new-operator.txt

NEW OPERATOR

The "new" operator can have a parameter, which specifies the type of memory to
be allocated:

* HEAP_LOW: memory below 1 GByte
* HEAP_HIGH: memory above 1 GByte (on Raspberry Pi 4 only)
* HEAP_ANY: memory above 1 GB (if available) or memory below 1 GB (otherwise)
* HEAP_DMA30: 30-bit DMA-able memory (alias for HEAP_LOW)

This is especially important on the Raspberry Pi 4, which supports different
SDRAM memory regions. For instance one can specify to allocate a 256 byte memory
block above 1 GByte:

	#include <circle/new.h>

	unsigned char *p = new (HEAP_HIGH) unsigned char[256];

If "new" is used without a memory type parameter, HEAP_LOW is used as default
type. This can be changed with the system option HEAP_DEFAULT_NEW in the file
include/circle/sysconfig.h.

The memory type parameter HEAP_DMA30 has to be selected to allocate memory
buffers, which will be used for DMA transfers to/from devices, which support
only 30-bit addressing (e.g. the platform DMA controller). Even if on Raspberry
Pi models before the Raspberry Pi 4 a memory type parameter is not required, the
HEAP_DMA30 type should be specified (if needed) to ensure, that the code does
work on the Raspberry Pi 4 too.

There is a difference in the handling of out-of-memory conditions. If explicitly
requesting a HEAP_HIGH memory block fails, zero is returned. If requesting any
other memory type block fails, because the memory is full, the system generates
an "Out of memory" panic message.

Memory blocks returned by the "new" operator are aligned to the maximum line
size of the data caches in the system in any case, which is always greater or
equal to 16 bytes.

Circle can currently handle up to 2 GBytes of high memory, which should be
enough for common bare metal applications. If your Raspberry Pi 4 has 4 GBytes
SDRAM, the highest 1 GByte memory region is unused. This is necessary, because
memory above the 3 GByte boundary may require a specific DMA-handling, which is
not implemented so far.

The C-functions malloc() and calloc() allocate a memory block from HEAP_LOW by
default.