GENERIC DESIGN OF THE PROTECTION MODULE
                            design.mps.prot
                             incomplete doc
                             drj 1997-04-02

INTRODUCTION

.readership: Any MPS developer.

.intro: This is the generic design of the Protection Module.  The protection 
module provides protection services to other parts of the MPS.  It is expected 
that different operating systems will have different implementations of this 
module.


INTERFACE

.if.setup:

void ProtSetup(void);

ProtSetup will be called exactly once (per process).  It will be called as part 
of the initialization of the first space that is created.  It should arrange 
for the setup and initialization of any datastructures or services that are 
necessary in order to implement the protection module.  (On UNIX it expected 
that it will install a signal handler, on Windows it will do nothing)

.if.set:

void ProtSet(Addr base, Addr limit, AccessSet mode)

ProtSet should set the protection of the memory between base and limit, 
including base, but not including limit (ie the half-open interval 
[base,limit)) to that specified by mode.
The mode parameter should have the AccessWrite bit set if write accesses to the 
page are to be forbidden, and should have the AccessRead bit set if read 
accesses to the page are to be forbidden.  A request to forbid read accesses 
(ie AccessRead is set) may also forbid write accesses, but read accesses will 
not be forbidden unless AccessRead is set.

.if.tramp:

void ProtTramp(void **resultReturn, void *(*f)(void *, size_t), void *p, size_t 
s);

.if.sync:

void ProtSync(Space space);

ProtSync is called to ensure that the actual protection of each segment (as 
determined by the OS) is in accordance with the segments's pm field.

.if.context-type:

typedef struct MutatorFaultContextStruct *MutatorFaultContext;

This abstract type is implemented by the protection module (impl.c.prot*).  It 
represents the continuation of the mutator which is restored after a mutator 
fault has been handled.  The functions ProtCanStepInstruction (.if.canstep 
below) and ProtStepInstruction (.if.step below) inspect and manipulate the 
context.

.if.canstep:

Bool ProtCanStepInstruction(MutatorFaultContext context);

Examines the context to determine whether the protection module can single-step 
the instruction which is causing the fault. Should return TRUE if and only if 
the instruction can be single-stepped (ie ProtStepInstruction can be called).

.if.step:

Bool Res ProtStepInstruction(MutatorFaultContext context);

Single-steps the instruction which is causing the fault.  This function should 
only be called if ProtCanStepInstruction applied to
the context returned TRUE.  It should return ResUNIMPL if the instruction 
cannot be single-stepped.  It should return ResOK if the
instruction is single-stepped.

The mutator context will be updated by the emulation/execution of the 
instruction such that resuming the mutator will not cause the
instruction which was causing the fault to be executed.