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C64 Notes
From: (Jim Brain)
Newsgroups: comp.sys.cbm
Subject: PC's as 64 HD's and the 6522 chip!
Date: 7 Apr 1994 13:48:35 GMT

Right after I posted about hooking a 6522 VIA up to a PC parallel
port to turn the PC into a "char-banger" instead of a "bit-banger",
someone posted that they had thought the 6522 had a problem with the shift
register. The poster also said that Jim Butterfield had alerted him and
others to it.

Well, I have been in correspondence with "The Commodore Man",
so I asked him to elaborate on the topic. Here is the two responses:

> Yes, it's true. Although I didn't get official confirmation of this
>long after, when a Spectrum article quoted the designers.
> As you know, the first Commodore computers used the IEEE bus to connect to
>peripherals such as disk and printer. I understand that these were available
>only from one source: Belden cables. A couple of years into Commodore's
>computer career, Belden went out of stock on such cables (military contract?
>who knows?). In any case, Commodore were in quite a fix: they made
>computers and disk drives, but couldn't hook 'em together!
> So Tramiel issued the order: "On our next computer, get off that bus.
>Make it a cable anyone can manufacture". And so, starting with the VIC-20
>the serial bus was born. It was intended to be just as fast as the IEEE-488 it replaced.
> Technically, the idea was sound: the 6522 VIA chip has a "shift register" circuit that,
> if tickled with the right signals (data and clock)
>will cheerfully collect 8 bits of data without any help from the CPU.
>At that time, it would signal that it had a byte to be collected, and
>the processor would do so, using an automatic handshake built into the
>6522 to trigger the next incoming byte. Things worked in a similar way
>outgoing from the computer, too.
> We early PET/CBM freaks knew, from playing music, that there was something
>wrong with the 6522's shift register: it interfered with other functions.
>The rule was: turn off the music before you start the tape! (The shift
>register was a popular sound generator). But the Commodore engineers,
>who only made the chip, didn't know this. Until they got into final checkout of the VIC-20.
> By this time, the VIC-20 board was in manufacture. A new chip could
>be designed in a few months (yes, the silicon guys had application notes
>about the problem, long since), but it was TOO LATE! A major software rewrite had to take
> place that changed the VIC-20 into a "bit-catcher" rather than a "character-catcher". It called for
>eight times as much work on the part of the CPU; and unlike the shift
>register plan, there was no timing/handshake slack time. The whole
>thing slowed down by a factor of approximately 5 to 6.
> There's more (the follow-on C64 catastrophe), but that's where it happened.

And the saga continues ...

> When the 64 came out, the problem VIA 6522 chip had been
>replaced by the CIA 6526. This did not have the shift register problem
>which had caused trouble on the VIC-20, and at that time it would have
>been possible to restore plan 1, a fast serial bus. Note that this would
>have called for a redesign of the 1540 disk drive, which also used a VIA.
> As best I can estimate - and an article in the IEEE Spectrum magazine
>supports this - the matter was discussed within Commodore, and it was
>decided that VIC-20 compatibility was more important than disk speed.
>Perhaps the prospect of a 1541 redesign was an important part of the
>decision, since current inventories needed to be taken into account.
> But to keep the Commodore 64 as a "bit-banger", a new problem arose.
>The higher-resolution screen of the 64 (as compared to the VIC-20)
>could not be supported without stopping the CPU every once in a while.
>To be exact: Every 8 screen raster lines (each line of text), the CPU
>had to be put into a WAIT condition for 42 microseconds, so as to allow
>the next line of screen text and color nybbles to be swept into the chip.
>(More time would be needed if sprites were being used).
> But the bits were coming in on the serial bus faster than that:
>a bit would come in about every 20 microseconds! So the poor
> CPU, frozen for longer than that, would miss some serial bits completely!
> Commodore's solution was to slow down the serial bus even more.
>That's why the VIC-20 has a faster serial bus than the 64, even though
>the 64 was capable, technically, of running many times faster.
> Fast disk finally came into its own with the Commodore 128.


Now someone also told me at one time that they had seen a fastloader
that same someone said he thought it was odd that the author of the
loader had credited Commodore with the routines. Well, I can hazard a guess
that the routines were the ones they had wanted to put in the 6522, but had
to scrap due to the 6522 problem. Now I have no idea what the problem
is/was, but I am eager to find out. However, I rescind my plans to build
something around the 6522 until we find out what the problem is.

Jim "Just the Facts" Brain


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