alternative? */
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/*ljmp $CODE_SEG, $b32*/
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/*
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This is needed to set `%cs` to 8.
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8 means the second entry, since each entry is 8 bytes wide,
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and we have an initial null entry.
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*/
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ljmp $0x08, $b32
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.code32
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print32:
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pusha
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# Video memory address.
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mov $0xb8000, %edx
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print32.loop:
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mov (%ebx), %al
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# White on black.
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mov $0x0f, %ah
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cmp $0, %al
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je print32.done
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mov %ax, (%edx)
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add $1, %ebx
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add $2, %edx
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jmp print32.loop
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print32.done:
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popa
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ret
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b32:
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/* Setup the other segments. */
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mov $DATA_SEG, %ax
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mov %ax, %ds
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mov %ax, %es
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mov %ax, %fs
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mov %ax, %gs
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mov %ax, %ss
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/* TODO: what is the maximum we can put were? Why does `FFFFFFFF` fail? */
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mov $0XFFFFFF00, %ebp
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mov %ebp, %esp
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mov $message, %ebx
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call print32
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jmp .
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gdt_start:
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gdt_null:
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.long 0x0
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.long 0x0
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gdt_code:
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.word 0xffff
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.word 0x0
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.byte 0x0
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.byte 0b10011010
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.byte 0b11001111
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.byte 0x0
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gdt_data:
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.word 0xffff
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.word 0x0
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.byte 0x0
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.byte 0b10010010
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.byte 0b11001111
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.byte 0x0
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gdt_end:
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gdt_descriptor:
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.word gdt_end - gdt_start
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.long gdt_start
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.equ CODE_SEG, gdt_code - gdt_start
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.equ DATA_SEG, gdt_data - gdt_start
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message: .asciz "hello world"
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