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- Path: conch.aa.msen.com!not-for-mail
- From: brain@mail.msen.com (Jim Brain)
- Newsgroups: alt.folklore.computers,comp.sys.cbm
- Subject: Re: Vic-20/C-64 serial ports
- Date: 15 Jan 1996 17:14:03 -0500
- Organization: Brain Innovations, Incorporated
- Sender: brain@msen.com
- Message-ID: <jWm+wwyjto7O088yn@mail.msen.com>
- References: <4cn3ba$ifj@celebrian.otago.ac.nz> <4d563t$rcm@whidbey.whidbey.com>
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-
- In article <4d563t$rcm@whidbey.whidbey.com>,
- "Douglas Beattie Jr." <beattidp@svc.ctc.edu> wrote:
- >simon.brady@stonebow.otago.ac.nz (The Arch-Deviant) wrote:
- > (snip!)
- >>...tape I/O was interrupt-driven, and to
- >>finally get to the point of this post I'm struck by the irony of a system that
- >>has interrupt-driven code for tape but software polling for disk I/O.
- >>
- >>My question is, why did Commodore implement the serial protocol in software?
- >>Both the 6522 and 6526 had hardware shift registers and associated interrupt
- >>capability, but until the 128's fast serial mode these were never used. Does
- >>anyone know why not?
- >>
- >
- >Commodore wanted to use the 6551 ACIA in their original VIC-20 design,
- >but MOS Technology could not supply the part on time for the VIC-20's
- >first production. The intention was to add the 6551 to a later VIC
- >design; with minor modifications, the Kernel was to be portable.
- >
- >Therefore they used EMULATION. All of that "bit-banging" code is a
- >6551 emulator which has VIRTUAL REGISTERS at absolute offsets from a
- >base address. I think the C64 was made in the likeness of the VIC-20
- >and used a similar method...
-
- Areyou sure about this. No one has ever confirmed the real reason, but
- I was told the most probable reason was simply MONEY. CBM didn't think the
- VIC would need fast RS232 or DISK, so those two parts were skimped on in the
- end.
-
- JIm
-
- >
- >--
- >Douglas Beattie Jr. http://www.whidbey.net/~beattidp
- >
- >
-
-
- --
- Jim Brain, Embedded Systems Designer, Brain Innovations, Inc. (BII)
- brain@mail.msen.com "Above views DO reflect my employer, since I'm my employer"
- Dabbling in WWW, Embedded Systems, VR, Old CBM computers, and Good Times! -Me-
- <a href=http://www.msen.com/~brain/>BII, VR, CBM, and personal info</a>
-