This call allocates multiple segments as a huge block of memory.
DosAllocHuge (NumSeg, Size, Selector, MaxNumSeg, AllocFlags)
- NumSeg (USHORT) - input
- Number of 65536-byte segments to be allocated.
- Size (USHORT) - input
- Number of bytes to be allocated in the last (non-65536-byte) segment. A value of zero indicates none.
- Selector (PSEL) - output
- Address where the selector of the first segment allocated is returned.
- MaxNumSeg (USHORT) - input
- Maximum number of 65536-byte segments this object occupies as a result of any subsequent DosReallocHuge. If MaxNumSeg is 0, OS/2 assumes this segment will never be increased by DosReallocHuge beyond its original size, though it may be decreased. This value is ignored in the DOS mode.
- AllocFlags (USHORT) - input
- Bit indicators describing the characteristics of the segment allocated. The bits that can be set and their meanings are:
- rc (USHORT) - return
- Return code descriptions are:
- 0 NO_ERROR
- 8 ERROR_NOT_ENOUGH_MEMORY
- 87 ERROR_INVALID_PARAMETER
- 212 ERROR_LOCKED
DosAllocHuge allows a process to allocate a large amount of memory by telling the system how many 64KB segments it needs and whether it requires an additional partial segment. The system allocates the memory, which is movable and swappable, and returns a selector to the first segment. When this selector is used with a call, the requested function is performed for the entire block of memory.
Each segment of a huge memory allocation has a unique selector. To determine the remaining selectors of a huge memory allocation, issue DosGetHugeShift, which returns a shift count. To compute the next sequential selector, take the value 1 and shift it left by the number of bits specified in shift count. Use the resulting value as an increment to add to the previous selector, using the selector returned by DosAllocHuge as the first selector. For example:
- Assume DosAllocHuge is issued with NumSeg equal to 3, and that the number 63 is returned for the selector of the first segment.
- If DosGetHugeShift returns a shift count of 4, shifting the value "1" by this amount results in an increment of 16.
- Adding this increment to selector number 63 produces 79 for the second selector. Adding the same increment to selector number 79 yields 95 for the third selector.
Like single segment memory allocated with DosAllocSeg, huge memory can be designated as shareable by other processes and discardable by the system when no longer needed. Allocating a huge block of memory as discardable automatically locks the memory for use by the caller. When one segment of a huge allocation is discarded by the system, this forces the discard of all the other segments. See DosAllocSeg for more information relating to discardable and shared segments.
Applications should be discretionary in claiming large memory when doing so can impair system performance. To test system memory availability, issue DosMemAvail. This call returns the size of the largest block of unallocated memory. Although this value can change at any time because of system activity, it can provide a good indication of the system memory state.
Memory allocated by DosAllocHuge is freed by DosFreeSeg. One call to DosFreeSeg, passing the selector returned from a DosAllocHuge, frees all of the memory allocated.
- This request may be issued from privilege level 2. However, the segment is allocated as a privilege level 3 segment.
Family API Considerations
Some options operate differently in the DOS mode than in the OS/2 mode. Therefore, the following considerations apply to DosAllocHuge when coding for the DOS mode:
- Requested size value is rounded up to the next paragraph (16-byte)
- Selector is the actual segment address allocated.
#define INCL_DOSMEMMGR USHORT rc = DosAllocHuge(NumSeg, Size, Selector, MaxNumSeg, AllocFlags); USHORT NumSeg; /* Number of 65536-byte segments */ USHORT Size; /* Number of bytes in last segment */ PSEL Selector; /* The first Selector allocated (returned) */ USHORT MaxNumSeg; /* Max number of 65536-byte segments */ USHORT AllocFlags; /* Allocation flags */ USHORT rc; /* return code */
EXTRN DosAllocHuge:FAR INCL_DOSMEMMGR EQU 1 PUSH WORD NumSeg ;Number of 65536-byte segments PUSH WORD Size ;Number of bytes in last segment PUSH@ WORD Selector ;The first Selector allocated (returned) PUSH WORD MaxNumSeg ;Max number of 65536-byte segments PUSH WORD AllocFlags ;Allocation flags CALL DosAllocHuge Returns WORD
This example requests a block of memory with 4 segments, the last segment having 1,040 bytes. The block of memory will never be larger than 8 segments. The memory can be shared with DosGiveSeg API calls. The system can discard the memory if it needs too.
#define INCL_DOSMEMMGR #define NUMBER_OF_SEGMENTS 4 #define BYTES_IN_LAST_SEGMENT 1040 #define MAXIMUM_SEG_SIZE 8 #define ALLOC_FLAG SEG_GIVEABLE | SEG_DISCARDABLE SEL Selector; USHORT rc; rc = DosAllocHuge(NUMBER_OF_SEGMENTS, /* # of 65536-byte segments */ BYTES_IN_LAST_SEGMENT, /* # of bytes in last segment */ &Selector, /* The 1st selector allocated */ MAXIMUM_SEG_SIZE, /* Max number of segments */ ALLOC_FLAG); /* Allocation flags */