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== Installing an Executable Directly Into Memory

If you do not want to use the bootloader (or the on-chip debugger) for executable upload or if your setup does not provide
a serial interface for that, you can also directly install an application into embedded memory.

This concept uses the "Direct Boot" scenario that implements the processor-internal IMEM as ROM, which is
pre-initialized with the application's executable during synthesis. Hence, it provides _non-volatile_ storage of the
executable inside the processor. This storage cannot be altered during runtime and any source code modification of
the application requires to re-program the FPGA via the bitstream.

[TIP]
See datasheet section https://stnolting.github.io/neorv32/#_direct_boot[Direct Boot] for more information.


Using the IMEM as ROM:

* for this boot concept the bootloader is no longer required
* this concept only works for the internal IMEM (but can be extended to work with external memories coupled via the processor's bus interface)
* make sure that the memory components (like block RAM) the IMEM is mapped to support an initialization via the bitstream

[start=1]
. At first, make sure your processor setup actually implements the internal IMEM: the `MEM_INT_IMEM_EN` generics has to be set to `true`:

.Processor top entity configuration - enable internal IMEM
[source,vhdl]
----
  -- Internal Instruction memory --
  MEM_INT_IMEM_EN => true, -- implement processor-internal instruction memory
----

[start=2]
. For this setup we do not want the bootloader to be implemented at all. Disable implementation of the bootloader by setting the
`INT_BOOTLOADER_EN` generic to `false`. This will also modify the processor-internal IMEM so it is initialized with the executable during synthesis.

.Processor top entity configuration - disable internal bootloader
[source,vhdl]
----
  -- General --
  INT_BOOTLOADER_EN => false, -- boot configuration: false = boot from int/ext (I)MEM
----

[start=3]
. To generate an "initialization image" for the IMEM that contains the actual application, run the `install` target when compiling your application:

[source,bash]
----
neorv32/sw/example/blink_led$ make clean_all install
Memory utilization:
   text    data     bss     dec     hex filename
   3176       0     120    3296     ce0 main.elf
Compiling ../../../sw/image_gen/image_gen
Installing application image to ../../../rtl/core/neorv32_application_image.vhd
----

[start=4]
. The `install` target has compiled all the application sources but instead of creating an executable (`neorv32_exe.bit`) that can be uploaded via the
bootloader, it has created a VHDL memory initialization image `core/neorv32_application_image.vhd`.
. This VHDL file is automatically copied to the core's rtl folder (`rtl/core`) so it will be included for the next synthesis.
. Perform a new synthesis. The IMEM will be build as pre-initialized ROM (inferring embedded memories if possible).
. Upload your bitstream. Your application code now resides unchangeable in the processor's IMEM and is directly executed after reset.


The synthesis tool / simulator will print asserts to inform about the (IMEM) memory / boot configuration:

[source]
----
NEORV32 PROCESSOR CONFIG NOTE: Boot configuration: Direct boot from memory (processor-internal IMEM).
NEORV32 PROCESSOR CONFIG NOTE: Implementing processor-internal IMEM as ROM (3176 bytes), pre-initialized with application.
----