DNJZ Address Question

Pete Miles

Okay, here is a simple question for your guys

DNJZ fr, addr

If addr = $ then the command jumps back to itself right?

Now if addr = $-2 where does this jump to? 2 lines prior or two words prior the the command?

Pete

Jon Williams

I'm pretty sure it's two words, Pete.

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Jon Williams
Applications Engineer, Parallax
Dallas Office

BrianW

Simple answer?

Consider the problem in mathematical terms. $ is the program counter value (physical memory address) for the current instruction. therefore the jump is to is this address - 2. In the case of the SX chip thats two words. Thats how assemblers usually work it out. To jump to a specific line of the source code then just put a label on the required line and DJNZ fr,label

Brian

Pete Miles

Thanks,

Some of the "tricks" are just not spelled out in the documentation.

Pete

Guenther Daubach

Pete,

DJNZ fr, addr is a so-called "compund" statement, i.e. the SX does not "understand" it directy. Instead, the Assembler generates two subsequent instructions as replacement:

DECSZ fr
JMP addr

In case you specify $ for addr, the Assembler is clever enough to replace $ with the address of the DECSZ instruction, and not with the address of the JMP instruction which would not make too much sense.

The same is true for an argument like $-2, IOW, the Assembler always replaces $ with the address of the first instruction of a compount statement.

BTW, a good method to test the behavior of certain instructions is writing a simple program, and then single-stepping it with the debugger to see what happens.

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Greetings from Germany,


G

James Newton

Guenther Daubach said...

In case you specify $ for addr, the Assembler is clever enough to replace $ with the address of the DECSZ instruction, and not with the address of the JMP instruction which would not make too much sense.

The same is true for an argument like $-2, IOW, the Assembler always replaces $ with the address of the first instruction of a compount statement.

BTW, a good method to test the behavior of certain instructions is writing a simple program, and then single-stepping it with the debugger to see what happens.

Good point Guenther! I played around with it a bit, and the results are as you said. Except that when using the SXKey internal assembler (rather than SASM) if you define a macro and pass it an argument of e.g. $-2, the $ is relative to the start of the macro and in SASM it is relative to the start of the instruction. I believe this is because SASM doesn't evaluate macro parameters until they are used and the SXKey asm evaluates them before the macro is called. A minor but interesting point.

In general, unless you REALLY know what you are doing, don't use $. Put in a lable instead.

     3  =00000010               org $10
     4  0010  0000              nop
     5  0011  0000              nop
     6  0012  02E9              djnz 9, $-2
        0013  [color=green]0A10[/color]
     7                  
     8  =00000020               org $20
     9  0020  0000              nop
    10  0021  0000              nop
    11  0022  02E9              decsz 9
    12  0023  [color=red]0A21[/color]              jmp $-2
    13                  
    14                  untl macro 2
    15                          decsz \1
    16                          jmp \2
    17                          endm
    18                  
[color=blue]At this point the output differs:[/color]
[color=#0000ff]        SASM evaluates the $-2 in the macro   The SXKey asm evaluates it before[/color]
    19  =00000030               org $30       =00000030               org $30 
    20  0030  0000              nop           0030  0000              nop           
    21  0031  0000              nop           0031  0000              nop
    22                          untl 9, $-2                           untl 9, $-2
    23  0032  02E9   m          decsz 9       0032  02E9   m          decsz 9
    24  0033  [color=red]0A31[/color]   m          jmp $-2       0033  [color=green]0A30[/color]   m          jmp $-2
    26                  
    27                  untl2 macro 2
    28                          djnz \1,\2
    29                          endm
    30                  
    31  =00000040               org $40
    32  0040  0000              nop
    33  0041  0000              nop
    34                          untl2 9, $-2
    35  0042  02E9   m          djnz 9,$-2
        0043  [color=green]0A40[/color]

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