hdr_ptr->hdr.bytes_left = 0;
  mem->large_list[pool_id] = hdr_ptr;

  return (void FAR *) (hdr_ptr + 1); /* point to first data byte in pool */
}


/*
 * Creation of 2-D sample arrays.
 * The pointers are in near heap, the samples themselves in FAR heap.
 *
 * To minimize allocation overhead and to allow I/O of large contiguous
 * blocks, we allocate the sample rows in groups of as many rows as possible
 * without exceeding MAX_ALLOC_CHUNK total bytes per allocation request.
 * NB: the virtual array control routines, later in this file, know about
 * this chunking of rows.  The rowsperchunk value is left in the mem manager
 * object so that it can be saved away if this sarray is the workspace for
 * a virtual array.
 */

METHODDEF(JSAMPARRAY)
alloc_sarray (j_common_ptr cinfo, int pool_id,
	      JDIMENSION samplesperrow, JDIMENSION numrows)
/* Allocate a 2-D sample array */
{
  my_mem_ptr mem = (my_mem_ptr) cinfo->mem;
  JSAMPARRAY result;
  JSAMPROW workspace;
  JDIMENSION rowsperchunk, currow, i;
  long ltemp;

  /* Calculate max # of rows allowed in one allocation chunk */
  ltemp = (MAX_ALLOC_CHUNK-SIZEOF(large_pool_hdr)) /
	  ((long) samplesperrow * SIZEOF(JSAMPLE));
  if (ltemp <= 0)
    ERREXIT(cinfo, JERR_WIDTH_OVERFLOW);
  if (ltemp < (long) numrows)
    rowsperchunk = (JDIMENSION) ltemp;
  else
    rowsperchunk = numrows;
  mem->last_rowsperchunk = rowsperchunk;

  /* Get space for row pointers (small object) */
  result = (JSAMPARRAY) alloc_small(cinfo, pool_id,
				    (size_t) (numrows * SIZEOF(JSAMPROW)));

  /* Get the rows themselves (large objects) */
  currow = 0;
  while (currow < numrows) {
    rowsperchunk = MIN(rowsperchunk, numrows - currow);
    workspace = (JSAMPROW) alloc_large(cinfo, pool_id,
	(size_t) ((size_t) rowsperchunk * (size_t) samplesperrow
		  * SIZEOF(JSAMPLE)));
    for (i = rowsperchunk; i > 0; i--) {
      result[currow++] = workspace;
      workspace += samplesperrow;
    }
  }

  return result;
}


/*
 * Creation of 2-D coefficient-block arrays.
 * This is essentially the same as the code for sample arrays, above.
 */

METHODDEF(JBLOCKARRAY)
alloc_barray (j_common_ptr cinfo, int pool_id,
	      JDIMENSION blocksperrow, JDIMENSION numrows)
/* Allocate a 2-D coefficient-block array */
{
  my_mem_ptr mem = (my_mem_ptr) cinfo->mem;
  JBLOCKARRAY result;
  JBLOCKROW workspace;
  JDIMENSION rowsperchunk, currow, i;
  long ltemp;

  /* Calculate max # of rows allowed in one allocation chunk */
  ltemp = (MAX_ALLOC_CHUNK-SIZEOF(large_pool_hdr)) /
	  ((long) blocksperrow * SIZEOF(JBLOCK));
  if (ltemp <= 0)
    ERREXIT(cinfo, JERR_WIDTH_OVERFLOW);
  if (ltemp < (long) numrows)
    rowsperchunk = (JDIMENSION) ltemp;
  else
    rowsperchunk = numrows;
  mem->last_rowsperchunk = rowsperchunk;

  /* Get space for row pointers (small object) */
  result = (JBLOCKARRAY) alloc_small(cinfo, pool_id,
				     (size_t) (numrows * SIZEOF(JBLOCKROW)));

