nuttx/arch/Kconfig

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#
# For a description of the syntax of this configuration file,
# see the file kconfig-language.txt in the NuttX tools repository.
#
choice
prompt "CPU Architecture"
default ARCH_ARM
config ARCH_ARM
bool "ARM"
select ARCH_HAVE_INTERRUPTSTACK
select ARCH_HAVE_TLS
select ARCH_HAVE_VFORK
select ARCH_HAVE_STACKCHECK
select ARCH_HAVE_CUSTOMOPT
---help---
The ARM architectures
config ARCH_AVR
bool "AVR"
select ARCH_NOINTC
select ARCH_HAVE_INTERRUPTSTACK
select ARCH_HAVE_CUSTOMOPT
---help---
Atmel 8-bit bit AVR and 32-bit AVR32 architectures
config ARCH_HC
bool "Freescale HC"
select ARCH_NOINTC
select ARCH_HAVE_INTERRUPTSTACK
---help---
Freescale HC architectures (M9S12)
config ARCH_MIPS
bool "MIPS"
select ARCH_HAVE_INTERRUPTSTACK
select ARCH_HAVE_CUSTOMOPT
---help---
MIPS architectures (PIC32)
config ARCH_RGMP
bool "RGMP"
---help---
RTOS and GPOS on Multi-Processor (RGMP) architecture. See
http://rgmp.sourceforge.net/wiki/index.php/Main_Page.
config ARCH_RENESAS
bool "Renesas"
select ARCH_NOINTC
select ARCH_HAVE_INTERRUPTSTACK
---help---
Renesas architectures (SH and M16C).
config ARCH_RISCV
bool "RISC-V"
select ARCH_HAVE_INTERRUPTSTACK
select ARCH_HAVE_CUSTOMOPT
---help---
RISC-V 32 and 64-bit RV32 / RV64 architectures.
config ARCH_SIM
bool "Simulation"
select ARCH_HAVE_MULTICPU
select ARCH_HAVE_TLS
select ARCH_HAVE_TICKLESS
select ARCH_HAVE_POWEROFF
select SERIAL_CONSOLE
---help---
Linux/Cywgin user-mode simulation.
config ARCH_X86
bool "x86"
---help---
Intel x86 architectures.
config ARCH_Z16
bool "ZNEO"
select ARCH_HAVE_HEAP2
---help---
ZiLOG ZNEO 16-bit architectures (z16f).
config ARCH_Z80
bool "z80"
select ARCH_HAVE_HEAP2
---help---
ZiLOG 8-bit architectures (z80, ez80, z8).
endchoice
config ARCH
string
default "arm" if ARCH_ARM
default "avr" if ARCH_AVR
default "hc" if ARCH_HC
default "mips" if ARCH_MIPS
default "rgmp" if ARCH_RGMP
default "renesas" if ARCH_RENESAS
default "risc-v" if ARCH_RISCV
default "sim" if ARCH_SIM
default "x86" if ARCH_X86
default "z16" if ARCH_Z16
default "z80" if ARCH_Z80
source arch/arm/Kconfig
source arch/avr/Kconfig
source arch/hc/Kconfig
source arch/mips/Kconfig
source arch/rgmp/Kconfig
source arch/renesas/Kconfig
source arch/risc-v/Kconfig
source arch/sim/Kconfig
source arch/x86/Kconfig
source arch/z16/Kconfig
source arch/z80/Kconfig
comment "Architecture Options"
config ARCH_NOINTC
bool
default n
config ARCH_VECNOTIRQ
bool
default n
config ARCH_DMA
bool
default n
config ARCH_HAVE_IRQPRIO
bool
default n
config ARCH_L2CACHE
bool
default n
config ARCH_HAVE_COHERENT_DCACHE
bool
default n
config ARCH_HAVE_ADDRENV
bool
default n
config ARCH_NEED_ADDRENV_MAPPING
bool
default n
config ARCH_HAVE_MULTICPU
bool
default n
config ARCH_HAVE_VFORK
bool
default n
config ARCH_HAVE_MMU
bool
default n
config ARCH_HAVE_MPU
bool
default n
config ARCH_NAND_HWECC
bool
default n
config ARCH_HAVE_EXTCLK
bool
default n
config ARCH_HAVE_POWEROFF
bool
default n
config ARCH_HAVE_RESET
bool
default n
config ARCH_USE_MMU
bool "Enable MMU"
default n
depends on ARCH_HAVE_MMU
---help---
The architecture supports supports an MMU. Enable this option in
order to enable use of the MMU. For most architectures, this is
not really an option: It is required to use the MMU. In those
cases, this selection will always be forced.
