Path: utzoo!utgpu!attcan!uunet!lll-winken!lll-tis!ames!pasteur!ucbvax!electro.UUCP!ignac
From: ignac@electro.UUCP (Ignac Kolenko)
Newsgroups: comp.sys.m68k.pc
Subject: LDS/UDS lines on 68k
Summary: what exactly do they do??
Message-ID: <227@electro.UUCP>
Date: 15 Dec 88 17:46:33 GMT
Sender: mwm@ucbvax.BERKELEY.EDU
Reply-To: Ignac Kolenko <electro!ignac>
Organization: Electrohome Ltd., Kitchener, ON
Lines: 27
Approved: info-68k@ucbvax.berkeley.edu



hello. i have a question regarding the exact function of the upper data/lower
data strobes on the m68k. can the LDS be used in place of the missing A0 line?
in other words, if i do a byte access to an even addressed byte, the LDS signal
will be low, right. and if i do a byte access to an odd byte, the LDS signal
will be high, right?? can someone please verify my theory.

also, if the above is correct, can i do the following:

MOVEA.L $FB0000,A0
TST.B (A0,D0.W)

where D0 contains either an even or an odd integer??
in case anyone is wonderin' why i'm asking, i want to try to build an interface
to my atari 1040st cartridge port, which doesn't have write access capability.
i therefore have to use the bottom 8 lines of the address bus as a "data bus"
when i want to write to the cartridge port address space, and using one of the
higher address lines a pseudo read/write line. thus i need to somehow pass the
least significant bit on the address bus, which led me to believe the LDS would
do the trick.

thanx ahead of time for any info.

-- 
Ignac A. Kolenko                   "Squeeze my lemon, let the juice run
watmath!watcgl!electro!ignac        down my leg!" - Led Zep (Lemon Song)
"Ask me no further questions, I WILL NOT answer!!" - Spock (ST:TOS Amok Time)