  /* Get the rows themselves (large objects) */
  currow = 0;
  while (currow < numrows) {
    rowsperchunk = MIN(rowsperchunk, numrows - currow);
    workspace = (JBLOCKROW) alloc_large(cinfo, pool_id,
	(size_t) ((size_t) rowsperchunk * (size_t) blocksperrow
		  * SIZEOF(JBLOCK)));
    for (i = rowsperchunk; i > 0; i--) {
      result[currow++] = workspace;
      workspace += blocksperrow;
    }
  }

  return result;
}


/*
 * About virtual array management:
 *
 * The above "normal" array routines are only used to allocate strip buffers
 * (as wide as the image, but just a few rows high).  Full-image-sized buffers
 * are handled as "virtual" arrays.  The array is still accessed a strip at a
 * time, but the memory manager must save the whole array for repeated
 * accesses.  The intended implementation is that there is a strip buffer in
 * memory (as high as is possible given the desired memory limit), plus a
 * backing file that holds the rest of the array.
 *
 * The request_virt_array routines are told the total size of the image and
 * the maximum number of rows that will be accessed at once.  The in-memory
 * buffer must be at least as large as the maxaccess value.
 *
 * The request routines create control blocks but not the in-memory buffers.
 * That is postponed until realize_virt_arrays is called.  At that time the
 * total amount of space needed is known (approximately, anyway), so free
 * memory can be divided up fairly.
 *
 * The access_virt_array routines are responsible for making a specific strip
 * area accessible (after reading or writing the backing file, if necessary).
 * Note that the access routines are told whether the caller intends to modify
 * the accessed strip; during a read-only pass this saves having to rewrite
 * data to disk.  The access routines are also responsible for pre-zeroing
 * any newly accessed rows, if pre-zeroing was requested.
 *
 * In current usage, the access requests are usually for nonoverlapping
 * strips; that is, successive access start_row numbers differ by exactly
 * num_rows = maxaccess.  This means we can get good performance with simple
 * buffer dump/reload logic, by making the in-memory buffer be a multiple
 * of the access height; then there will never be accesses across bufferload
 * boundaries.  The code will still work with overlapping access requests,
 * but it doesn't handle bufferload overlaps very efficiently.
 */


METHODDEF(jvirt_sarray_ptr)
request_virt_sarray (j_common_ptr cinfo, int pool_id, boolean pre_zero,
		     JDIMENSION samplesperrow, JDIMENSION numrows,
		     JDIMENSION maxaccess)
/* Request a virtual 2-D sample array */
{
  my_mem_ptr mem = (my_mem_ptr) cinfo->mem;
  jvirt_sarray_ptr result;

  /* Only IMAGE-lifetime virtual arrays are currently supported */
  if (pool_id != JPOOL_IMAGE)
    ERREXIT1(cinfo, JERR_BAD_POOL_ID, pool_id);	/* safety check */

  /* get control block */
  result = (jvirt_sarray_ptr) alloc_small(cinfo, pool_id,
					  SIZEOF(struct jvirt_sarray_control));

  result->mem_buffer = NULL;	/* marks array not yet realized */
  result->rows_in_array = numrows;
  result->samplesperrow = samplesperrow;
  result->maxaccess = maxaccess;
  result->pre_zero = pre_zero;
  result->b_s_open = FALSE;	/* no associated backing-store object */
  result->next = mem->virt_sarray_list; /* add to list of virtual arrays */
  mem->virt_sarray_list = result;

  return result;
}


METHODDEF(jvirt_barray_ptr)
request_virt_barray (j_common_ptr cinfo, int pool_id, boolean pre_zero,
		     JDIMENSION blocksperrow, JDIMENSION numrows,
		     JDIMENSION maxaccess)
/* Request a virtual 2-D coefficient-block array */
{
  my_mem_ptr mem = (my_mem_ptr) cinfo->mem;
  jvirt_barray_ptr result;

  /* Only IMAGE-lifetime virtual arrays are currently supported */
  if (pool_id != JPOOL_IMAGE)
    ERREXIT1(cinfo, JERR_BAD_POOL_ID, pool_id);	/* safety check */

