Amiga® RKM Libraries: 20 Exec Memory Allocation

Exec manages all of the free memory currently available in the system.
Using linked list structures, Exec keeps track of memory and provides the
functions to allocate and access it.

When an application needs some memory, it can either declare the memory
statically within the program or it can ask Exec for some memory.  When
Exec receives a request for memory, it looks through its list of free
memory regions to find a suitably sized block that matches the size and
attributes requested.

 Memory Functions                     Other Memory Functions 
 Allocating Multiple Memory Blocks    Function Reference 

20 Exec Memory Allocation / Memory Functions

Normally, an application uses the AllocMem() function to ask for memory:

    APTR AllocMem(ULONG byteSize, ULONG attributes);

The byteSize argument is the amount of memory the application needs and
attributes is a bit field which specifies any special memory
characteristics (described later).  If AllocMem() is successful, it
returns a pointer to a block of memory.  The memory allocation will fail
if the system cannot find a big enough block with the requested
attributes.  If AllocMem() fails, it returns NULL.

Because the system only keeps track of how much free memory is available
and not how much is in use, it has no idea what memory has been allocated
by any task.  This means an application has to explicitly return, or
deallocate, any memory it has allocated so the system can return that
memory to the free memory list.  If an application does not return a block
of memory to the system, the system will not be able to reallocate that
memory to some other task.  That block of memory will be lost until the
Amiga is reset.  If you are using AllocMem() to allocate memory, a call to
FreeMem() will return that memory to the system:

    void FreeMem(APTR mymemblock, ULONG byteSize);

Here mymemblock is a pointer to the memory block the application is
returning to the system and byteSize is the same size that was passed when
the memory was allocated with AllocMem().

Unlike some compiler memory allocation functions, the Amiga system memory
allocation functions return memory blocks that are at least longword
aligned.  This means that the allocated memory will always start on an
address which is at least evenly divisible by four.  This alignment makes
the memory suitable for any system structures or buffers which require
word or longword alignment, and also provides optimal alignment for stacks
and memory copying.

 Memory Attributes 
 Allocating System Memory 
 Freeing System Memory 
 Memory Information Functions 
 Using Memory Copy Functions 
 Summary of System Controlled Memory Handling Routines 

20 / Memory Functions / Memory Attributes

When asking the system for memory, an application can ask for memory with
certain attributes.  The currently supported flags are listed below. Flags
marked "V37" are new memory attributes for Release 2.  Allocations which
specify these new bits may fail on earlier systems.

    This indicates that there is no requirement for either Fast or Chip
    memory.  In this case, while there is Fast memory available, Exec
    will only allocate Fast memory.  Exec will allocate Chip memory if
    there is not enough Fast memory.

    This indicates the application wants a block of chip memory, meaning
    it wants memory addressable by the Amiga custom chips.  Chip memory
    is required for any data that will be accessed by custom chip DMA.
    This includes screen memory, images that will be blitted, sprite
    data, copper lists, and audio data, and pre-V37 floppy disk buffers.
    If this flag is not specified when allocating memory for these types
    of data, your code will fail on machines with expanded memory.

    This indicates a memory block outside of the range that the special
    purpose chips can access.  "FAST" means that the special-purpose
    chips do not have access to the memory and thus cannot cause
    processor bus contention, therefore processor access will likely be
    faster.  Since the flag specifies memory that the custom chips cannot
    access, this flag is mutually exclusive with the MEMF_CHIP flag.  If
    you specify the MEMF_FAST flag, your allocation will fail on any
    Amiga that has only CHIP memory.  Use MEMF_ANY if you would prefer
    FAST memory.

    This indicates that the memory should be accessible to other tasks.
    Although this flag doesn't do anything right now, using this flag
    will help ensure compatibility with possible future features of the
    OS (like virtual memory and memory protection).

    This indicates that the memory should be initialized with zeros.

    This indicates memory which is located on the motherboard which is
    not initialized on reset.

