DosAllocMem Parameter - flag

flag (ULONG) - input

Allocate attribute and desired access protection flags.

A set of flags describing the allocation attributes and desired access protection for the private memory object. Possible values are shown in the lists below:

Allocation Attributes

PAG_COMMIT (0x00000010)

All pages in the private memory object are initially committed.
OBJ_TILE (0x00000040)
The private memory object must be allocated in the first 512MB of virtual-address space, with 16-bit selectors mapping the memory object.
OBJ_ANY (0x01000000)
The private memory object may reside anywhere in the available virtual-address space. 16-bit programs will not be able to access this object.
OBJ_TILE and OBJ_ANY are incompatible options. Specifying both for the same memory object will result in an error.

OBJ_ANY indicates that the memory may be allocated anywhere in the available virtual address space. In OS/2 Warp Server for SMP, OBJ_ANY memory allocations will first be attempted in the high memory area, which is the area of the virtual address space above 512 MB. If there is insufficient memory to satisfy the request from the high memory area, the memory will be allocated in the first 512 MB of the address space. IBM reserves the right to change this allocation strategy in future releases of OS/2.

If a memory object is allocated in the high memory area above 512 MB, 16-bit programs will not be able to address or access the object.

Memory should be allocated with the OBJ_ANY attribute wherever possible, since it provides the most efficient use of both the per-process address space as well as the system-wide virtual memory resources. It is intended that an application would make infrequent requests for large blocks of memory using DosAllocMem and then use DosSubSetMem along with DosSubAllocMem and DosSubFreeMem to manage the memory.

The maximum amount of free space in the high memory area can be determined from a program by using the DosQuerySysInfo function.

For OBJ_TILE memory objects, the 16-bit selectors are allocated to map the 32-bit object at 64KB boundaries. The figure below shows how the 16-bit alias selectors map the 32-bit object.

    32-bit             32-bit               16-bit alias
    Offset             Object               Selectors

   BaseAddress+000KB┌──────────────┐ ───── Sel
                    │              │
   BaseAddress+064KB├──────────────┤ ───── Sel+HugeInc
                    │              │
   BaseAddress+128KB├──────────────┤ ───── Sel+HugeInc*2
                    │              │
   BaseAddress+192KB├──────────────┤ ───── Sel+HugeInc*3
                    │              │

HugeInc is the huge increment used for DosAllocHuge.

Desired Access Protection

PAG_EXECUTE (0x00000004)

Execute access to the committed pages in the private memory object
PAG_READ (0x00000001)
Read access
PAG_WRITE (0x00000002)
Write access
PAG_GUARD (0x00000008)
Access to the committed pages in the private memory object causes a "guard page entered" condition to be raised in the subject process.
At least one of the bits PAG_READ, PAG_WRITE, or PAG_EXECUTE must be set.

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