Which data bus is used after physical remap to RAM in STM32F4?

Question

STM32F4 controllers (with ARM Cortex M4 CPU) allow a so called physical remap of the lowest addresses in the memory space (0x00000000 to 0x03FFFFFF) using the SYSCFG_MEMRMP register. What I do understand is that the register selects which memory (FLASH/RAM/etc.) is aliased to the lowest addresses and therefore from which memory the reset vector and stack pointer is fetched after reset.

The documentation [1] also mentions that

In remap mode, the CPU can access the external memory via ICode bus instead of System bus which boosts up the performance.

This means that after a remap e.g. to RAM an instruction fetched from within the alias address space (0x00000000 to 0x03FFFFFF) the ICode bus will be used.

Now my question: After such a remap operation e.g. to RAM, will a fetch to the non-aliased location of the RAM use the system bus or the ICode bus?

The background of the question is that I want to write a linker script for an image executing from RAM only (under control of a debugger). To which memory area should the .text section go? The alias space or the physical space?

[1] ST DocID018909 Rev 7

Thanks to Sean I could find the answer in the ARM® Cortex®‑M4 Processor Technical Reference Manual section 2.3.1 Bus interfaces:

ICode memory interface: Instruction fetches from Code memory space, 0x00000000 to 0x1FFFFFFC, are performed over the [sic!: this] 32-bit AHB-Lite bus.

DCode memory interface: Data and debug accesses to Code memory space, 0x00000000 to 0x1FFFFFFF, are performed over the [sic!: this] 32-bit AHB-Lite bus.

System interface: Instruction fetches and data and debug accesses to address ranges 0x20000000 to 0xDFFFFFFF and 0xE0100000 to 0xFFFFFFFF are performed over the [sic!: this] 32-bit AHB-Lite bus.

This also makes clear, that the flash memory of STM32F4 MCUs located at 0x08000000 is always accessed (by the CPU core) using the ICode/DCode busses, regardless if it is remapped. This is because both, the original location and the remapped location are within the code memory space (0x00000000 to 0x1FFFFFFF).

However, if the code is located in SRAM at 0x20000000 then access to the remapped location at 0x00000000 uses the ICode/DCode busses while access to the original location (outside the code memory space) uses the system bus.

Sean Houlihane Sean Houlihane · Accepted Answer · 2017-11-23T11:47:58

The choice of bus interface on the core depends on the addresses accessed. If you access an instruction at 0x00000004, this is done on the ICode bus. An access to 0x20000004 is done using the System bus.

What the REMAP function does is change the physical memory system so that an access to 0x00000004 (ICode bus) will use the same RAM as you can also access on the system bus. Any access to 0x20000004 will be unaffected, and still be generated on the System bus by the core.

Which data bus is used after physical remap to RAM in STM32F4?

1 Answers