    This indicates that the memory should be allocated within the 24 bit
    address space, so that the memory can be used in Zorro-II expansion
    device DMA transactions.  This bit is for use by Zorro-II DMA devices
    only.  It is not for general use by applications.

    Indicates that the memory list should be searched backwards for the
    highest address memory chunk which can be used for the memory

If an application does not specify any attributes when allocating memory,
the system tries to satisfy the request with the first memory available on
the system memory lists, which is MEMF_FAST if available, followed by

    Make Sure You Have Memory.
    Always check the result of any memory allocation to be sure the type
    and amount of memory requested is available.  Failure to do so will
    lead to trying to use an non-valid pointer.

20 / Memory Functions / Allocating System Memory

The following examples show how to allocate memory.

    APTR  apointer,anotherptr, yap;

    if (!(apointer = AllocMem(100, MEMF_ANY)))
        {  /* COULDN'T GET MEMORY, EXIT */ }

AllocMem() returns the address of the first byte of a memory block that is
at least 100 bytes in size or NULL if there is not that much free memory.
Because the requirement field is specified as MEMF_ANY (zero), memory will
be allocated from any one of the system-managed memory regions.

    if (!(anotherptr = (APTR)AllocMem(1000, MEMF_CHIP | MEMF_CLEAR)))
        {  /* COULDN'T GET MEMORY, EXIT */ }

The example above allocates only chip-accessible memory, which the system
fills with zeros before it lets the application use the memory.  If the
system free memory list does not contain enough contiguous memory bytes in
an area matching your requirements, AllocMem() returns a zero.  You must
check for this condition.

If you are using Release 2, you can use the AllocVec() function to
allocate memory.  In addition to allocating a block of memory, this
function keeps track of the size of the memory block, so your application
doesn't have to remember it when it deallocates that memory block.  The
AllocVec() function allocates a little more memory to store the size of
the memory allocation request.

    if (!(yap = (APTR)AllocVec(512, MEMF_CLEAR)))
        {  /* COULDN'T GET MEMORY, EXIT */ }

20 / Memory Functions / Freeing System Memory

The following examples free the memory chunks shown in the previous calls
to AllocMem().

    FreeMem(apointer, 100);
    FreeMem(anotherptr, 1000);

A memory block allocated with AllocVec() must be returned to the system
pool with the FreeVec().  This function uses the stored size in the
allocation to free the memory block, so there is no need to specify the
size of the memory block to free.


FreeMem() and FreeVec() return no status.  However, if you attempt to free
a memory block in the middle of a chunk that the system believes is
already free, you will cause a system crash. Applications must free the
same size memory blocks that they allocated.  An allocated block may not
be deallocated as smaller pieces.  Due to the internal way the system
rounds up and aligns allocations.  Partial deallocations can corrupt the
system memory list.

    Leave Memory Allocations Out Of Interrupt Code.
    Do not allocate or deallocate system memory from within interrupt
    code.  The "Exec Interrupts" chapter explains that an interrupt may
    occur at any time, even during a memory allocation process.  As a
    result, system data structures may not be internally consistent at
    this time.

20 / Memory Functions / Memory Information Functions

The memory information routines AvailMem() and TypeOfMem() can provide the
amount of memory available in the system, and the attributes of a
particular block of memory.

 Memory Requirements    Calling Memory Information Functions 

20 / / Memory Information Functions / Memory Requirements

The same attribute flags used in memory allocation routines are valid for
the memory information routines.  There is also an additional flag,
MEMF_LARGEST, which can be used in the AvailMem() routine to find out what
the largest available memory block of a particular type is.  Specifying
the MEMF_TOTAL flag will return the total amount of memory currently

20 / / Memory Info Functions / Calling Memory Information Functions

The following example shows how to find out how much memory of a
particular type is available.

    ULONG size;

    size = AvailMem(MEMF_CHIP|MEMF_LARGEST);

AvailMem() returns the size of the largest chunk of available chip memory.

    AvailMem() May Not Be Totally Accurate.
    Because of multitasking, the return value from AvailMem() may be
    inaccurate by the time you receive it.

The following example shows how to determine the type of memory of a
specified memory address.

