ARMCC: LOCATE CONSTANTS TO FIXED LOCATIONS

Information in this article applies to:

RealView C Compiler Version 3

QUESTION

I am using a Philips LPC2000 device. To read protect the chip, I want to locate a constant at a fixed memory location (0x1FC) in the Flash ROM.

What is the best way to do this?

ANSWER

To locate a constant at a fixed address you must create a small module that contains the constant, and modify the scatter file to reference that module. Here are the steps:

Write the following C module called ABS_ADDRESS.C to define the constant:
```
#pragma push
#pragma Ono_remove_unused_constdata
unsigned int const Security_Value = 0x87654321;
#pragma pop
```
Make sure that you reduce optimization with:
```
#pragma Ono_remove_unused_constdata
```
Without this directive, the linker could remove the constant from the final object.
Locate the project's scatter file. µVision creates a scatter file (with the extension *.SCT) when the Use Memory Layout from Target Dialog option (Project — Options for Target — Linker) is checked. The scatter file looks like this:
```
LR_IROM1 0x00000000 0x00080000  {  ; load region
  ER_IROM1 0x00000000  {       ; load address = execution address
   *.o (RESET, +First)
   * (+RO)
  }
  RW_IRAM1 0x40000000 0x00008000  {  ; RW data
   * (+RW +ZI)
  }
}
```

Disable the Use Memory Layout from Target Dialog option, then specify the scatter file under Project — Options — Linker — Scatter File and change the content of this file to:

LR_IROM1 0x00000000 0x00080000 { ; load region
  ER_IROM1 0x00000000 { ; load address = execution address
    *.o (RESET, +First)
    .ANY (*)
  }

  ABS_ADDRESS 0x1FC FIXED 4 {
    abs_address.o (*)
  }

  ER_IROM2 +0 0x80000 {
    * (+RO)
    .ANY(*)
  }

  RW_IRAM1 0x40000000 0x00008000 { ; RW data
    * (+RW +ZI)
  }
}

This will locate the constant in the file ABS_ADDRESS.C to memory location 0x1FC.

The linker will still remove unused sections. To avoid that you need to enter the directive --keep abs_address.o(.constdata) under Project — Options — Linker — Misc controls.

MORE INFORMATION

RealView Linker and Utilities User's Guide

FORUM THREADS

The following Discussion Forum threads may provide information related to this topic.

Last Reviewed: Wednesday, November 15, 2006