Hi,

I've recently been writing bare metal assembly language software for the FRDM KL25Z board and using the online MBED ARM assembler. I've managed to get a number of different examples working ok, so have passed the initial hurdle of getting things running.

I'm using assembly language (rather than C++) as I'm developing some teaching material for an undergrad architecture course.

One problem I've come across is with using the value of labels in assembler directives and as operands to instructions. The value of the label appearing in the binary appears to be the actual value plus 0x8000.

The following code is a minimal example of this:

    AREA asm_func, CODE, READONLY
    THUMB  
    DCD    0x20000000   // Top of Stack 
    DCD    main               // Reset Handler
    
main
    LDR     R0,=main
    B       main

This assembles to the following series of bytes:

00 00 00 20 09 80 00 00 00 48 FD E7 09 80 00 00

which if disassembled, gives the following:

0000    20000000
0004    00008009
0008    4800             LDR     R0,=0x000C
000A    E7FD            B          0008
000C    00008009

The reset vector at address 0004 should be 0x9 (8 + 1 for thumb) - whereas we have 0x8009.

Again we see the same problem with the LDR instruction. The assembler is dropping the value of the operand at the end of code, as expected, at address 000C and accessing this as an offset on PC. Again however the value in the binary is 0x8009 rather than 9.

I can get around these problems by plugging in constants, but it would be nice to refer to labels in many cases. [The particular example where I would like to reference the value of a label is so that I can pass the address of an interrupt handler to a piece of code which updates a copy of the interrupt vector table which has been relocated to RAM.]

Has anyone come across this, or know what the problem is?

cheers Gerald

Hi Gerald,

This is because you are missing a scatter file to specify memory layout. The ARM compiler is likely falling back to a default where ROM starts at 0x8000. If you add your own scatter file starting at address zero then you will get the expected assembly output. Something like this should work:

LR_IROM1 0x00000000 0x20000  {    ; load region size_region (32k)
  ER_IROM1 0x00000000 0x20000  {  ; load address = execution address
   .ANY (+RO)
  }
  RW_IRAM1 0x1FFFF000 0x4000 {
   .ANY (+RW +ZI)
  }
}

Note - The online compiler might give you an error if you try and create a .sct file online. To work around this create the .sct file locally and then drag-n-drop it into your online project.

Finally, make sure you have the correct configuration bits set at address 0x400 on the KL25Z. If they are wrong or the default of 0xFF they will either lock your device or DAPLink will refuse to program the device and give you an error.