    ULONG memtype;

    memtype = TypeOfMem((APTR)0x090000);
    if ((memtype & MEMF_CHIP) == MEMF_CHIP) {/* ...It's chip memory... */}

TypeOfMem() returns the attributes of the memory at a specific address. If
it is passed an invalid memory address, TypeOfMem() returns NULL. This
routine is normally used to determine if a particular chunk of memory is
in chip memory.

20 / Memory Functions / Using Memory Copy Functions

For memory block copies, the CopyMem() and CopyMemQuick() functions can be

 Copying System Memory 

20 / / Using Memory Copy Functions / Copying System Memory

The following samples show how to use the copying routines.

    APTR source, target;

    source = AllocMem(1000, MEMF_CLEAR);
    target = AllocMem(1000, MEMF_CHIP);
    CopyMem(source, target, 1000);

CopyMem() copies the specified number of bytes from the source data region
to the target data region.  The pointers to the regions can be aligned on
arbitrary address boundaries.  CopyMem() will attempt to copy the memory
as efficiently as it can according to the alignment of the memory blocks,
and the amount of data that it has to transfer.  These functions are
optimized for copying large blocks of memory which can result in
unnecessary overhead if used to transfer very small blocks of memory.

    CopyMemQuick(source, target, 1000);

CopyMemQuick() performs an optimized copy of the specified number of bytes
from the source data region to the target data region.  The source and
target pointers must be longword aligned and the size (in bytes) must be
divisible by four.

    Not All Copies Are Supported.
    Neither CopyMem() nor CopyMemQuick() supports copying between
    regions that overlap.

20 / / Summary of System Controlled Memory Handling Routines

AllocMem() and FreeMem()
    These are system-wide memory allocation and deallocation routines.
    They use a memory free-list owned and managed by the system.

    This routine returns the number of free bytes in a specified type of

    This routine returns the memory attributes of a specified memory

    CopyMem() is a general purpose memory copy routine. CopyMemQuick() is
    an optimized version of CopyMemQuick(), but has restrictions on the
    size and alignment of the arguments.

20 Exec Memory Allocation / Allocating Multiple Memory Blocks

Exec provides the routines AllocEntry() and FreeEntry() to allocate
multiple memory blocks in a single call. AllocEntry() accepts a data
structure called a MemList, which contains the information about the size
of the memory blocks to be allocated and the requirements, if any, that
you have regarding the allocation. The MemList structure is found in the
include file <exec/memory.h> and is defined as follows:

    struct MemList
        struct Node     ml_Node;
        UWORD           ml_NumEntries;      /* number of MemEntrys */
        struct MemEntry ml_ME[1];           /* where the MemEntrys begin*/

    allows you to link together multiple MemLists.  However, the node is
    ignored by the routines AllocEntry() and FreeEntry().

    tells the system how many MemEntry sets are contained in this
    MemList.  Notice that a MemList is a variable-length structure and
    can contain as many sets of entries as you wish.

The MemEntry structure looks like this:

    struct MemEntry
        union {
            ULONG   meu_Reqs;   /* the AllocMem requirements */
            APTR    meu_Addr;   /* address of your memory */
            } me_Un;
        ULONG   me_Length;      /* the size of this request */

 Sample Code for Allocating Multiple Memory Blocks 
 Result of Allocating Multiple Memory Blocks 
 Multiple Memory Blocks and Tasks 
 Summary of Multiple Memory Blocks Allocation Routines 

20 / Allocating Multiple Memory Blocks / Sample Code

Here's an example of showing how to use the AllocEntry() with multiple
blocks of memory.


AllocEntry() returns a pointer to a new MemList of the same size as the
MemList that you passed to it.  For example, ROM code can provide a
MemList containing the requirements of a task and create a RAM-resident
copy of the list containing the addresses of the allocated entries.  The
pointer to the MemList is used as the argument for FreeEntry() to free the
memory blocks.

    Assembly Does Not Have MemEntry.
    The MemList structure used by assembly programmers is slightly
    different; it has only a label for the start of the MemEntry
    array.  See the Exec AllocEntry() Autodoc for an example of using
    AllocEntry() from assembler.

