debug using bochs

[david]

Code: Select all

(0) [0x00007d16] 07c0:0116 (unk. ctxt): mov ds, ax               ; 8ed8
<bochs:40> dump_cpu
......
ds:s=0x0010, dl=0x0000ffff, dh=0x00009200, valid=1
......
<bochs:41> s
Next at t=2082197
(0) [0x00007d18] 07c0:0118 (unk. ctxt): mov es, ax                ; 8ec0
<bochs:42> dump_cpu
......
ds:s=0x07c0, dl=0x7c00ffff, dh=0x00009300, valid=1
......

source code:

Code: Select all

; set DS=ES=CS after jump from protect mode to real mode
mov ax, cs
mov ds, ax
mov es, ax

why dh's value changed after 'mov ds, ax' in real mode?
what's the meaning of 'valid'?

[Troy Martin]

why dh's value changed after 'mov ds, ax' in real mode?

Umm, cause your code sets DS to AX.

[david]

I remembered that it's impossible to change the value in real mode, but it changed, so i doubt.

it's allowed to edit the value in real mode?

[Troy Martin]

Here's a breakdown of the code:

Code: Select all

mov ax, cs    ; makes AX = CS
mov ds, ax    ; makes DS = AX
mov es, ax    ; makes ES = AX

Can't get any more simpler than that.

[david]

Code: Select all

<bochs:26> u
00007cfa: (                    ): mov ds, ax                ; 8ed8
<bochs:27> dump_cpu
......
ds:s=0x07c0, dl=0x7c00ffff, dh=0x00009300, valid=1
......
<bochs:28> s
Next at t=2082187
(0) [0x00007cfc] 0008:00000000000000fc (unk. ctxt): mov es, ax
<bochs:29> dump_cpu
......
ds:s=0x0010, dl=0x0000ffff, dh=0x00009200, valid=1
......

source code:

Code: Select all

; DS=ES=SS=FS=GS
; GDT_Data_Sel is a selector
mov ax, GDT_Data_Sel
mov ds, ax
mov es, ax
mov ss, ax
mov fs, ax
mov gs, ax

when i set ds to a selector, the A(bit0) should be 1, so dh should be 0x00009300, but its value is 0x00009200. i can't understand. the code in protect mode.

[Helu]

Hi,

ds:s=0x07c0, dl=0x7c00ffff, dh=0x00009300, valid=1
...........................................
ds:s=0x0010, dl=0x0000ffff, dh=0x00009200, valid=1

Comparing the above two,not only dh has been changed from 0x00009300 to 0x00009200,but dl has been changed too.
I suggest you post more code ,so we can simulate and help you.
HeLU

[Brendan]

Hi,

ds:s=0x07c0, dl=0x7c00ffff, dh=0x00009300, valid=1
...........................................
ds:s=0x0010, dl=0x0000ffff, dh=0x00009200, valid=1

Comparing the above two,not only dh has been changed from 0x00009300 to 0x00009200,but dl has been changed too.

Sure - the base address of the segment changed from 0x7C00 to 0x0000 in dl, and the segment type changed from "read/write accessed" to "read/write" in dh. I'd assume that as soon as you use the DS segment for something it'll change back to "read/write accessed".

Note: The segment limit, DPL, granularity flag, default operand size flag and "available" flag didn't change.

It all looks perfectly normal (for real mode) to me...


Cheers,

Brendan

[Helu]

Hi,
Seems it is in PMode?

; GDT_Data_Sel is a selector
mov ax, GDT_Data_Sel

It mentions GDT... ,after all.

[david]

GDT:

Code: Select all

GDT		label	byte	
GDT_Null	s_RestoreDescriptor  <>
GDT_Code	s_RestoreDescriptor  <0FFFFh, 7C00h, 0, 10011010b, 0, 0>	; Base=7C00h 	Limit=0FFFFh 
GDT_Code_Sel = offset GDT_Code - offset GDT		

GDT_Data	s_RestoreDescriptor  <0FFFFh, 0, 0, 10010010b, 0, 0>		; Base=0h 		Limit=0FFFFh 
GDT_Data_Sel = offset GDT_Data - offset GDT		

Code: Select all

s_RestoreDescriptor		struc

	LimitL		dw 	0			; Segment Limit 0~15		
	BaseL		dw	0			; Segment Base	 0~15
	BaseM		db	0			; Segment Base	 16~23		
	AttrL		db	0			; Low Attr
	AttrLimitH	db	0			; High Attr and Limit 16~19
	BaseH		db	0			; Base 24~31

s_RestoreDescriptor	ends