MMU - Adjust the Ratio of ICACHE to IRAM


The ESP8266 has a total of 64K of instruction memory, IRAM. This 64K of IRAM is composed of one dedicated 32K block of IRAM and two 16K blocks of IRAM. The last two 16K blocks of IRAM are flexible in the sense that it can be used as a transparent cache for external flash memory. These blocks can either be used for IRAM or an instruction cache for executing code out of flash, ICACHE.

The code generated for a sketch is divided up into two groups, ICACHE and IRAM. IRAM offers faster execution. It is used for interrupt service routines, exception handling, and time-critical code. The ICACHE allows for the execution of up to 1MB of code stored in flash. On a cache miss, a delay occurs as the instructions are read from flash via the SPI bus.

There is 98KB of DRAM space. This memory can be accessed as byte, short, or a 32-bit word. Access must be aligned according to the data type size. A 16bit short must be on a multiple of 2-byte address boundary. Likewise, a 32-bit word must be on a multiple of 4-byte address boundary. In contrast, data access in IRAM or ICACHE must always be a full 32-bit word and aligned. We will discuss a non32-bit exception handler for this later.

Option Summary

The Arduino IDE Tools menu option, MMU has the following selections:

  1. 32KB cache + 32KB IRAM (balanced)

    • This is the legacy ratio.

    • Try this option 1st.

  2. 16KB cache + 48KB IRAM (IRAM)

    • With just 16KB cache, execution of code out of flash may be slowed by more cache misses when compared to 32KB. The slowness will vary with the sketch.

    • Use this if you need a little more IRAM space, and you have enough DRAM space.

  3. 16KB cache + 48KB IRAM and 2nd Heap (shared)

    • This option builds on the previous option and creates a 2nd Heap made with IRAM.

    • The 2nd Heap size will vary with free IRAM.

    • This option is flexible. IRAM usage for code can overflow into the additional 16KB IRAM region, shrinking the 2nd Heap below 16KB. Or IRAM can be under 32KB, allowing the 2nd Heap to be larger than 16KB.

    • Installs a Non-32-Bit Access handler for IRAM. This allows for byte and 16-bit aligned short access.

    • This 2nd Heap is supported by the standard malloc APIs.

    • Heap selection is handled through a HeapSelect class. This allows a specific heap selection for the duration of a scope.

    • Use this option, if you are still running out of DRAM space after you have moved as many of your constant strings/data elements that you can to PROGMEM.

  4. 16KB cache + 32KB IRAM + 16KB 2nd Heap (not shared)

    • Not managed by the umm_malloc heap library

    • If required, non-32-Bit Access for IRAM must be enabled separately.

    • Enables a 16KB block of unmanaged IRAM memory

    • Data persist across reboots, but not deep sleep.

    • Works well for when you need a simple large chunk of memory. This option will reduce the resources required to support a shared 2nd Heap.

MMU related build defines and possible values. These values change as indicated with the menu options above:




shared (IRAM and Heap)

not shared (IRAM and Heap)












defined, enablesumm_malloc









** This define is to an inline function that calculates the value, based on unused code space, requires #include <mmu_iram.h>.

The Arduino IDE Tools menu option, Non-32-Bit Access has the following selections:

  • Use pgm_read macros for IRAM/PROGMEM

  • Byte/Word access to IRAM/PROGMEM (very slow)

    • This option adds a non32-bit exception handler to your build.

    • Handles read/writes to IRAM and reads to ICACHE.

    • Supports short and byte access to IRAM

    • Not recommended for high-frequency access data, use DRAM if you can.

    • Expect it to be slower than DRAM, each character access, will require a complete save and restore of all 16+ registers.

    • Processing an exception uses 256 bytes of stack space just to get started. The actual handler will add a little more.

    • This option is implicitly enabled and required when you select MMU option 16KB cache + 48KB IRAM and 2nd Heap (shared).

IRAM, unlike DRAM, must be accessed as aligned full 32-bit words, no byte or short access. The pgm_read macros are an option; however, the store operation remains an issue. For a block copy, ets_memcpy appears to work well as long as the byte count is rounded up to be evenly divided by 4, and source and destination addresses are 4 bytes aligned.

A word of caution, I have seen one case with the new toolchain 10.1 where code that reads a 32-bit word to extract a byte was optimized away to be a byte read. Using volatile on the pointer stopped the over-optimization.

To get a sense of how memory access time is effected, see examples MMU48K and irammem in ESP8266.


For calls to umm_malloc with interrupts disabled.

  • malloc will always allocate from the DRAM heap when called with interrupts disabled.

  • realloc with a NULL pointer will use malloc and return a DRAM heap allocation. Note, calling realloc with interrupts disabled is not officially supported. You are on your own if you do this.

  • If you must use IRAM memory in your ISR, allocate the memory in your init code. To reduce the time spent in the ISR, avoid non32-bit access that would trigger the exception handler. For short or byte access, consider using the inline functions described in section “Performance Functions” below.

How to Select Heap

The MMU selection 16KB cache + 48KB IRAM and 2nd Heap (shared) allows you to use the standard heap API function calls (malloc, calloc, free, … ). to allocate memory from DRAM or IRAM. This selection can be made by instantiating the class HeapSelectIram or HeapSelectDram. The usage is similar to that of the InterruptLock class. The default/initial heap source is DRAM. The class is in umm_malloc/umm_heap_select.h.

  char *bufferDram;
  bufferDram = (char *)malloc(33);
  char *bufferIram;
      HeapSelectIram ephemeral;
      bufferIram = (char *)malloc(33);

free will always return memory to the correct heap. There is no need for tracking and selecting before freeing.

realloc with a non-NULL pointer will always resize the allocation from the original heap it was allocated from. When the supplied pointer is NULL, then the current heap selection is used.

Low-level primitives for selecting a heap. These are used by the above Classes:

  • umm_get_current_heap_id()

  • umm_set_heap_by_id( ID value )

  • Possible ID values



Also, an alternate stack select method API is available. This is not as easy as the class method; however, for some small set of cases, it may provide some additional control:

  • ESP.setIramHeap() Pushes current heap ID onto a stack and sets Heap API for an IRAM selection.

  • ESP.setDramHeap() Pushes current heap ID onto a stack and sets Heap API for a DRAM selection.

  • ESP.resetHeap() Restores previously pushed heap. ### Identify Memory

These always inlined functions can be used to determine the resource of a pointer:

bool mmu_is_iram(const void *addr);
bool mmu_is_dram(const void *addr);
bool mmu_is_icache(const void *addr);

Performance Functions

While these always inlined functions, will bypass the need for the exception handler reducing execution time and stack use, it comes at the cost of increased code size.

These are an alternative to the pgm_read macros for reading from IRAM. When compiled with ‘Debug Level: core’ range checks are performed on the pointer value to make sure you are reading from the address range of IRAM, DRAM, or ICACHE.

uint8_t mmu_get_uint8(const void *p8);
uint16_t mmu_get_uint16(const uint16_t *p16);
int16_t mmu_get_int16(const int16_t *p16);

While these functions are intended for writing to IRAM, they will work with DRAM. When compiled with ‘Debug Level: core’, range checks are performed on the pointer value to make sure you are writing to the address range of IRAM or DRAM.

uint8_t mmu_set_uint8(void *p8, const uint8_t val);
uint16_t mmu_set_uint16(uint16_t *p16, const uint16_t val);
int16_t mmu_set_int16(int16_t *p16, const int16_t val);