  /* get control block */
  result = (jvirt_barray_ptr) alloc_small(cinfo, pool_id,
					  SIZEOF(struct jvirt_barray_control));

  result->mem_buffer = NULL;	/* marks array not yet realized */
  result->rows_in_array = numrows;
  result->blocksperrow = blocksperrow;
  result->maxaccess = maxaccess;
  result->pre_zero = pre_zero;
  result->b_s_open = FALSE;	/* no associated backing-store object */
  result->next = mem->virt_barray_list; /* add to list of virtual arrays */
  mem->virt_barray_list = result;

  return result;
}


METHODDEF(void)
realize_virt_arrays (j_common_ptr cinfo)
/* Allocate the in-memory buffers for any unrealized virtual arrays */
{
  my_mem_ptr mem = (my_mem_ptr) cinfo->mem;
  long space_per_minheight, maximum_space, avail_mem;
  long minheights, max_minheights;
  jvirt_sarray_ptr sptr;
  jvirt_barray_ptr bptr;

  /* Compute the minimum space needed (maxaccess rows in each buffer)
   * and the maximum space needed (full image height in each buffer).
   * These may be of use to the system-dependent jpeg_mem_available routine.
   */
  space_per_minheight = 0;
  maximum_space = 0;
  for (sptr = mem->virt_sarray_list; sptr != NULL; sptr = sptr->next) {
    if (sptr->mem_buffer == NULL) { /* if not realized yet */
      space_per_minheight += (long) sptr->maxaccess *
			     (long) sptr->samplesperrow * SIZEOF(JSAMPLE);
      maximum_space += (long) sptr->rows_in_array *
		       (long) sptr->samplesperrow * SIZEOF(JSAMPLE);
    }
  }
  for (bptr = mem->virt_barray_list; bptr != NULL; bptr = bptr->next) {
    if (bptr->mem_buffer == NULL) { /* if not realized yet */
      space_per_minheight += (long) bptr->maxaccess *
			     (long) bptr->blocksperrow * SIZEOF(JBLOCK);
      maximum_space += (long) bptr->rows_in_array *
		       (long) bptr->blocksperrow * SIZEOF(JBLOCK);
    }
  }

  if (space_per_minheight <= 0)
    return;			/* no unrealized arrays, no work */

  /* Determine amount of memory to actually use; this is system-dependent. */
  avail_mem = jpeg_mem_available(cinfo, space_per_minheight, maximum_space,
				 mem->total_space_allocated);

  /* If the maximum space needed is available, make all the buffers full
   * height; otherwise parcel it out with the same number of minheights
   * in each buffer.
   */
  if (avail_mem >= maximum_space)
    max_minheights = 1000000000L;
  else {
    max_minheights = avail_mem / space_per_minheight;
    /* If there doesn't seem to be enough space, try to get the minimum
     * anyway.  This allows a "stub" implementation of jpeg_mem_available().
     */
    if (max_minheights <= 0)
      max_minheights = 1;
  }

  /* Allocate the in-memory buffers and initialize backing store as needed. */

  for (sptr = mem->virt_sarray_list; sptr != NULL; sptr = sptr->next) {
    if (sptr->mem_buffer == NULL) { /* if not realized yet */
      minheights = ((long) sptr->rows_in_array - 1L) / sptr->maxaccess + 1L;
      if (minheights <= max_minheights) {
	/* This buffer fits in memory */
	sptr->rows_in_mem = sptr->rows_in_array;
      } else {
	/* It doesn't fit in memory, create backing store. */
	sptr->rows_in_mem = (JDIMENSION) (max_minheights * sptr->maxaccess);
	jpeg_open_backing_store(cinfo, & sptr->b_s_info,
				(long) sptr->rows_in_array *
				(long) sptr->samplesperrow *
				(long) SIZEOF(JSAMPLE));
	sptr->b_s_open = TRUE;
      }
      sptr->mem_buffer = alloc_sarray(cinfo, JPOOL_IMAGE,
				      sptr->samplesperrow, sptr->rows_in_mem);
      sptr->rowsperchunk = mem->last_rowsperchunk;
      sptr->cur_start_row = 0;
      sptr->first_undef_row = 0;
      sptr->dirty = FALSE;
    }
  }

