Data defined with DS statement

Question

I'm doing a simple program that reads UART buffer (S0BUF) on an incoming transmission and then transmits it back via the UART when I press a button. I'm using the Nordic Semiconductor nRF24LE1 chip.

I get it to work with C and I get it to work when I use one of the R0-R7 registers. What I want to do is to use a relocatable byte instead of the normal R-registers.

Here is a snippet of code that works.

UART_ISR: ; (Store incoming transmission in R0)
        JNB    RI0,    UART_ISR_A
        MOV    R0,     S0BUF
        SETB   P00
        CLR    RI0
        JMP    UART_ISR_END
UART_ISR_A:
        SETB   P01
        CLR    TI0
UART_ISR_END:           
        RETI


Main:
        MOV    A,       P2
        XRL    A,       #0x0FE
        JZ     Main_Case_A
        XRL    A,       #(0x0FE XOR 0x0FD)
        JZ     Main_Case_B
        ...

Main_Case_A:
        MOV    S0BUF,   R0
        SJMP   Main

But if I do the following changes (marked with <--- THIS) it wont work. I know I am missing out some segments and stuff in this snippet, but I don't want to many lines.

?DT?Data    segment     data                 ; <--- THIS

rseg     ?DT?Data                            ; <--- THIS
         RX_Message:    ds    1              ; <--- THIS

UART_ISR: ; (Store incoming transmission in R0)
            JNB    RI0,    UART_ISR_A
            MOV    RX_Message,     S0BUF        ; <-- THIS
            SETB   P00
            CLR    RI0
            JMP    UART_ISR_END
UART_ISR_A:
            SETB   P01
            CLR    TI0
UART_ISR_END:           
            RETI


Main:
        MOV    A,       P2
        XRL    A,       #0x0FE
        JZ     Main_Case_A
        XRL    A,       #(0x0FE XOR 0x0FD)
        JZ     Main_Case_B
        ...

Main_Case_A:
        MOV    S0BUF,   RX_Message     ;      <--- AND THIS
        SJMP   Main

It assembles and everything without warnings or errors but the result is not the same.

What I want is to let the assembler/linker to decide where in memory to put my RX_Message byte. If I look in the list file I can see that it assigned it to address 0x00 which is fine by me since I'm not using R0 anywhere.. But I assumed that if I used R0 somewhere it would set my RX_Message address to 0x01 (or something else).. but that didn't happen.

What am I missing out? Or am I doing it the wrong way?

Answer 1

Solution.. The stack is of course involved since I'm using interrupts (didn't mention that because I didn't think that was important.. Adding some code to reposition the stack solved the issue. Although I hoped that the linker wouldn't have put my RX_Message label on the stacks default address..