config ARCH_USE_MPU
bool "Enable MPU"
default n
depends on ARCH_HAVE_MPU
---help---
The architecture supports supports an MPU. Enable this option in
order to enable use of the MPU. For most architectures, this option
is enabled by other, platform-specific logic. In those cases, this
selection will always be forced.
menuconfig ARCH_ADDRENV
bool "Address environments"
default n
depends on ARCH_HAVE_ADDRENV
---help---
Support per-task address environments using the MMU... i.e., support
"processes"
if ARCH_ADDRENV && ARCH_NEED_ADDRENV_MAPPING
config ARCH_TEXT_VBASE
hex "Virtual .text base"
---help---
The virtual address of the beginning the .text region
config ARCH_DATA_VBASE
hex "Virtual .bss/.data base"
---help---
The virtual address of the beginning of the .bss/.data region.
config ARCH_HEAP_VBASE
hex "Virtual heap base"
---help---
The virtual address of the beginning of the heap region.
config ARCH_SHM_VBASE
hex "Shared memory base"
depends on MM_SHM
---help---
The virtual address of the beginning of the shared memory region.
config ARCH_TEXT_NPAGES
int "Max .text pages"
default 1
---help---
The maximum number of pages that can allocated for the .text region.
This, along with knowledge of the page size, determines the size of
the .text virtual address space. Default is 1.
config ARCH_DATA_NPAGES
int "Max .bss/.data pages"
default 1
---help---
The maximum number of pages that can allocated for the .bss/.data
region. This, along with knowledge of the page size, determines the
size of the .bss/.data virtual address space. Default is 1.
config ARCH_HEAP_NPAGES
int "Max heap pages"
default 1
---help---
The maximum number of pages that can allocated for the heap region.
This, along with knowledge of the page size, determines the size of
the heap virtual address space. Default is 1.
if MM_SHM
config ARCH_SHM_MAXREGIONS
int "Max shared memory regions"
default 1
---help---
The maximum number of regions that can allocated for the shared
memory space. This hard-coded value permits static allocation of
the shared memory data structures and serves no other purpose.
Default is 1.
The size of the virtual shared memory address space is then
determined by the product of the maximum number of regions, the
maximum number of pages per region, and the configured size of
each page.
config ARCH_SHM_NPAGES
int "Max shared memory pages"
default 1
---help---
The maximum number of pages that can allocated per region for the shared memory
region. Default is 1.
The size of the virtual shared memory address space is then
determined by the product of the maximum number of regions, the
maximum number of pages per region, and the configured size of
each page.
endif # MM_SHM
config ARCH_STACK_DYNAMIC
bool "Dynamic user stack"
default n
depends on BUILD_KERNEL && EXPERIMENTAL
---help---
Select this option if the user process stack resides in its own
address space. The naming of this selection implies that dynamic
stack allocation is supported. Certainly this option must be set if
dynamic stack allocation is supported by a platform. But the more
general meaning of this configuration environment is simply that the
stack has its own address space.
NOTE: This option not yet fully implemented in the code base.
Hence, it is marked EXPERIMENTAL: Do not enable it unless you plan
finish the implementation.
if ARCH_STACK_DYNAMIC
config ARCH_STACK_VBASE
hex "Virtual stack base"
---help---
The virtual address of the beginning the stack region
config ARCH_STACK_NPAGES
int "Max. stack pages"
default 1
---help---
The maximum number of pages that can allocated for the stack region.
This, along with knowledge of the page size, determines the size of
the stack virtual address space. Default is 1.
endif # ARCH_STACK_DYNAMIC
config ARCH_KERNEL_STACK
bool "Kernel process stack"
default n if !LIBC_EXECFUNCS
default y if LIBC_EXECFUNCS
depends on BUILD_KERNEL
---help---
It this option is selected, then every user process will have two
stacks: A large, potentially dynamically sized user stack and small
kernel stack that is used during system call process.
