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study/sabre/os/files/ProtectedMode/DPMI1.0Spec/ch4.3.html
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study/sabre/os/files/ProtectedMode/DPMI1.0Spec/ch4.3.html
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At one point in its execution or another, every DPMI client runs on
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four different stacks: an application stack, a locked protected mode
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stack, a real mode stack, and a DPMI host stack. It is important to
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understand how the host maintains these stacks to fully understand the
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protected mode environment.<p>
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The <i>application stack</i> is the primary stack that the DPMI client
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executes on. It is initially the real mode stack that the client was
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on when it switched into protected mode, although nothing prevents the
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client from switching protected mode stacks at any point after the
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initial mode switch. Th application stack can be unlocked if desired.
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Software interrupts executed in protected mode are reflected on this
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stack.<p>
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The <i>locked protected mode stack</i> is provided by the DPMI host.
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The host automatically switches to this stack during servicing of
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hardware interrupts, software interrupts 1CH, 23H, and 24H, all
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exceptions, and during the execution of real mode callbacks.
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Subsequent nested interrupts or calls will not cause a stack switch.
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If the client switches off this stack, the new stack must also be
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locked and will become the protected mode stack until it switches
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back. When the interrupt or call returns, the host switches back to
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the original protected mode stack. Note that the host must provide a
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minimum of one 4 KB locked stack, and that software interrupts other
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than 1CH, 23H, and 24H do <i>not</i> use this stack. (Refer Appendix
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D for descriptor usage rule of locked stack.)<p>
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The <i>real mode stack</i> is also provided by the DPMI host, and is
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usually located in the DPMI host data area allocated by the client
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prior to its initial switch into protected mode. The real mode stack
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is at least 200H bytes in size and is always located in locked memory.
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Interrupts that are reflected to real mode, as well as calls to real
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mode interrupt handlers or procedures via <a
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href="api/310300.html">Int 31H Functions 0300H</a>, <a
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href="api/310301.html">0301H</a>, or <a
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href="api/310302.html">0302H</a>, will use this stack.<p>
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A <i>DPMI host stack</i> is only accessible to the DPMI host; it is
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used by the host to handle interrupts and exceptions that occur while
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the host is executing on behalf of the client. For example, when the
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client requests a mode switch, the original SS:(E)SP of the protected
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mode program can be saved on the host stack while the DPMI host
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switches onto the locked protected mode stack.<p>
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There are four different ways that a client can force a mode switch
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between protected and real mode:
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<ul>
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<li> Execute the default interrupt reflection handler (all interrupts
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other than Int 31H and Int 21H Function 4CH are initialized by the
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DPMI host to point to a handler that reflects the interrupt to real
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mode);<p>
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<li> Use the DPMI translation services (<a href="api/310303.html">Int
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31H Function 0303H</a>, <a href="api/310301.html">0301H</a>, and <a
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href="api/310302.html">0302H</a>) to call a real mode interrupt
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handler or procedure;<p>
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<li> Allocate a real mode callback address with <a
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href="api/310303.html">Int 31H Function 0303H</a>; when a real mode
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program transfer control to the callback address, the DPMI host will
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switch the CPU from real mode into protected mode;<p>
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<li> Use the DPMI raw mode switch functions, whose addresses are
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obtained with <a href="api/310306.html">Int 31H Function 0306H</a>.<p>
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</ul>
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All of these mode switches except for the raw mode switches may save
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some information on the DPMI host's stack. This means that clients
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should not terminate within nested mode switches unless they are using
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the raw mode switching services. However, even clients that use raw
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mode switches should not attempt to terminate from a hardware
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interrupt or exception handler or real mode callback because the DPMI
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host performs automatic mode and stack switching during these
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events.<p>
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Clients that use the raw mode switch services and perform nested mode
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switches must use the DPMI state save/restore functions (whose
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addresses may be obtained with <a href="api/310305.html">Int 31H
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Function 0305H</a>), causing the host to maintain information on the
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"other" mode's current state. This information includes the CS:(E)IP,
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SS:(E)SP, and other segment register contents; values that the client
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has no way to access directly. For example, during the service of a
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hardware interrupt that occurs in real mode, the DPMI host may
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preserve the real mode CS:(E)IP, SS:(E)SP, and segment registers on
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the host stack, causing a return to the wrong address when the handler
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finally executes the IRET.<p>
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<b>Example:</b> This example illustrates code that saves the state of
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the real mode registers using the DPMI save/restore function, switches
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to real mode using the raw mode switch service, issues a DOS call to
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open a file, switches back to protected mode using the raw mode switch
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service, and restores the state of the real mode registers using the
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save/restore function. The protected mode registers are saved by
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pushing them on the stack in the usual fashion. The example is
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intended only to show the logical sequence of execution; in a real
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program, the real mode and protection mode variables and functions
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would likely reside in separate segments.
