memory questions

Armored Cars

If I set org $0A and then set a list of variables with "variable· ds·· 1" once the ds 1 reaches $10 will that variable be in bank 0 or bank 1?

Armored Cars

Anybody?

Paul Baker

How about compiling an example then looking at the generated asm file and list file? The answer may be dependent on whether you are using a SX28 and SX52 since the addressing mode in SX52 is handled differently and bank 0 is a special case.

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·1+1=10

Armored Cars

I'll try that.· I am using an SX 52 though.

Guenther Daubach

Letting the variables "overflow" into the next bank can cause you a lot of sleepless nights. Here is an example:

      org    $8
aaa          ds 1
bbb          ds 1
ccc          ds 1
ddd          ds 1
eee          ds 1
fff          ds 1
ggg          ds 1
hhh          ds 1
iii          ds 1

      org    $10
jjj          ds 1

When you look at the list file, you will notice that the assembler has defined address $10 for variable iii due to "overflowing" the bank border but it also has defined address $10 for variable jjj as it is defined immediately following the org $10 directive.

SASM does not generate an error because in general, it is absolutely legal referring to one value by more than one symbol. The disaster will come up when you indeed assume that iii and jjj are two different variables occupying different RAM locations. As this is not the case, modifying the contents of variable iii also changes the contents of variable jjj, and vice-versa, which is not what you might expect. Therefore, it's a good idea to check the list file for such possible pitfalls.

BTW, the same is true for code-generation across page boundaries.

    org        $2f3
ClearVars
    clr   !wdt
    sb      fsr.4               
      setb  fsr.3               
    clr     ind                 
    incsz   fsr                 
      jmp   ClearVars
      
    bank    PWM
    mov     PWM_Prescaler, #PWMPeriod    

    mov     !option, #INT_ON
:MainLoop
    bank    Analog
    mov     w, Analog_ADCresult
    mov     rb, w
    bank    PWM
    mov     PWM_DutyCycle, w
    jmp     :MainLoop

When you assemble this code, and look at the list file, you will notice that code generation "overflows" into the next page, i.e. the instruction code for "mov w, Analog_ADCResult" will be located at $02ff, and the instruction code for "mov rb, w" is at $300, with the next instruction codes following at subsequent locations in page $0300. To make the disaster complete, see what happens when the final "jmp :MainLoop" is executed. As the SX-internal program counter is simply incremented as long as no skips, jumps, or calls are executed, there is no problem, crossing the page when this code is executed the very first time. But the final jmp instruction targets to a location which is not in the page currently addressed by the program counter. So, this jump will not go back to address $2fe, the first instruction following the label ":MainLoop" but the jump will go to $3FE instead, i.e. right into "nowhere land".

Here is a "trick" to let the assembler generate an error on such situations: Place nop instructions at the very first location of each page, as long as you don't need that page for "real" code. When you follow the lines of this sample code with

org $300
nop

SASM will generate an "Overwriting same program counter location" error in this case.

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Greetings from Germany,

G