Re: A86: djnz, jr and jp

To: assembly-86@lists.ticalc.org
Subject: Re: A86: djnz, jr and jp
From: Jimmy =?iso-8859-1?Q?M=E5rdell?= <yarin@acc.umu.se>
Date: Sat, 09 Jan 1999 03:44:34 +0100
Delivered-To: assembly-86-outgoing@towerguard.unix.edu.sollentuna.se
In-Reply-To: <3.0.6.32.19990108183028.007b33b0@dowco.com>
References: <4227e43b.3696b595@aol.com>
Reply-To: assembly-86@lists.ticalc.org

At 18:30 1999-01-08 -0800, you wrote:
>despite all this choas and crap...I'm quite interested now to know if
>ANYONE knows how to use jp/jr/djnz when programming in hex.

jr and djnz uses relative addresses:

$D748   ld b,3          2 bytes
$D74A   <do something>
.
.
.
$D75A   djnz -$10       2 bytes  (loop to $D74A)

The second byte of the last instruction should be -$12 though,
because the relative address is added to PC after the instruction
has been read. Thus, PC is $D75C when djnz has been read,
and -$12 ($EE) is use.d

jr works the same way...

jp uses absolute addressing which mean you have to know the exact
location of where to jump:

$D748   <lots of code>
$D7A0   jp $D758        ; stored as $C3 $58 $D7

When programming in hex, you shouldn't insert code... rather, you will up
a lot of memory with $00 (nop) and overwrite code all the time, so you
don't have to change the jump addresses all the time.

--
Real name:     Jimmy Mårdell         
Email:         mailto:yarin@acc.umu.se
Homepage:      http://www.acc.umu.se/~yarin/

Icarus Productions homepage: http://icarus.ganymed.org/

References:

Re: A86: djnz, jr and jp

From: ComAsYuAre@aol.com

Re: A86: djnz, jr and jp

From: Dave VanEe <dvanee@dowco.com>