Noob 6809 question - offsets

[minsoft]

Hi all,

I am new to vectrex programming, 6809 and assembler in general really. I am working on a game and am doing ok so far, but there are a few things I'm not sure how to do which seems like there may be a simple solution to.

I have some data at #enemy_pattern_0. I need to load in the data from that address+an offset (offset is in 'enemy_step'). I have achieved this with the following code:

    

    STB enemy_step

    LDX #enemy_pattern_0
step_loop:
    LDA ,X+
    ...   

    CMPB #0
    BEQ enemy_save
    DECB  

    JMP step_loop

enemy_save:

    ...

This works - I end up with what I want in A, but is very inefficient as I'm looping through the no. of times in 'enemy_step' to get the offset I need by doing 'LDA ,X+'. I would rather not have a loop at all.

I thought I might be able to do this:

LDX enemy_step, #enemy_pattern

...but this doesn't work. I assume this is because a pointer cannot be used as the offset parameter.

First instinct was to use INCX / ADDX but of course they don't exist. I am not sure the best way to achieve what I'm trying to do. Any tips?

[kokovec]

Maybe use something like this?

LDX #enemy_pattern_0
LDA enemy_step
LDB A,X

[minsoft]

Sept 5, 2020 19:57:42 GMT -5 kokovec said:

Maybe use something like this?

LDX #enemy_pattern_0
LDA enemy_step
LDB A,X

It's that easy isn't it...not sure why I didn't do that. Thanks very much!