http://www.sizecoding.org/index.php?action=history&feed=atom&title=Talk%3AGetting_StartedTalk:Getting Started - Revision history2026-04-22T23:22:18ZRevision history for this page on the wikiMediaWiki 1.27.0http://www.sizecoding.org/index.php?title=Talk:Getting_Started&diff=849&oldid=prevMaroonbells: Initial AX/BX register values2021-06-04T19:09:29Z<p>Initial AX/BX register values</p>
<p><b>New page</b></p><div>Initial register values AX=0000 and BX=0000 is not 100% correct. Through several DOS versions, I've observed them to be:<br />
<br />
AH=see explanation<br />
AL=see explanation<br />
BX=AX<br />
<br />
The value for AL is linked to the 1st command line parameter, and the value for AH is linked to the 2nd parameter.<br />
The value for AL is 00 unless the 1st command line parameter begins with a path pointing to an unmapped drive letter.<br />
The value for AH is created the same way, except for linking to the 2nd command line parameter instead of the 1st.<br />
examples:<br />
<br />
program.com foo bar<br />
or<br />
program.com<br />
<br />
AX=0000 because of the 1st or 2nd command line parameters neither are/begin-with drive letters<br />
<br />
program.com z:<br />
program.com foo z:<br />
Assuming the Z drive is not mapped, the 1st example sets AX=00FF, while the 2nd example sets AX=FF00.<br />
<br />
In all examples, the BX register is defined to match whatever value was placed into AX.</div>Maroonbells