• State Not Answered
  • Locked Locked
  • Replies 7 replies
  • Subscribers 22 subscribers
  • Views 2027 views
  • Users 0 members are here
Options
Related
How was your experience today?
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

Remapping interrupts to RAM

Cactus Blip

Hello,
I am mapping interrupts to RAM (MEMMAP = 2). I want to overwrite some of the contents of 0x40000000 with my own address (namely, 0x40000008, that is the SWI handler). I know what I want: I need to install a branch instruction like

LDR PC, SWI_Handler


But when I look at address 0x0 (just for an example of how it is done in flash), I see the following repeating pattern:

0x0 18F09FE5
0x4 18F09FE5
0x8 18F09FE5
0xC 18F09FE5

I would expect to find there different instructions!
What gives?

Parents
  • 0 in reply to John Linq

    > But I "guess", maybe it is due to that, you do not
    > implement the related xxxx_Handler. So KEIL just
    > simply optimizes them.

    I was stupid. Though I haven't become smarter, I get better understanding about the exception handlers.

    The machine code 18F09FE5 (E59FF018) means
    LDR PC, [PC, #0x0018] This is a "PC - relative addressing".

    Each triggered exception handler has a different PC value, so the processor will fetch a different instruction from a different address.

    For example:

    Vectors         LDR     PC, Reset_Addr
                LDR     PC, Undef_Addr
                LDR     PC, SWI_Addr
                LDR     PC, PAbt_Addr
                LDR     PC, DAbt_Addr
                NOP                            ; Reserved Vector
                LDR     PC, [PC, #-0x0120]     ; Vector from VicVectAddr
                LDR     PC, FIQ_Addr


    Reset_Addr at 0x00000000 is an instruction -> LDR PC, [PC, #0x0018]
    After it is executed, it will load the content of address (0x0000 + 0x4 + 0x4 + 0x0018) 0x0020 into PC. In my case, the content of address 0x0020 is 0x0054, So the value of Program Counter will become 0x0054.

Reply
  • 0 in reply to John Linq

    > But I "guess", maybe it is due to that, you do not
    > implement the related xxxx_Handler. So KEIL just
    > simply optimizes them.

    I was stupid. Though I haven't become smarter, I get better understanding about the exception handlers.

    The machine code 18F09FE5 (E59FF018) means
    LDR PC, [PC, #0x0018] This is a "PC - relative addressing".

    Each triggered exception handler has a different PC value, so the processor will fetch a different instruction from a different address.

    For example:

    Vectors         LDR     PC, Reset_Addr
                LDR     PC, Undef_Addr
                LDR     PC, SWI_Addr
                LDR     PC, PAbt_Addr
                LDR     PC, DAbt_Addr
                NOP                            ; Reserved Vector
                LDR     PC, [PC, #-0x0120]     ; Vector from VicVectAddr
                LDR     PC, FIQ_Addr


    Reset_Addr at 0x00000000 is an instruction -> LDR PC, [PC, #0x0018]
    After it is executed, it will load the content of address (0x0000 + 0x4 + 0x4 + 0x0018) 0x0020 into PC. In my case, the content of address 0x0020 is 0x0054, So the value of Program Counter will become 0x0054.

Children