ARM Technical Support Knowledge Articles

Can the MMU set a piece of memory space (SDRAM) to read only?

Applies to: ARM720T

Answer

Yes. You can do this in one of 2 ways:

  • Set the Access Permission bits associated with the section or pages describing the virtual to physical address mapping of the SDRAM, to '10'. This marks the memory as 'read only' in user mode, but allows privileged modes to read and write the area.
  • Set the Access Permission bits to '00' and have the 'S' bit set to 0 and the 'R' bit set to 1 within CP15 Register 1. (Control register bits 8 & 9). This tells the MMU that these pages are ROM and can only be read and never written. This method is generally deprecated and is provided for backward compatibility.
  • For both of these cases, the domain access control must be set to 'client'.

Article last edited on: 2008-09-09 15:47:37

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