If this option is not selected, then kernel system calls will simply
use the caller's user stack. So, in most cases, this option is not
required. However, this option is *required* if both BUILD_KERNEL
and LIBC_EXECFUNCS are selected. Why? Because when we instantiate
and initialize the address environment of the new user process, we
will temporarily lose the address environment of the old user
process, including its stack contents. The kernel C logic will
crash immediately with no valid stack in place.
When this option is selected, the smaller kernel stack stays in
place during system call processing event though the original user
stack may or may not be accessible.
if ARCH_KERNEL_STACK
config ARCH_KERNEL_STACKSIZE
int "Kernel stack size"
default 1568
---help---
The common size of each process' kernel stack
endif # ARCH_KERNEL_STACK
config ARCH_PGPOOL_MAPPING
bool "Have page pool mapping"
default n
---help---
If there is a MMU mapping in place for the page pool memory, then
this mapping can be utilized to simplify some page table operations.
Otherwise, a temporary mapping will have to be established each time
it is necessary to modify the contents of a page.
if ARCH_PGPOOL_MAPPING
config ARCH_PGPOOL_PBASE
hex "Page pool physical address"
default 0x0
---help---
The physical address of the start of the page pool memory. This
setting is probably equivalent to other platform specific definitions
but is required again in order to modularize the common address
environment logic.
config ARCH_PGPOOL_VBASE
hex "Page pool virtual address"
default 0x0
---help---
The virtual address of the start of the page pool memory. This
setting is probably equivalent to other platform specific definitions
but is required again in order to modularize the common address
environment logic.
config ARCH_PGPOOL_SIZE
int "Page pool size (byes)"
default 0
---help---
The size of the page pool memory in bytes. This setting is probably
equivalent to other platform specific definitions but is required again
in order to modularize the common address environment logic.
endif # ARCH_PGPOOL_MAPPING
endif # ARCH_ADDRENV && ARCH_NEED_ADDRENV_MAPPING
menuconfig PAGING
bool "On-demand paging"
default n
depends on ARCH_USE_MMU && !ARCH_ROMPGTABLE
---help---
If set =y in your configation file, this setting will enable the on-demand
paging feature as described in http://www.nuttx.org/NuttXDemandPaging.html.
if PAGING
config PAGING_PAGESIZE
int "Page size (bytes)"
default 4096
---help---
The size of one managed page. This must be a value supported by the
processor's memory management unit
config PAGING_NLOCKED
int "Number of locked pages"
default 48
---help---
This is the number of locked pages in the memory map.
config PAGING_CUSTOM_BASE
bool "Custom paging base address"
default n
---help---
By default, the page begins at RAM_START/VSTART. That base address
can be changed if this value is selected.
if PAGING_CUSTOM_BASE
config PAGING_LOCKED_PBASE
hex "Physical base address"
config PAGING_LOCKED_VBASE
hex "Virtual base address"
endif # PAGING_CUSTOM_BASE
config PAGING_NPPAGED
int "Number of physical pages"
default 256
---help---
This is the number of physical pages available to support the paged
text region.
config PAGING_NVPAGED
int "Number of virtual pages"
default 1024
---help---
This actual size of the virtual paged text region (in pages). This
is also the number of virtual pages required to span the entire
paged region. The on-demand paging feature is intended to support
only the case where the virtual paged text area is much larger the
available physical pages. Otherwise, why would you enable on-demand paging?
config PAGING_NDATA
int "Number of data pages"
default 256
---help---
This is the number of data pages in the memory map. The data region
will extend to the end of RAM unless overridden by a setting in the
configuration file.
NOTE: In some architectures, it may be necessary to take some memory
from the end of RAM for page tables or other system usage. The
configuration settings and linker directives must be cognizant of
that: PAGING_NDATA should be defined to prevent the data region from
extending all the way to the end of memory.
config PAGING_DEFPRIO
int "Page fill worker thread priority"
default 100
---help---
The default, minimum priority of the page fill worker thread. The
priority of the page fill work thread will be boosted boosted
dynamically so that it matches the priority of the task on behalf
of which it performs the fill. This defines the minimum priority
that will be used. Default: 100.
config PAGING_STACKSIZE
int "Page fill worker thread stack size"
default 1024
---help---
Defines the size of the allocated stack for the page fill worker
thread. Default: 1024.
config PAGING_BLOCKINGFILL
bool "Blocking fill"
default n
---help---
The architecture specific up_fillpage() function may be blocking
or non-blocking. If defined, this setting indicates that the
up_fillpage() implementation will block until the transfer is
completed. Default: Undefined (non-blocking).