20 / Allocating Multiple Memory Blocks / Result

The MemList created by AllocEntry() contains MemEntry entries.  MemEntrys
are defined by a union statement, which allows one memory space to be
defined in more than one way.

If AllocEntry() returns a value with bit 31 clear, then all of the
meu_Addr positions in the returned MemList will contain valid memory
addresses meeting the requirements you have provided.  To use this memory
area, you would use code similar to the following:

    #define ALLOCERROR 0x80000000
    struct  MemList *ml;
    APTR    data, moredata;

    if ( ! ((ULONG)ml & ALLOCERROR)))    /* After calling AllocEntry to */
                                         /* allocate ml */
        data     = ml->ml_ME[0].me_Addr;
        moredata = ml->ml_ME[1].me_Addr;
    else  exit(200);                     /* error during AllocEntry */

If AllocEntry() has problems while trying to allocate the memory you have
requested, instead of the address of a new MemList, it will return the
memory requirements value with which it had the problem.  Bit 31 of the
value returned will be set, and no memory will be allocated.  Entries in
the list that were already allocated will be freed.  For example, a failed
allocation of cleared Chip memory (MEMF_CLEAR | MEMF_CHIP) could be
indicated with 0x80010002, where bit 31 indicates failure, bit 16 is the
MEMF_CLEAR flag and bit 1 is the MEMF_CHIP flag.

20 / Allocating Multiple Memory / Multiple Memory Blocks and Tasks

If you want to take advantage of Exec's automatic cleanup, use the MemList
and AllocEntry() facility to do your dynamic memory allocation.

In the Task control block structure, there is a list header named
tc_MemEntry. This is the list header that you initialize to include
MemLists that your task has created by call(s) to AllocEntry().  Here is a
short program segment that handles task memory list header initialization
only.  It assumes that you have already run AllocEntry() as shown in the
simple AllocEntry() example above.

    struct Task *tc;
    struct MemList *ml;

    /* First initialize the task pointer and AllocEntry() the memlist ml */

        NewList(tc->tc_MemEntry);  /* Initialize the task's memory    */
                                   /* list header. Do this once only! */
    AddTail(tc->tc_MemEntry, ml);

Assuming that you have only used the AllocEntry() method (or AllocMem()
and built your own custom MemList), the system now knows where to find the
blocks of memory that your task has dynamically allocated.  The RemTask()
function automatically frees all memory found on tc_MemEntry.

    CreateTask() Sets Up A MemList.
    The amiga.lib CreateTask() function, and other system task and
    process creation functions use a MemList in tc_MemEntry so that
    the Task structure and stack will be automatically deallocated when
    the Task is removed.

20 / Allocating Multiple Memory / Summary of Allocation Routines

AllocEntry() and FreeEntry()
    These are routines for allocating and freeing multiple memory blocks
    with a single call.

    This routine initializes memory from data and offset values in a
    table.  Typically only assembly language programs benefit from using
    this routine.  For more details see the Amiga ROM Kernel Reference
    Manual: Include & Autodocs.

20 Exec Memory Allocation / Other Memory Functions

Allocate() and Deallocate() use a memory region header, called MemHeader,
as part of the calling sequence.  You can build your own local header to
manage memory locally. This structure takes the form:

    struct MemHeader {
        struct Node       mh_Node;
        UWORD             mh_Attributes; /* characteristics of region  */
        struct  MemChunk *mh_First;      /* first free region          */
        APTR              mh_Lower;      /* lower memory bound         */
        APTR              mh_Upper;      /* upper memory bound + 1     */
        ULONG             mh_Free;       /* total number of free bytes */

    is ignored by Allocate() and Deallocate().

    is the pointer to the first MemChunk structure.

    is the lowest address within the memory block. This must be a
    multiple of eight bytes.

    is the highest address within the memory block + 1.  The highest
    address will itself be a multiple of eight if the block was allocated
    to you by AllocMem().

    is the total free space.

