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study/sabre/os/files/ProtectedMode/DPMI1.0Spec/ch4.1.html
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study/sabre/os/files/ProtectedMode/DPMI1.0Spec/ch4.1.html
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DPMI clients are loaded in real mode. In order to enter protected
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mode, the client must first call <a href="api/2f1687.html">Int 2FH
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Function 1687H</a> to test for the presence of a DPMI host and obtain
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the address of the real-to-protected mode switch entry point. <a
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href="api/2f1687.html">Function 1687H</a> also returns information
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about the DPMI host's capabilities and the size of a private data area
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which will be used by the host to hold client-specific data
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structures. After allocating the required private data area via the
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normal real mode memory allocation interface (DOS Int 21H Function
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48H), the client makes a <tt>FAR CALL</tt> to the mode switch entry
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point with the following parameters: <p>
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<table border=1 cellspacing=0 cellpadding=4>
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<tr><td rowspan=5 align=center>AX</td><td colspan=2>flags</td></tr>
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<tr><th align=center>Bit</th><th>Significance</th></tr>
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<tr><td align=center rowspan=2>0</td><td align=left>0 = 16-bit application</td></tr>
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<tr><td align=left>1 = 32-bit application</td></tr>
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<tr><td align=center rowspan=1>1-15</td><td align=left>Reserved, should be zero</td></tr>
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<tr><td align=center>ES</td><td colspan=2>real mode segment of DPMI
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host private data area (must be at least as large as the size returned
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in SI by <a href="api/2f1687.html">Int 2FH Function 1687H</a>; ES is
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ignored if the size was zero.</td></tr>
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</table><p>
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If the Carry flag is set upon return, the mode switch was
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unsuccessful, the client is still running in real mode, and register
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AX contains one of the following error codes: <p>
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<table border=1 cellspacing=0 cellpadding=4>
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<tr><td align=center>8011H</td><td align=left>descriptor unavailable
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(cannot allocate descriptors for CS, DS, ES, SS, PSP, and environment
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pointer)</td></tr>
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<tr><td align=center>8021H</td><td align=left>invalid value (32-bit
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program specified but not supported)</td></tr>
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</table><p>
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If the mode switch was successful, the mode switch routine returns to
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the call in protected mode with the Carry flag clear and the following
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results:<p>
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<table border=1 cellspacing=0 cellpadding=4>
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<tr><td align=center>CS</td><td align=left>16-bit selector with base
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of real mode CS and a 64KB limit</td></tr>
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<tr><td align=center>SS</td><td align=left>selector with base of real
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mode SS and a 64 KB limit</td></tr>
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<tr><td align=center>DS</td><td align=left>selector with base of real
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mode DS and a 64 KB limit</td></tr>
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<tr><td align=center>ES</td><td align=left>selector to program's PSP
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with a 100H byte limit</td></tr>
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<tr><td align=center>FS</td><td align=left>0 (if running on an 80386
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or later)</td></tr>
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<tr><td align=center>GS</td><td align=left>0 (if running on an 80386
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or later)</td></tr>
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</table><p>
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All other registers are preserved, except that for 32-bit clients the
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high word of ESP will be forced to zero. 32-bit clients will
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initially run with a 16-bit code segment, but all Int 31H calls will
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still require 48-bit pointers, and the stack and data descriptors will
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be 32-bit (the Big bit will be set in the descriptors). Note that if
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DS+SS at the time of the mode switch call, only one descriptor may be
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allocated, and the same selector may be returned in DS and SS. The
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client is allowed to modify or free the CS, DS and SS descriptors
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allocated by the mode switch routine. <p>
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The environment pointer in the client program's PSP is automatically
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converted to a selector during the mode switch. If the client wishes
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to free the memory occupied by the environment, it should do so <i>
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before </i> entering protected mode and zero the word at PSP:2CH
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(segment address of the environment>. The client may change the
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environment point in the PSP after entering protected mode but it must
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restore it to the selector created by the DPMI host before
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terminating. The client should <i>not</I> modify or free the PSP or
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environment descriptors supplied by the DPMI host.<p>
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<b> Example:</b> The following code illustrates how a DPMI client
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would obtain the address of the mode switch entry point, allocate the
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DPMI host private data area, and enter protected mode.
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<pre>
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modesw dd 0 ; far pointer to DPMI host's
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; mode switch entry point
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.
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.
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.
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mov ax,1687h ; get address of DPMI host's
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<a href="api/2f1687.html">int 2fh</a> ; mode switch entry point
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or ax,ax ; exit if no DPMI host
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jnz error
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mov word ptr modesw,di ; save far pointer to host's
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mov word ptr modesw+2,es ; mode switch entry point
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or si,si ; check private data area size
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jz @@1 ; jump if no private data area
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mov bx,si ; allocate DPMI private area
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mov ah,48h ; allocate memory
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int 21h ; transfer to DOS
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jc error ; jump, allocation failed
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mov es,ax ; let ES=segment of data area
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@@1: mov ax,0 ; bit 0=0 indicates 16-bit app
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call modesw ; switch to protected mode
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jc error ; jump if mode switch failed
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; else we're in prot. mode now
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.
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.
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.
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</pre>
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