config PAGING_WORKPERIOD
int "Work period (usec)"
default 500000
---help---
The page fill worker thread will wake periodically even if there
is no mapping to do. This selection controls that wake-up period
(in microseconds). This wake-up a failsafe that will handle any
cases where a single is lost (that would really be a bug and
shouldn't happen!) and also supports timeouts for case of non-
blocking, asynchronous fills (see CONFIG_PAGING_TIMEOUT_TICKS).
config PAGING_TIMEOUT
bool "Paging timeout"
default n
---help---
If defined, the implementation will monitor the (asynchronous) page
fill logic. If the fill takes longer than than a timeout value,
then a fatal error will be declared. Default: No timeouts monitored
config PAGING_TIMEOUT_TICKS
int "Paging timeout ticks"
default 10
depends on PAGING_TIMEOUT
---help---
If PAGING_TIMEOUT is defined, then implementation will monitor the
(asynchronous) page fill logic. If the fill takes longer than this
number if microseconds, then a fatal error will be declared.
Default: No timeouts monitored
endif # PAGING
config ARCH_IRQPRIO
bool "Prioritized interrupt support"
default n
depends on ARCH_HAVE_IRQPRIO
---help---
Enable support for prioritized interrupts.
NOTE: The use of interrupt priorities implies that you also have
support for nested interrupts. Most architectures do not support
nesting of interrupts or, if they do, they only supported nested
interrupts with certain configuration options. So this selection
should be used with caution.
config ARCH_STACKDUMP
bool "Dump stack on assertions"
default n
select DEBUG_ALERT
---help---
Enable to do stack dumps after assertions
config ARCH_USBDUMP
bool "Dump USB trace data"
default n
depends on USBDEV_TRACE
---help---
Enable to do USB trace after assertions
config ENDIAN_BIG
bool "Big Endian Architecture"
default n
depends on !ARCH_RISCV
---help---
Select if architecture operates using big-endian byte ordering.
config ARCH_IDLE_CUSTOM
bool "Custom IDLE loop"
default n
---help---
Each architecture provides a "default" IDLE loop that exits when the
MCU has nothing else to do. This default IDLE loop can be replaced
by a custom, board-specific IDLE loop by setting this option. Such
a custom IDLE loop may do things like a continuous built-in test or
perhaps or IDLE low power operations.
NOTE: As of this writing, this capability is only supported by the
STM32. However, the implementation is trivial: If CONFIG_ARCH_IDLE_CUSTOM,
then the default IDLE loop file is not included in the MCU-specific
Make.defs file.
config ARCH_CUSTOM_PMINIT
bool "Custom PM initialization"
default n
depends on PM
---help---
Each architecture provides default power management (PM)
initialization that is called automatically when the system is
started. This default PM initialization can be replaced by custom,
board-specific PM initialization by setting this option. Such a
custom initialization may do additional PM-related initialization
that is unique to the board power management requirements.
NOTE: As of this writing, this capability is only supported by the
STM32. However, the implementation is trivial: If CONFIG_ARCH_CUSTOM_PMINIT,
then the default PM initialization is not included in the MCU-specific
Make.defs file.
config ARCH_HAVE_RAMFUNCS
bool
default n
config ARCH_RAMFUNCS
bool "Copy functions to RAM on startup"
default y
depends on ARCH_HAVE_RAMFUNCS
---help---
Copy some functions to RAM at boot time. This is done in some
architectures to improve performance. In other cases, it is done
so that FLASH can be reconfigured while the MCU executes out of
SRAM.
config ARCH_HAVE_RAMVECTORS
bool
default n
config ARCH_RAMVECTORS
bool "Support RAM interrupt vectors"
default n
depends on ARCH_HAVE_RAMVECTORS
---help---
If ARCH_RAMVECTORS is defined, then the architecture will support
modifiable vectors in a RAM-based vector table.
comment "Board Settings"
config BOARD_LOOPSPERMSEC
int "Delay loops per millisecond"
default 5000
---help---
Simple delay loops are used by some logic, especially during boot-up,
driver initialization. These delay loops must be calibrated for each
board in order to assure accurate timing by the delay loops.
