I am in the process of rewriting some code to tidy it up a bit, and I have run into a major brick wall causing more problems than when I originally figured out how to do this.
As usual, its in the switch from real mode to protected mode.
Bochs dumpped:
00000525302e[CPU ] jump_protected: S=1: descriptor not executable
00000525302p[CPU ] >>PANIC<< exception(): 3rd (13) exception with no resolution
00000525302i[SYS ] Last time is 1062374731
00000525302i[CPU ] protected mode
00000525302i[CPU ] CS.d_b = 16 bit
00000525302i[CPU ] SS.d_b = 16 bit
00000525302i[CPU ] | EAX=60000011 EBX=00000004 ECX=000c0fff EDX=00000100
00000525302i[CPU ] | ESP=00001000 EBP=00000000 ESI=0000010c EDI=0000000c
00000525302i[CPU ] | IOPL=0 NV UP DI PL NZ NA PE NC
00000525302i[CPU ] | SEG selector base limit G D
00000525302i[CPU ] | SEG sltr(index|ti|rpl) base limit G D
00000525302i[CPU ] | DS:1000( 0000| 0| 0) 00010000 0000ffff 0 0
00000525302i[CPU ] | ES:8000( 0000| 0| 0) 00080000 0000ffff 0 0
00000525302i[CPU ] | FS:0000( 0000| 0| 0) 00000000 0000ffff 0 0
00000525302i[CPU ] | GS:ffff( 0000| 0| 0) 000ffff0 0000ffff 0 0
00000525302i[CPU ] | SS:9000( 0000| 0| 0) 00090000 0000ffff 0 0
00000525302i[CPU ] | CS:1000( 0000| 0| 0) 00010000 0000ffff 0 0
00000525302i[CPU ] | EIP=000000a3 (000000a3)
00000525302i[CPU ] | CR0=0x60000011 CR1=0x00000000 CR2=0x00000000
00000525302i[CPU ] | CR3=0x00000000 CR4=0x00000000
From what I can figure, it is trying to jump to protected mode, but the selector is not valid for a code segment???
This is the code that set's up protected mode (A20 is also enabled earlier)
CLI
DB 066h
LGDT [FWORD PTR GDT_48]
MOV EAX, CR0
OR AL, 1
MOV CR0, EAX
JMP $+2
NOP
;-------------------------------------------------------------------------------
; CPU is now in Protected Mode
; JMP FAR to Selector1:System32 to clear pipeline
;-------------------------------------------------------------------------------
DB 67h
DB 66h
DB 0EAh
DD 30000h
DW 0008
And the GDT...
GDT_48:
DW 4*8-1 ; GDT = 4 Entries
DD BIG_GDT ; GDT Base
BIG_GDT:
;-------------------------------------------------------------------------------
; #0 Segment Descriptor - DUMMY
; Intel Specification
;-------------------------------------------------------------------------------
DB 0,0,0,0
DB 0,0,0,0
;-------------------------------------------------------------------------------
; #1 Segment Descriptor - CODE SEGMENT (All 4Gb)
; Index=0x08H
;-------------------------------------------------------------------------------
DW 0FFFFh
DW 00000h
DB 00h
DB 09Ah
DB 0CFh
DB 00h
As I said, this is a rewrite of existing code and all this code is unchanged (and worked before). Can anyone see anything obvious, or make some other suggestions?
Cheers
Code re-write problem
-
Sentient
RE:Code re-write problem
I did notice one overlooked problem which just didnt appear before.
The GDT was not aligned, so I set it to Align 16.
Now Bochs seems to die with:
00000525302e[CPU ] jump_protected: dpl > CPL
00000525302p[CPU ] >>PANIC<< exception(): 3rd (13) exception with no resolution
The GDT was not aligned, so I set it to Align 16.
Now Bochs seems to die with:
00000525302e[CPU ] jump_protected: dpl > CPL
00000525302p[CPU ] >>PANIC<< exception(): 3rd (13) exception with no resolution
-
Sentient
RE:Code re-write problem
Hmmm.. I don't know how I avoided this in my original code, but the GDTR was loading the wrong value
GDT_48:
DW 4*8-1 ; GDT = 4 Entries
DD BIG_GDT ; GDT Base
should have been:
GDT_48:
DW 4*8-1 ; GDT = 4 Entries
DD SEG<<4 + BIG_GDT ; GDT Base
I have no idea how that actually worked in my original code, but oh well, all is good now.
GDT_48:
DW 4*8-1 ; GDT = 4 Entries
DD BIG_GDT ; GDT Base
should have been:
GDT_48:
DW 4*8-1 ; GDT = 4 Entries
DD SEG<<4 + BIG_GDT ; GDT Base
I have no idea how that actually worked in my original code, but oh well, all is good now.