Memory Region Descriptors

The ChorusOS operating system offers various services which enable an actor to extend its address space dynamically by allocating memory regions. An actor may also shrink its address space by freeing memory regions. An area of memory to be allocated or freed is described to the system through a region descriptor of the following type:

typedef struct {
    VmAddr    startAddr;
    VmSize    size;
    VmFlags   options;
    VmAddr    endAddr;
    void*     opaque1;
    VmFlags   opaque2;
} KnRgnDesc;

The startAddr field defines the starting address of the memory region. The size field defines its length expressed in bytes. The options field enables a low level control on the attributes of the memory region to be allocated. The opaque1 and opaque2 fields should be set to NULL if they are not being used by the application.

The options field is a combination of flags, of which the following are the most important:

• K_WRITABLE tells the system that the memory region to be created must be writable, otherwise, the memory region will be read only.

• K_FILLZERO tells the system that the content of the memory region to be created must be filled with zeroes upon creation. If this flag is not set, the content of the memory region at creation time is unspecified.

• K_ANYWHERE tells the system that the actual address used to allocate the region is not critical to the application. An appropriate address will be selected by the system and returned to the application. This avoids the need for the application to find out which addresses are already in use within the actor address space. It also permits memory to be allocated within a library without any possible conflict with an existing address space.

• K_SUPERVISOR tells the system that the memory to be allocated will be part of the supervisor address space rather than of the user address space. This flag is usually set by actors running in supervisor mode.

On a ChorusOS operating system configured with the Virtual Memory feature, further options are available. The most important one allows control of the swapping policy to be applied to the pages of the created region:

• K_NODEMAND tells the system that no page fault should ever occur on such a memory region. Physical pages are allocated to the region at creation time and they will never be swapped out. Thus the region is locked in memory.