OSDev.org

;Inputs:
;        AL    -- initial sequential value to write
;       WIDESI -- 4096 bytes page base address
;       WIDEDI -- buffer to preserve original page contents
;
;;
check_page_existence__0000:
owide pushf


;INIT: Cache the original page contents
;INIT: Cache the original page contents
;INIT: Cache the original page contents
;INIT: Cache the original page contents

   push widecx
   push widesi
   push widedi

     xor widecx,widecx
     mov cx,(4096/WIDEWORD_SZ)
     rep movswide

   pop widedi
   pop widesi
   pop widecx

;END:  Cache the original page contents
;END:  Cache the original page contents
;END:  Cache the original page contents
;END:  Cache the original page contents




;INIT: Write the page
;INIT: Write the page
;INIT: Write the page
;INIT: Write the page

   ;NOTE: Since the initial test value will be normally random,
   ;      we could also normally call the function without setting
   ;      AL/WIDEAX and using whatever initial value is there,
   ;      but also saving it for being able to compare it later.
   ;;
    push wideax
    push widecx
    push widesi
    xor widecx,widecx
    mov cx,4096
    .l_alterpage:

     ;Prepare writing 4096 individual bytes
     ;from the original WIDESI, and increase
     ;the byte value and the address on each
     ;iteration to prove whether this physical
     ;page can maintain memory contents. It will
     ;probably be better to always allocate pages
     ;in this way even if it's slower overall for
     ;our system, but at least it will allow us to
     ;never use damaged memory even if it becomes
     ;damaged at run time under our OS, so this test
     ;for every time that we allocate or swap out
     ;a page will give us increased robustness.
     ;;
      mov byte[widesi],al
      inc widesi
      inc al
   
    dec widecx
    jnz .l_alterpage
    pop widesi
    pop widecx
    pop wideax

;END:  Write the page
;END:  Write the page
;END:  Write the page
;END:  Write the page




;INIT: See if it held its contents
;INIT: See if it held its contents
;INIT: See if it held its contents
;INIT: See if it held its contents


    push widecx
    push widesi
    xor widecx,widecx
    mov cx,4096
    .l_checkpage:

     ;Prepare reading 4096 individual bytes
     ;from the original WIDESI, and increase
     ;the byte value and the address on each
     ;iteration to see whether this physical
     ;page can maintain memory contents.
     ;
     ;If the current byte doesn't contain the
     ;value that it should, end indicating that
     ;the page didn't hold its contents; otherwise
     ;continue the test of 4096 bytes:
     ;;
      cmp byte[widesi],al
      jne .l_checkpage_end_notequal
      inc widesi
      inc al
   
    dec widecx
    jnz .l_checkpage
    .l_checkpage_end_equal:
     xor wideax,wideax   ;Indicate that the page held its contents
     inc al     


    jmp .l_checkpage_END
    .l_checkpage_end_notequal:
     xor wideax,wideax   ;Indicate that the page didn't hold its contents



    .l_checkpage_END:
    pop widesi
    pop widecx


;At this point, WIDEAX contains TRUE or FALSE
;for this function:
;;


;END:  See if it held its contents
;END:  See if it held its contents
;END:  See if it held its contents
;END:  See if it held its contents



;INIT: Restore original page contents
;INIT: Restore original page contents
;INIT: Restore original page contents
;INIT: Restore original page contents

   
   push widecx
   push widesi
   push widedi

    ;Make the cached page buffer the source memory
    ;and the original address the destination memory:
    ;;
     xchg widesi,widedi

    ;Copy it to restore:
    ;;
     xor widecx,widecx
     mov cx,(4096/WIDEWORD_SZ)
     rep movswide

   pop widedi
   pop widesi
   pop widecx


;END:  Restore original page contents
;END:  Restore original page contents
;END:  Restore original page contents
;END:  Restore original page contents




owide popf
retwide


;Should implement RETWIDE to automatically select between 16, 32 or 64-bit stack
;return address POPs.






;EOF