config ARCH_CALIBRATION
bool "Calibrate delay loop"
default n
---help---
Enables some built in instrumentation that causes a 100 second delay
during boot-up. This 100 second delay serves no purpose other than it
allows you to calibrate BOARD_LOOPSPERMSEC. You simply use a stop
watch to measure the actual delay then adjust BOARD_LOOPSPERMSEC until
the actual delay is 100 seconds.
comment "Interrupt options"
config ARCH_HAVE_INTERRUPTSTACK
bool
default n
config ARCH_INTERRUPTSTACK
int "Interrupt Stack Size"
depends on ARCH_HAVE_INTERRUPTSTACK
default 0
---help---
This architecture supports an interrupt stack. If defined, this symbol
will be the size of the interrupt stack in bytes. If not defined (or
defined to be zero), the user task stacks will be used during interrupt
handling.
config ARCH_HAVE_HIPRI_INTERRUPT
bool
default n
config ARCH_HIPRI_INTERRUPT
bool "High priority interrupts"
default n
depends on ARCH_HAVE_HIPRI_INTERRUPT && ARCH_HAVE_IRQPRIO
select ARMV7M_USEBASEPRI
select ARCH_IRQPRIO
---help---
NOTE: This description is currently unique to the Cortex-M family
which is the only family that currently supports this feature. The
general feature is not conceptually unique to the Cortex-M but it
is extended to any other family, then this discussion will have to
be generalized.
If ARMV7M_USEBASEPRI is selected, then interrupts will be disabled
by setting the BASEPRI register to NVIC_SYSH_DISABLE_PRIORITY so
that most interrupts will not have execution priority. SVCall must
have execution priority in all cases.
In the normal cases, interrupts are not nest-able and all interrupts
run at an execution priority between NVIC_SYSH_PRIORITY_MIN and
NVIC_SYSH_PRIORITY_MAX (with NVIC_SYSH_PRIORITY_MAX reserved for
SVCall).
If, in addition, ARCH_HIPRI_INTERRUPT is defined, then special high
priority interrupts are supported. These are not "nested" in the
normal sense of the word. These high priority interrupts can
interrupt normal processing but execute outside of OS (although they
can "get back into the game" via a PendSV interrupt).
How do you specify a high priority interrupt? You need to do two
things:
1) You need to change the address in the vector table so that
the high priority interrupt vectors to your special C
interrupt handler. There are two ways to do this:
a) If you select CONFIG_ARCH_RAMVECTORS, then vectors will
be kept in RAM and the system will support the interface:
int up_ramvec_attach(int irq, up_vector_t vector)
that can be used to attach your C interrupt handler to the
vector at run time.
b) Alternatively, you could keep your vectors in FLASH but in
order to this, you would have to develop your own custom
vector table.
2) Then set the priority of your interrupt to NVIC to
NVIC_SYSH_HIGH_PRIORITY using the standard interface:
int up_prioritize_irq(int irq, int priority)
NOTE: ARCH_INTERRUPTSTACK must be set in kernel mode (BUILD_KERNEL).
In kernel mode without an interrupt stack, the interrupt handler
will set the MSP to the stack pointer of the interrupted thread. If
the interrupted thread was a privileged thread, that will be the MSP
otherwise it will be the PSP. If the PSP is used, then the value of
the MSP will be invalid when the interrupt handler returns because
it will be a pointer to an old position in the unprivileged stack.
Then when the high priority interrupt occurs and uses this stale MSP,
there will most likely be a system failure.
If the interrupt stack is selected, on the other hand, then the
interrupt handler will always set the the MSP to the interrupt
stack. So when the high priority interrupt occurs, it will either
use the MSP of the last privileged thread to run or, in the case of
the nested interrupt, the interrupt stack if no privileged task has
run
config ARCH_INT_DISABLEALL
bool "Disable high priority interrupts"
default y
depends on ARCH_HIPRI_INTERRUPT && EXPERIMENTAL
---help---
If ARCH_HIPRI_INTERRUPT is defined, then special high priority
interrupts are supported. These are not "nested" in the normal
sense of the word. These high priority interrupts can interrupt
normal processing but execute outside of OS (although they can "get
back into the game" via a PendSV interrupt).
