https://wikiti.brandonw.net/index.php?action=history&feed=atom&title=83Plus%3ABCALLs%3A40D883Plus:BCALLs:40D8 - Revision history2026-04-24T11:24:16ZRevision history for this page on the wikiMediaWiki 1.23.5https://wikiti.brandonw.net/index.php?title=83Plus:BCALLs:40D8&diff=4745&oldid=prevFloppusMaximus: A curious discovery2006-06-01T04:19:48Z<p>A curious discovery</p>
<p><b>New page</b></p><div>[[Category:83Plus:BCALLs:By Name:FP_Math|ATan2Rad]] [[Category:83Plus:BCALLs:By Name|ATan2Rad]] [[Category:83Plus:BCALLs:By Address|40D8 - ATan2Rad]]<br />
== Synopsis ==<br />
'''Official Name:''' ATan2Rad<br />
<br />
'''BCALL Address:''' 40D8<br />
<br />
Compute the arctangent of [[83Plus:RAM:8478|OP1]] divided by [[83Plus:RAM:8483|OP2]], placing the result in the correct quadrant according to the signs of its arguments.<br />
<br />
=== Inputs ===<br />
* [[83Plus:RAM:8478|OP1]] = Y coordinate<br />
* [[83Plus:RAM:8483|OP2]] = X coordinate<br />
* D = 0 (see below)<br />
<br />
=== Outputs ===<br />
* [[83Plus:RAM:8478|OP1]] = angle in radians<br />
<br />
=== Destroys ===<br />
* AF, BC, DE, HL<br />
* OP1-OP5<br />
<br />
== Comments ==<br />
The official documentation makes no mention of the effect of the D register; the initialization of D seems to have been left out accidentally. To obtain the documented behavior, D should be set to zero. In any case, D affects the output as follows:<br />
<br />
* If D is between 60h and DFh, the inverse hyperbolic tangent is computed instead.<br />
* Otherwise, if bit 5 of D is set, the answer will be returned in degrees or radians depending on the mode. If bit 5 is clear, the answer will always be returned in radians.<br />
<br />
Furthermore, this routine also uses the low bits of D to encode the original octant. Specifically:<br />
<br />
* If the input point is below the X-axis, 0Ch will be added to D.<br />
* If it is left of the Y-axis, D will be XORed with 4.<br />
* If it is closer to the Y-axis than to the X-axis, D will be ORed with 10h.<br />
<br />
As a result, if the low bits of D are not initially zero, strange things will happen to the output octant.</div>FloppusMaximus