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proc    user_ring stdcall, size:dword

locals

        virt_ptr        dd ?

        phys_ptr        dd ?

        num_pages       dd ?

endl

; Size must be an exact multiple of pagesize

        mov     eax, size

        test    eax, PAGE_SIZE-1

        jnz     .exit

; We must have at least one complete page

        shr     eax, 12

        jz      .exit

        mov     [num_pages], eax

; Allocate double the virtual memory

        mov     eax, [size]

        shl     eax, 1

        jz      .exit

        stdcall user_alloc, eax

        test    eax, eax

        jz      .exit

        mov     [virt_ptr], eax

; Now allocate physical memory

        stdcall alloc_pages, [num_pages]

        test    eax, eax

        jz      .exit_free_virt

        mov     [phys_ptr], eax

; Map first half of virtual memory to physical memory

        push    ecx esi edi

        mov     ecx, [num_pages]

        mov     esi, [virt_ptr]

        mov     edi, [phys_ptr]

  .loop1:

        stdcall map_page, esi, edi, PG_UWR

        add     esi, PAGE_SIZE

        add     edi, PAGE_SIZE

        dec     ecx

        jnz     .loop1

; Map second half of virtual memory to same physical memory

        mov     ecx, [pages]

        mov     edi, [phys_ptr]

  .loop2:

        stdcall map_page, esi, edi, PG_UWR

        add     esi, PAGE_SIZE

        add     edi, PAGE_SIZE

        dec     ecx

        jnz     .loop2

        pop     edi esi ecx

        mov     eax, [virt_ptr]

        ret

  .exit_free_virt:

        stdcall user_free, [virt_ptr]

  .exit:

        xor     eax, eax

        ret