In the normal course of things, interrupts must occasionally be
disabled using the up_irq_save() inline function to prevent contention
in use of resources that may be shared between interrupt level and
non-interrupt level logic. Now the question arises, if
ARCH_HIPRI_INTERRUPT, do we disable all interrupts (except SVCall),
or do we only disable the "normal" interrupts. Since the high
priority interrupts cannot interact with the OS, you may want to
permit the high priority interrupts even if interrupts are
disabled. The setting ARCH_INT_DISABLEALL can be used to select
either behavior:
----------------------------+--------------+----------------------------
CONFIG_ARCH_HIPRI_INTERRUPT | NO | YES
----------------------------+--------------+--------------+-------------
CONFIG_ARCH_INT_DISABLEALL | N/A | YES | NO
----------------------------+--------------+--------------+-------------
| | | SVCall
| SVCall | SVCall | HIGH
Disable here and below --------> MAXNORMAL ---> HIGH --------> MAXNORMAL
| | MAXNORMAL |
----------------------------+--------------+--------------+-------------
NOTE: This does not work now because interrupts get disabled in the
standard interrupt handling, prohibiting nesting. Fix is simple: Need
to used more priority levels so that we can make a cleaner distinction
with the standard interrupt handler.
comment "Boot options"
choice
prompt "Boot Mode"
default BOOT_RUNFROMFLASH
config BOOT_RUNFROMEXTSRAM
bool "Run from external SRAM"
---help---
Some configuration support booting and running from external SRAM.
config BOOT_RUNFROMFLASH
bool "Boot and run from flash"
---help---
Most configurations support XIP operation from FLASH but must copy
initialized .data sections to RAM. (This is the default).
config BOOT_RUNFROMISRAM
bool "Boot and run from internal SRAM"
---help---
Some configuration support booting and running from internal SRAM.
config BOOT_RUNFROMSDRAM
bool "Boot and run from external SDRAM"
---help---
Some configuration support booting and running from external SDRAM.
config BOOT_COPYTORAM
bool "Boot from FLASH but copy to ram"
---help---
Some configurations boot in FLASH but copy themselves entirely into
RAM for better performance.
endchoice
menu "Boot Memory Configuration"
config RAM_START
hex "Primary RAM start address (physical)"
default 0x0
help
The physical start address of primary installed RAM. "Primary" RAM
refers to the RAM that you link program code into. If program code
does not execute out of RAM but from FLASH, then you may designate
any block of RAM as "primary."
config RAM_VSTART
hex "Primary RAM start address (virtual)"
default 0x0
depends on ARCH_USE_MMU
help
The virtual start address of installed primary RAM. "Primary" RAM
refers to the RAM that you link program code into. If program code
does not execute out of RAM but from FLASH, then you may designate
any block of RAM as "primary."
config RAM_SIZE
int "Primary RAM size"
default 0
help
The size in bytes of the installed primary RAM. "Primary" RAM
refers to the RAM that you link program code into. If program code
does not execute out of RAM but from FLASH, then you may designate
any block of RAM as "primary."
if BOOT_RUNFROMFLASH && ARCH_USE_MMU
config FLASH_START
hex "Boot FLASH start address (physical)"
default 0x0
help
The physical start address of installed boot FLASH. "Boot" FLASH
refers to the FLASH that you link program code into.
config FLASH_VSTART
hex "Boot FLASH start address (virtual)"
default 0x0
help
The virtual start address of installed boot FLASH. "Boot" FLASH
refers to the FLASH that you link program code into.
config FLASH_SIZE
int "Boot FLASH size"
default 0
help
The size in bytes of the installed boot FLASH. "Boot" FLASH
refers to the FLASH that you link program code into.
endif # BOOT_RUNFROMFLASH && ARCH_USE_MMU
config ARCH_HAVE_SDRAM
bool
default n
config BOOT_SDRAM_DATA
bool "Data in SDRAM"
default n
depends on ARCH_HAVE_SDRAM && !BOOT_RUNFROMSDRAM
---help---
This selection should be set if data lies in SDRAM (vs. SRAM) and if
SDRAM was not previously initialized by a loader. Obviously, this
does not apply if we booting from SDRAM because SDRAM must have been
initialized priority to loading NuttX into SDRAM.
In the case where SDRAM must be initialized by NuttX, the
initialization sequence is a little different: Normally, .data and
.bss must be initialized before starting the system. But in this
case SDRAM must be configured by board-specific logic before the
.data and .bss sections can be initialized.
endmenu # Boot Memory Configuration