Re: A83: rom redundancy

To: assembly-83@lists.ticalc.org
Subject: Re: A83: rom redundancy
From: AutismUK@aol.com
Date: Sat, 29 Apr 2000 13:59:10 EDT
Reply-To: assembly-83@lists.ticalc.org


In a message dated 29/04/00 09:10:41 GMT Daylight Time, profzoom@hotmail.com 
writes:

<< I was messing around, comparing the disassembled roms of the 82 and 83, 
and 
 I noticed something interesting.  Most of the 83's commonly used rom calls 
 in the $4000's actually just call another routine earlier on in the rom.  
 For instance, take a look at _getcsc, located at $4014:
 call $268
 ret
 I tested out a little program using $268 instead of $4014, and of course it 
 worked perfectly.  Plus I saved a few clock cycles.  Any idea why TI might 
 have done this? >>

So that when the ROM is upgraded or changed the positions don't change.
At $4014 there will be a table of JMPs which head off to various bits of the
ROM. 

If you didn't have a vector table then the routine would have to be at $268 ;
if you wanted the preceding routine to be two bytes longer you'd then have
mayhem.

For an extreme example, look at the Amiga.

Follow-Ups:

Re: A83: rom redundancy

From: Linus Akesson <lairfight@softhome.net>