BlockoS

EEK! 6 months !

It’s not that I had nothing to write. On the contrary, I have a lot to post. I just didn’t found the time to write… Erm.. Let’s go with a 7 months old stuff.

While working on the tracker, I wanted to add some kind of “live” mode. The user could choose which sequence to play, modify it, control volume, add an effect etc… So I came with the idea to associate a command to a key combo.

Combos are detected by walking down a tree. Each time a button is pressed, we look at the current node and check if one of its children is associated to this button. If that’s the case, this child becomes the new current node. Otherwise there’s no matching combo and we reset the current node to the tree root. If the button pressed didn’t not changed for a certain amount of time (timeout), we check there’s a child corresponding to the current button. This way, we can detect combo containing a sequence like “press A button during t seconds”.

Let’s look at a simple example.

Mkit will successively return : DOWN – DOWN+RIGHT – RIGHT – 0 (aka nothing) – A

We have 0 because when you go from RIGHT to A, there’s a moment when you don’t press any button. And as we check keys every vsync, we necessary get 0 (unless if you are Superman on Redbull).

Consider we want to handle the following combos :

The combo tree is very similar to a lexical tree. A node can have 0 to N children but only one parent.

The tree is store in level-order with the longest path first. This means that you’ll have to be very careful and store node children by their depth. We have for our example :

Root – Down – A – Down+Right – Down+Left – Nothing – Left – Right – A – Nothing – Nothing – Leaf – A – B – Leaf – Leaf

In the current implement, the tree is stored in 2 arrays. The first one contains the index to first child of a node. And the second the key for each node.

combo_tree : 1, 3, 5, 6, 7, 8, 9, 10, 128 | 2, 11, 12, 128 | 0, 128 | 1
combo_keys: .db 255, JOY_DOWN, JOY_I, JOY_DOWN | JOY_RIGHT, JOY_DOWN | JOY_LEFT
.db 0, JOY_RIGHT, JOY_LEFT, JOY_I, 0, 0, JOY_I, JOY_II

The root is always at index 0 and we associate the key value 255 to it.
The tree traversal algorithm is very simple. For a given node i, we know that its children are at indices j from combo_tree[i] to combo_tree[i+1]-1. And that we can reach them only if the current key is combo_key[j]. If the bit 7 of a value in the children list is set, this means that we have a transition a leaf. The remaining bits are used as an index to a jump table.
The traversal looks like this:

i // current node index
for(j=combo_tree[i]; jCOMBO_DEFAULT_DELAY in combo.asm). Note that in current code combo_tree, combo_keys and combo_jumpTable are in RAM. So you’ll have to initialize them before-hand.

• [rom]

If you have any question, feel free to leave a comment or go and join the [PCEdev Forum].

Talking about web stuff, I finally updated the gallery. Hurray!

In my quest for demo effects I’m studying zoom. 2 days ago while looking at the code for right shifting signed bytes, I realized that this technique can be used to “double” a byte.

Here’s a little schema showing byte “doubling”:

How can signed byte bit right shifting can help us?
Signed numbers are represented in two’s complement. I won’t explain it here. But when you are shifting a negative value to the right the most significant bit is replaced by 0. However this bit is always 1 for negative values. A way to solve this issue is to use the right bit rotation instruction (ROR) instead of the right bit shift instruction (LSR). ROR shifts the bits on position to the right. The carry flag is shifted to bit 7 and the bit 0 is shifted to the carry flag. We must copy the value of the 7th bit to the carry flag. We’ll use the CMP for this. The carry flag is set if the value in the accumulator is equal or greater to the compared value. So comparing the accumulator to #$80 (128) will do the trick.

We’ll use this technique to “double” the bits. First we shit the source byte to the right. The least significant bit will be in the carry flag. Then we rotate the accumulator to the right. The carry flag is then shifted to the 7th bit of the accumulator. We compare the accumulator to #$08. The carry flag is now equal to the 7th bit of A. Rotate the accumulator to the right one more time. Et voila! 🙂

This macro reads a bit from the source byte and adds it twice to the destination.

doubleBit .macro
    lsr <__src
    ror A
    cmp #$80
    ror A
          .endm

And finally here’s the code to “double” a 8 pixels long line. You’ll have to repeat it for each lines. If you want to double a tile on pc-engine you’ll have to call this of code for each “line” (32 times).

    cla

    doubleBit    ; bit 0
    doubleBit    ; bit 1
    doubleBit    ; bit 2
    doubleBit    ; bit 3
    sta <__dest

    doubleBit    ; bit 4
    doubleBit    ; bit 5
    doubleBit    ; bit 6
    doubleBit    ; bit 7
    sta <__dest+1

We now have the basis to make a complete zoom-in routine! 🙂