I change source data segment attribute,but PC is down,why?

[leetow2003]

;16 jump
jump macro selector,offsetv
  db 0eah
  dw offsetv
  dw selector
endm

;display a char
echoch macro ascii
  mov ah,2
  mov dl,ascii
  int 21h
endm


descriptor struc
  limitl dw 0
  basel dw 0
  basem db 0
  attributes dw 0
  baseh db 0
descriptor ends

pdesc struc
  limit dw 0
  base dd 0
pdesc ends


atdw = 92h
atce = 98h
atdr = 90h ;data read only
  .386P
;--------------------------------------------------------

dseg segment use16
  gdt label byte
  dummy descriptor <>
  code descriptor <0ffffh,,,atce,>
  code_sel =code-gdt
  datas descriptor <0ffffh,0h,11h,atdw,0> ;If atdw change to atdr,PC will down
  datas_sel =datas-gdt
  datad descriptor <0ffffh,,,atdw,>
  datad_sel =datad-gdt
  gdtlen =$-gdt
;
  vgdtr pdesc <gdtlen-1,>
;
  bufferlen = 256
  buffer db bufferlen dup(0)
dseg ends

;-------------------------------------------------------------

cseg segment use16
  assume cs:cseg, ds:dseg
start:
  mov ax,dseg
  mov ds,ax
;
  mov bx,16
  mul bx
  add ax,offset gdt
  adc dx,0
  mov word ptr vgdtr.base,ax
  mov word ptr vgdtr.base+2,dx
;
  mov ax,cs
  mul bx
  mov code.basel,ax
  mov code.basem,dl
  mov code.baseh,dh
;
  mov ax,ds
  mul bx
  add ax,offset buffer
  adc dx,0
  mov datad.basel,ax
  mov datad.basem,dl
  mov datad.baseh,dh
;
  lgdt fword ptr vgdtr
;
  cli
  call ea20
;
  mov eax,cr0
  or eax,1
  mov cr0,eax
;
  jump <code_sel>,<offset virtual>
;
virtual:
  mov ax,datas_sel
  mov ds,ax
  mov ax,datad_sel
  mov es,ax
  cld
  xor si,si
  xor di,di
  mov cx,bufferlen/4
  repz movsd
;
  mov eax,cr0
  and eax,0fffffffeh
  mov cr0,eax
;
  jump <seg real>,<offset real>
;
real:
  call da20
  sti
;
  mov ax,dseg
  mov ds,ax
  mov si,offset buffer
  cld
  mov bp,bufferlen/16
nextline:
  mov cx,16
nextch:
  lodsb
  push ax
  shr al,4
  call toascii
  echoch al
  pop ax
  call toascii
  echoch al
  echoch ' '
  loop nextch
  echoch 0dh
  echoch 0ah
  dec bp
  jnz nextline
;
  mov ax,4c00h
  int 21h
;
toascii proc
  and al,0fh
  add al,'0'
  cmp al,'9'
  seta dl
  movzx dx,dl
  imul dx,7
  add al,dl
  ret   
toascii endp
;
;Open A20
ea20 proc
  push ax
  in al,92h
  or al,2
  out 92h,al
  pop ax
  ret
ea20 endp
;
;Close A20
da20 proc
  push ax
  in al,92h
  and al,0fdh ;0fdh=not 20h
  out 92h,al
  pop ax
  ret
da20 endp
;
cseg ends
  end start

This program will transfer data to another at 110000h in P-mode,and then display
in real mode,I find the source data don't be written,so I change the
segment attribute value:
datas descriptor <0ffffh,0h,11h,atdw,0> =>datas descriptor <0ffffh,0h,11h,atdr,0>
but the PC is down under pure DOS,why?

[hutch--]

leetow2003,

I have move this here because the code is 16 bit real mode code, it has nothing to do with protected mode programming under Windows.

[japheth]

datas descriptor <0ffffh,0h,11h,atdw,0> =>datas descriptor <0ffffh,0h,11h,atdr,0>
but the PC is down under pure DOS,why?

I guess the movsd in protected-mode itself has no problems with a read-only selector in DS. However, you'll have to restore DS to a R/W, 64 kB data selector BEFORE switching back to real-mode, because the attributes of DS can only be changed in protected-mode.

So, actually, your program crashes in real-mode because your program or DOS can't write to the DS segment anymore.

[leetow2003]

datas descriptor <0ffffh,0h,11h,atdw,0> =>datas descriptor <0ffffh,0h,11h,atdr,0>
but the PC is down under pure DOS,why?

I guess the movsd in protected-mode itself has no problems with a read-only selector in DS. However, you'll have to restore DS to a R/W, 64 kB data selector BEFORE switching back to real-mode, because the attributes of DS can only be changed in protected-mode.

So, actually, your program crashes in real-mode because your program or DOS can't write to the DS segment anymore.

Could you describe it  in detail? I am a little clear.Thank you

[japheth]

Could you describe it  in detail? I am a little clear.Thank you

But my explanation already was very detailed.

You'll have to set DS to a 64 kB R/W descriptor BEFORE switching back to real-mode!

for example:

Code Select Expand


  mov ax,datas_sel
  mov ds,ax
  mov ax,datad_sel
  mov es,ax
  cld
  xor si,si
  xor di,di
  mov cx,bufferlen/4
  repz movsd
;
  mov ax,datad_sel            ;<<<<<<<<<<<<<<<<<
  mov ds,ax                   ;<<<<<<<<<<<<<<<<<
  mov eax,cr0
  and eax,0fffffffeh
  mov cr0,eax