This structure is included in the include files <exec/memory.h> and

The following sample code fragment shows the correct initialization of a
MemHeader structure.  It assumes that you wish to allocate a block of
memory from the global pool and thereafter manage it yourself using
Allocate() and Deallocate().


    How Memory Is Tagged.
    Only free memory is "tagged" using a MemChunk linked list. Once
    memory is allocated, the system has no way of determining which task
    now has control of that memory.

If you allocate memory from the system, be sure to deallocate it when your
task exits.  You can accomplish this with matched deallocations, or by
adding a MemList to your task's tc_MemEntry, or you can deallocate the
memory in the finalPC routine (which can be specified if you perform
AddTask() yourself).

 Allocating Memory at an Absolute Address 
 Adding Memory to the System Pool 

20 / Other Memory Functions / Allocating Memory at an Absolute Address

For special advanced applications, AllocAbs() is provided. Using
AllocAbs(), an application can allocate a memory block starting at a
specified absolute memory address.  If the memory is already allocated or
if there is not enough memory available for the request, AllocAbs()
returns a zero.

Be aware that an absolute memory address which happens to be available on
one Amiga may not be available on a machine with a different configuration
or different operating system revision, or even on the same machine at a
different times.  For example, a piece of memory that is available during
expansion board configuration might not be available at earlier or later
times.  Here is an example call to AllocAbs():

    APTR absoluteptr;

    absoluteptr = (APTR)AllocAbs(10000, 0x2F0000);
    if (!(absoluteptr))
        { /* Couldn't get memory, act accordingly. */  }

    /* After we're done using it, we call FreeMem() to free the memory */
    /* block. */
    FreeMem(absoluteptr, 10000);

20 / Other Memory Functions / Adding Memory to the System Pool

When non-Autoconfig memory needs to be added to the system free pool, the
AddMemList() function can be used.  This function takes the size of the
memoryblock, its type, the priority for the memory list, the base address
and the name of the memory block.  A MemHeader structure will be placed at
the start of the memory block, the remainder of the memory block will be
made available for allocation. For example:

    AddMemList(0x200000, MEMF_FAST, 0, 0xF00000, "FZeroBoard");

will add a two megabyte memory block, starting at $F00000 to the system
free pool as Fast memory.  The memory list entry is identified with

20 Exec Memory Allocation / Function Reference

The following are brief descriptions of the Exec functions that handle
memory management.  See the Amiga ROM Kernel Reference Manual: Includes
and Autodocs for details on each call.

                    Table 20-1: Exec Memory Functions
 |                                                                        |
 |  Memory Function               Description                             |
 |      AllocMem()  Allocate memory with specified attributes.  If an     |
 |                  application needs to allocate some memory, it will    |
 |                  usually use this function.                            |
 |    AddMemList()  Add memory to the system free pool.                   |
 |      AllocAbs()  Allocate memory at a specified location.              |
 |      Allocate()  Allocate memory from a private memory pool.           |
 |    AllocEntry()  Allocate multiple memory blocks.                      |
 |      AllocVec()  Allocate memory with specified attributes and keep    |
 |                  track of the size (V36).                              |
 |      AvailMem()  Return the amount of free memory, given certain       |
 |                  conditions.                                           |
 |       CopyMem()  Copy memory block, which can be non-aligned and of    |
 |                  arbitrary length.                                     |
 |  CopyMemQuick()  Copy aligned memory block.                            |
 |    Deallocate()  Return memory block allocated, with Allocate() to the |
 |                  private memory pool.                                  |
 |     FreeEntry()  Free multiple memory blocks, allocated with           |
 |                  AllocEntry().                                         |
 |       FreeMem()  Free a memory block of specified size, allocated with |
 |                  AllocMem().                                           |
 |       FreeVec()  Free a memory block allocated with AllocVec().        |
 |    InitStruct()  Initialize memory from a table.                       |
 |     TypeOfMem()  Determine attributes of a specified memory address.   |

Converted on 22 Apr 2000 with RexxDoesAmigaGuide2HTML 2.1 by Michael Ranner.