  for (bptr = mem->virt_barray_list; bptr != NULL; bptr = bptr->next) {
    if (bptr->mem_buffer == NULL) { /* if not realized yet */
      minheights = ((long) bptr->rows_in_array - 1L) / bptr->maxaccess + 1L;
      if (minheights <= max_minheights) {
	/* This buffer fits in memory */
	bptr->rows_in_mem = bptr->rows_in_array;
      } else {
	/* It doesn't fit in memory, create backing store. */
	bptr->rows_in_mem = (JDIMENSION) (max_minheights * bptr->maxaccess);
	jpeg_open_backing_store(cinfo, & bptr->b_s_info,
				(long) bptr->rows_in_array *
				(long) bptr->blocksperrow *
				(long) SIZEOF(JBLOCK));
	bptr->b_s_open = TRUE;
      }
      bptr->mem_buffer = alloc_barray(cinfo, JPOOL_IMAGE,
				      bptr->blocksperrow, bptr->rows_in_mem);
      bptr->rowsperchunk = mem->last_rowsperchunk;
      bptr->cur_start_row = 0;
      bptr->first_undef_row = 0;
      bptr->dirty = FALSE;
    }
  }
}


LOCAL(void)
do_sarray_io (j_common_ptr cinfo, jvirt_sarray_ptr ptr, boolean writing)
/* Do backing store read or write of a virtual sample array */
{
  long bytesperrow, file_offset, byte_count, rows, thisrow, i;

  bytesperrow = (long) ptr->samplesperrow * SIZEOF(JSAMPLE);
  file_offset = ptr->cur_start_row * bytesperrow;
  /* Loop to read or write each allocation chunk in mem_buffer */
  for (i = 0; i < (long) ptr->rows_in_mem; i += ptr->rowsperchunk) {
    /* One chunk, but check for short chunk at end of buffer */
    rows = MIN((long) ptr->rowsperchunk, (long) ptr->rows_in_mem - i);
    /* Transfer no more than is currently defined */
    thisrow = (long) ptr->cur_start_row + i;
    rows = MIN(rows, (long) ptr->first_undef_row - thisrow);
    /* Transfer no more than fits in file */
    rows = MIN(rows, (long) ptr->rows_in_array - thisrow);
    if (rows <= 0)		/* this chunk might be past end of file! */
      break;
    byte_count = rows * bytesperrow;
    if (writing)
      (*ptr->b_s_info.write_backing_store) (cinfo, & ptr->b_s_info,
					    (void FAR *) ptr->mem_buffer[i],
					    file_offset, byte_count);
    else
      (*ptr->b_s_info.read_backing_store) (cinfo, & ptr->b_s_info,
					   (void FAR *) ptr->mem_buffer[i],
					   file_offset, byte_count);
    file_offset += byte_count;
  }
}


LOCAL(void)
do_barray_io (j_common_ptr cinfo, jvirt_barray_ptr ptr, boolean writing)
/* Do backing store read or write of a virtual coefficient-block array */
{
  long bytesperrow, file_offset, byte_count, rows, thisrow, i;

  bytesperrow = (long) ptr->blocksperrow * SIZEOF(JBLOCK);
  file_offset = ptr->cur_start_row * bytesperrow;
  /* Loop to read or write each allocation chunk in mem_buffer */
  for (i = 0; i < (long) ptr->rows_in_mem; i += ptr->rowsperchunk) {
    /* One chunk, but check for short chunk at end of buffer */
    rows = MIN((long) ptr->rowsperchunk, (long) ptr->rows_in_mem - i);
    /* Transfer no more than is currently defined */
    thisrow = (long) ptr->cur_start_row + i;
    rows = MIN(rows, (long) ptr->first_undef_row - thisrow);
    /* Transfer no more than fits in file */
    rows = MIN(rows, (long) ptr->rows_in_array - thisrow);
    if (rows <= 0)		/* this chunk might be past end of file! */
      break;
    byte_count = rows * bytesperrow;
    if (writing)
      (*ptr->b_s_info.write_backing_store) (cinfo, & ptr->b_s_info,
					    (void FAR *) ptr->mem_buffer[i],
					    file_offset, byte_count);
    else