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Commodore Trivia 08
Here are the answers to Commodore Trivia Edition #8 for July, 1994
Q $070) On a PET series computer, what visual power-on indication will tell
the user whether the computer has Revision 2 or Revision 3 ROMs?
A $070) Revision Level 2 ROMS (the ones with more bugs) power up with:
*** COMMODORE BASIC ***, with '*' in place of the more familiar
'#' character.
Q $071) The IEEE-488 interface is sometimes called the GPIB interface.
What does GPIB stand for?
A $071) General Purpose Interface Bus. Another name is Hewlett Packard
Interface Bus (HPIB), since HP developed this standard for its
istrumentation device networking.
Q $072) Commodore manufactured at least two hard drives with IEEE-488
interfaces. Can you name them?
A $072) The Commodore D9060 and D9090. From the cbmmodel.txt file:
* CBM D9060 5 MB Hard Drive, DOS3.0, Off-White, IEEE-488. GP
* CBM D9090 7.5 MB Hard Drive, DOS3.0, Off-White, IEEE-488. GP
The following model has been said to be in existence, though no one
has one on hand to prove it:
* CBM D9065 7.5 MB Hard Drive
And this model may never have made it past the prototype stage:
CBM D9062 Dual D9065.
Q $073) Why didn't buyers like the original PET-64?
A $073) It looked just like a old-style C-64. It had a "home" computer
look that the schools didn't care for. They liked the "business"
look of the PET series, so Commodore put refurbished and new 64
motherboards in PET cases and sold them as PET 64s. The repackaging
suited the schools.
Q $074) On a PET Revision 2 ROM, what was the largest single array size that
BASIC could handle?
A $074) An array can have a cumulative total of 256 elements. For single
dimension arrays, that means D(0) to D(255), but a 2D array can only
go from DD(0,0) to DD(1,127) etc. All types of arrays had this
limitation.
Q $075) On the stock 1541, data is transmitted one bit at a time. How many
bits are transferred at a time on the Commodore 1551 disk drive?
A $075) 3 bits were transmitted at a time. I assume that each byte had a
parity bit tacked on for error detection, so it would have taken
3 transfers to transmit a byte of information from the drives.
Q $076) On all Commodore floppy disk drives, how fast does the disk spin?
A $076) 300 RPM.
Q $077) Upon first reading the Commodore 1541 Error channel after turning
on the disk drive, what error number and text is returned?
A $077) 73, CBM DOS V2.6 1541, 0, 0
Q $078) What error number and text is returned on a 1551?
A $078) 73, CBM DOS V2.6TDISK, 0, 0 Notice that the new text JUST fits!
Q $079) Commodore printers are normally assigned to device #4, but they
can be also used as device #?
A $079) #5. The Commodore 1525 has a switch to do this, but not all printers
have such a switch.
Q $07A) What microprocessor is used in the Commodore 1551 disk drive?
A $07A) the 6510T. It is a slight variant on the 6510 microprocessor used
on the C64. Some say it runs at 2 MHz, but the specs drives spec
sheet doesn't say.
Q $07B) When the VIC-20 was designed, the serial port throughput was roughly
equivalent to the throughput of the IEEE-488 bus? Why isn't it
very fast in production VICs?
A $07B) Let's go back to question $04F:
<begin insert>
Q $04F) What was the primary reason Commodore went to a serial bus
with the introduction of the VIC-20?
A $04F) Jim Butterfield supplied me with this one:
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.
<end insert>
And here is what Jim Butterfield followed up with:
"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.
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."
--Jim
Q $07C) On Commodore computers, how much RAM is set aside as a tape buffer?
A $07C) 192 bytes is used as a tape buffer. Blocks of data on tape are 192
bytes long.
Q $07D) On Commodore computers, most every peripheral has a device number.
What is the device number of the screen?
A $07D) #3
Q $07E) What is the device number of the keyboard?
A $07E) #0
Q $07F) Commodore computers use 2's-complement notation to represent integers.
What is the 2's-complement hex representation of the signle byte -1?
A $07F) (This was not a Commodore specific question) Commodore computers
use this notation to represent integer quantities. In 2's complement
notation, a -1 looks like 11111111(binary) or $FF(hex).
End of Commodore Trivia Edition #8!
Jim Brain
brain@mail.msen.com
2306 B Hartland Road
Hartland, MI 48353
(810) 737-7300 x8528
--
Jim Brain, Embedded Systems Designer, Brain Innovations.
brain@msen.com
Dabbling in VR, Old Commodore Computers, and Good Times!
"The above views DO reflect my employer, since I am my employer" - Jim Brain
