Memory Map

Temporary Memory Layout

SBL Stage 1 temporary memory layout:

        Temporary Memory
+------------------------------+  Top of 4GB
|     Memory Mapped Flash      |
+------------------------------+  Memory-Mapped Flash Base
|     FSP-T Reserved Data      |
+------------------------------+  Top of Usable Temporary Memory
|         Available*           |
+------------------------------+  End of Stage 1 Data
|        Stage 1 Data          |
+------------------------------+  Stage 1 Stack Top/Stage 1 Data Base
|        Stage 1 Stack         |
+------------------------------+  Usable CAR Base/Stage 1 Stack Base

Memory Management

  • Once main memory has been initialized, Stage 1B migrates the Slim Bootloader stack from temporary memory to the permanent memory and also reserves a portion of memory to be used for global data structures (LdrGlobal, GDT, IDT), stack, heap, and payload.

Permanent Memory Map

SBL internal memory map layout (BootloaderCoreLib.h):

        Reserved MEM
+------------------------------+  Top of Low MEM
|       SOC Reserved MEM       |
+------------------------------+  Top of usable MEM Base
|       FSP Reserved MEM       |
+------------------------------+  FSP Reserved MEM Base
|       LDR Reserved MEM       |
+------------------------------+  LDR Reserved MEM Base
|         ACPI NVS MEM         |
+------------------------------+  ACPI NVS MEM Base
|       ACPI Reclaim MEM       |
+------------------------------+  ACPI Reclaim MEM Base
|       PLD Reserved MEM       |
+------------------------------+  PLD Reserved MEM Base

      Loader Reserved MEM
+------------------------------+  StackTop
|       LDR Stack (Down)       |
|                              |
|                              |
|                              |
|         LDR HOB (Up)         |
+------------------------------+  MemPoolEnd (Fixed)
|    Global Data structures    |      |
+------------------------------+      |
|                              |      |
|   Permanent MEM Pool (Down)  |      |
|                              |      v
+------------------------------+  MemPoolCurrTop (Moving down)
|                              |
+------------------------------+  MemPoolCurrBottom (Moving up)
|                              |      ^
|   Temporary MEM Pool (Up)    |      |
|                              |      |
+------------------------------+  MemPoolStart (Fixed)

Memory Allocation within Slim Bootloader

Slim Bootloader has two different libraries for allocating memory during various execution phases of the bootloader.

  • Stage 1A, Stage 1B, Stage 2 use MemoryAllocationLib present in BootloaderCorePkg.

  • OsLoader and FwUpdate use FullMemoryAllocationLib present in BootloaderCommonPkg.


MemoryAllocationLib allocates memory from the Loader Reserved Memory region. This can be seen diagrammatically in the “Loader Reserved MEM” layout shown above.

MemoryAllocationLib provides services for allocating two different types of memory -

  • Bootloader permanent memory:

    • Bootloader permanent memory is used by the bootloader to store device tables, S3 Data, SBL container headers, etc.

    • Memory allocated from bootloader permanent memory region cannot be freed

    • Note that permanent memory does not mean that it is persistent across reboots

    • AllocatePool(), AllocateZeroPool(), AllocatePages(), AllocateAlignedPages() allocate memory from the Loader Reserved region starting from MemPoolEnd which grows down towards MemPoolStart

      • Currently used allocated memory is between MemPoolEnd and MemPoolCurrTop

      • Loader Reserved Memory is reported to the OS as “Reserved” memory

  • Temporary Memory:

    • Temporary memory requested by a stage is expected to be used by that stage itself

    • Temporary memory is allocated starting from MemPoolStart and grows up towards MemPoolEnd.

      • Currently used temporary memory is between MemPoolStart and MemPoolEnd

    • Temporary Memory is freed by the FreeTemporaryMemory() API call.

      • Calling FreeTemporaryMemory() with NULL as a parameter will free all allocated memory within the temporary memory region.


  • Stage 2 passes the HOB list pointer, and the Payload executable base to the payload.

  • The HOB list pointer contains the pointer to the HOBs consumed by the payload. The MEMORY_MAP_INFO HOB will report memory available to the payload.

    • The payload reserved memory region from memory map info hob is identified by the MEM_MAP_TYPE_RESERVED entry type and the MEM_MAP_FLAG_PAYLOAD flag.

  • In case of tightly-coupled payloads, once the payload is loaded, PayloadInit() is called by PayloadEntryLib to initialize global data for the payload.

  • PayloadInit() in turn calls AddMemoryResourceRange() from FullMemoryAllocationLib to initialize the memory ranges from which memory will be allocated to the payload.

  • The size of the reserved memory, heap, and stack is determined by their respective PCDs defined in


  • Once the memory ranges are initialized, dynamic memory allocation will be done from the payload heap region.

  • The memory map of the Payload can be seen below:

                Payload Memmap
+--------------------------------------------+ TOLUM
|   Reserved memory for Slimboot core        |
+--------------------------------------------+ RsvdBase + RsvdSize
|   Reserved memory for Payload              |
+--------------------------------------------+ RsvdBase
|   + DMA buffer                             |
+--------------------------------------------+ DmaBase
|   + Payload heap                           |
+--------------------------------------------+ HeapBase
|   + Payload stack                          |
+--------------------------------------------+ StackBase
|   Free memory                              |
+--------------------------------------------+ 0