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<pre>
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savsiz dw 0 ; size of state information
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realsrs dd 0 ; far pointer to real mode
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; save/restore state entry point
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protsrs dd 0 ; far pointer to protected mode
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; save/restore state entry point
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realrms dd 0 ; far pointer to real mode
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; raw mode switch entry point
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protrms dd 0 ; far pointer to protected mode
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; raw mode switch entry point
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protdw dw 0 ; placeholder for protected mode DS
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protip dw 0 ; placeholder for protected mode IP
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protcs dw 0 ; placeholder for protected mode CS
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protsp dw 0 ; placeholder for protected mode SP
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protss dw 0 ; placeholder for protected mode SS
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.
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.
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.
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; this code is executed during
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; application initialization
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mov ax,305h ; get addresses of DPMI host's
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<a href="api/310305.html">int 31h</a> ; state save/restore entry points
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mov savsiz,ax ; save state info buffer size
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mov word ptr realsrs,cx ; BX:CX = state save/restore
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mov word ptr realsrs+2,bx ; entry point for real mode
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mov word ptr protsrs,di ; SI:DI = state save/restore
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mov word ptr protsrs+2,si ; entry point for protected mode
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mov ax,306h ; get address of DPMI host's
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<a href="api/310306.html">int 31h</a> ; raw mode switch entry points
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mov savsiz,ax ; save state info buffer size
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mov word ptr realrms,cx ; BX:CX = raw mode switch
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mov word ptr realrms+2,bx ; entry piont for real mode
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mov word ptr protrms,di ; SI:DI = raw mode switch
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mov word ptr protrms+2,si ; entry point for protected mode
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; must also initialize the
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; sp and realss variables
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.
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.
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.
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; this code is executed during
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; program execution
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callopenfile proc
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pusha ; save protected mode registers
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push es
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sub sp,savsiz ; allocate space on current stack
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mov di,sp ; to save real mode state
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mov ax,ss ; set ES:DI = address of buffer
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mov es,ax ; to receive state information
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xor al,al ; AL=0 for save state request
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call protsrs ; call state save/restore routine
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mov protds,ds ; save current DS for switch back
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mov protss,ss ; save current SS
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mov protsp,sp ; save current SP
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mov protip,offset returnfromreal ; save return IP
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mov protcs,cs ; save return CS
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mov ax,seg filename ; load real mode DS
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mov ds,realss ; load real mode SS
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mov bx,realsp ; load real mode SP
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mov si,seg openfile ; load real mode CS
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mov di,offset openfile ; load real mode IP
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jmp protrms ; go to openfile
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returnfromreal:
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mov ax,ss ; let ES:DI = address of buffer
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mov es,ax ; holding state information
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mov di,sp
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mov al,1 ; AL=1 to restore state
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call protsrs ; call state restore routine
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add sp,savsiz ; discard state info buffer
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pop es
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popa ; restore protected mode registers
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ret
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callopenfile endp
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.
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.
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.
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; this code is executed in real mode
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openfile proc
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mov dx,offset filename
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mov ah,3dh ; issue Open File DOS call
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int 21h
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jc openerr ; check for error (not shown here)
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mov filehandle,bx ; save file handle
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mov ax,protds ; load protected mode DS
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mov dx,protss ; load protected mode SS
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mov bx,protsp ; load protected mode SP
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mov si,protcs ; load protected mode CS
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mov di,protip ; load protected mode IP
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jmp realrms
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openfile endp
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</pre>
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