[solved] Error occured when jumping into ring3

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l4m2

Joined: 15 Jan 2015
Posts: 670

l4m2 04 Sep 2015, 13:42

I set GDT

Code:

dq 0
dw 0ffffh,0                     ;   4Gbytes, start at linear 0
db 0,10011010b,11001111b,0      ;   granularity = 64Kbytes, code segment, ring 0, read only,etc...
dw 0ffffh,0                     ;   4Gbytes, start at linear 0
db 0,10010010b,11001111b,0      ;   granularity = 64Kbytes, data segment, ring 0, read/write,etc...
dw 0ffffh,0                     ;   4Gbytes, start at linear 0
db 0,11111010b,11001111b,0      ;   granularity = 64Kbytes, code segment, ring 3, read only,etc...
dw 0ffffh,0                     ;   4Gbytes, start at linear 0
db 0,11110010b,11001111b,0      ;   granularity = 64Kbytes, data segment, ring 3, read/write,etc...
dw 8*5

and LDT(all be dq 0x****8e00********).When

Code:

cli
push 32
push 0x1000000
push 24
push @f
retf
@@:
jmp $

bochs says

Code:

00006403462e[CPU0  ] check_cs(0x0018): non-conforming code seg descriptor dpl != cpl, dpl=3, cpl=0
00006403468e[CPU0  ] iret: SS.rpl != CS.rpl
00006403469e[CPU0  ] fetch_raw_descriptor: GDT: index (3e7) 7c > limit (28)
00006403470e[CPU0  ] fetch_raw_descriptor: GDT: index (3e7) 7c > limit (28)
00006403471e[CPU0  ] fetch_raw_descriptor: GDT: index (3e7) 7c > limit (28)

but if

Code:

sti
jmp $

nothing happens

04 Sep 2015, 13:42

l4m2

Joined: 15 Jan 2015
Posts: 670

l4m2 05 Sep 2015, 01:35

Solved. jump into 32+3 and 24+3 and it is OK.
Now having trouble jumping back ...

05 Sep 2015, 01:35

Feryno

Joined: 23 Mar 2005
Posts: 515
Location: Czech republic, Slovak republic

Feryno 07 Sep 2015, 08:52

from ring3 to ring0 you can execute int (you must set up IDT and allow some interrupt for ring3, or you may wait for timer interrupt which is external interrupt)
don't forget to set TSS so CPU will know how to switch stacks from ring3 stack to ring0 stack
under x64 there is syscall to go from ring3 to ring0

07 Sep 2015, 08:52

l4m2

Joined: 15 Jan 2015
Posts: 670

l4m2 07 Sep 2015, 11:51

Feryno wrote:

from ring3 to ring0 you can execute int (you must set up IDT and allow some interrupt for ring3, or you may wait for timer interrupt which is external interrupt)
don't forget to set TSS so CPU will know how to switch stacks from ring3 stack to ring0 stack
under x64 there is syscall to go from ring3 to ring0

Just having the wrong IDT ... solved

07 Sep 2015, 11:51

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