Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!husc6!think!ames!sdcsvax!thor!hayes From: hayes@thor.ucsd.edu (James Hayes) Newsgroups: comp.sys.apple Subject: Re: ProDOS patch (write=read/write) Message-ID: <3746@sdcsvax.UCSD.EDU> Date: Wed, 26-Aug-87 21:41:33 EDT Article-I.D.: sdcsvax.3746 Posted: Wed Aug 26 21:41:33 1987 Date-Received: Sat, 29-Aug-87 04:03:10 EDT References: <8708241100.aa03291@SMOKE.BRL.ARPA> Sender: nobody@sdcsvax.UCSD.EDU Reply-To: hayes@thor.UCSD.EDU (James Hayes) Organization: UC San Diego Funny Farm. Lines: 31 mat6013@DMZRZU71.BITNET writes in article <8708241100.aa03291@SMOKE.BRL.ARPA>: >This weekend I took a detailed look at the ProDOS patcher program posted >5/8/87 to find out the changes applied to the "PRODOS" system file (I'm using >version 1.2). Adding the "LDA $C08C,X" makes sense to me, but *why* was "STA >$C08n,X" changed to "LDA $C08n,X" (n= 0, 2, 4, 6) ? These are for turning off >the four windings of the stepper motor which is responsible for the head >seeking. So what's the difference between a read and a write operation here ? >The best documentation I have about the softswitches of the disk ][ controller >is "Beneath Apple DOS"; it tells nothing about that. > > Matthias Kapffer > I don't know about the 6502's offspring, but a memory "write" reads the location before it writes the new value. If you have memory mapped I/O this can raise hell with it. _ If the harware decodes just the address and not the R/W line, it will see two references when the 6502 does a write, and one when it does a read. The external strobe on the game port demonstrates this problem. Write to it and you get two pulses about a uSec apart. Read from it and you get one pulse. Anybody know if this has been fixed? Jim Hayes, University of California at San Diego. BITNET: hayes%sdcsvax@WISCVM.BITNET ARPA: hayes@sdcsvax.ucsd.edu UUCP: {pick one close to berkeley}!sdcsvax!hayes