TAG read from register

[gamer]

Hi,

Can someone please explain how the functions GD.wr() and GD.rd() work, specifically relating to reading the TAG at a given position.
Referring to the frogger demo, in lines 218, 219, 
GD.wr(REG_TAG_X, frogx-8);
GD.wr(REG_TAG_Y, frogy);

and then in line 370,
byte tag = GD.rd(REG_TAG);

I see that GD.get_inputs() is being called just before the rd() call. Why is this so?

[bubu]

I tested this code some weeks ago myself and this is what I figured out:

GD.wr writes to Registers and GD.rd reads it.

First, the two GD.wr commands write coordinate values of the frog into corresponding registers for x and y.
Then GD.get_inputs - among other Tasks - proceses these registers and fills register REG_TAG with the value of the tag on that position(if there is any).
Without get_inputs there would be no Change to REG_TAG no matter what values you write to REG_TAG_X / REG_TAG_Y.
Byte tag=GD.rd(REG_TAG) then stores the value of the TAG in a variable which can be adressed by further commands.

So, if there is a car (drawn with a specific tag value) on coordinates frogx-8 and frogy, tag will have it´s tag value.

A simple "one pixel collision Routine".