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Commodore Trivia 15
(It is not late because I haven't worked on it. It is late because I cannot
find my information describing question #$0E4 and its answer. So, please, if
you gave me the info, send it again. I know it has an answer, but I can't
find my notes on it.)
Argh!
Not to leave you in the dark for too long, here are the trivia
answers to the previous edition of Commodore Trivia. I am posting the
answers at this time, and will post the scores and winners in a few
days. This time frame is set up to allow time for any discussions on the
correctness of these answers. By this time, the newest edition of
trivia has been posted. I encouarage you to enter it.
This edition of trivia answers has been posted to the USENET newsgroups:
comp.sys.cbm, alt.folklore.computers, and comp.sys.amiga.advocacy. It
ha also been posted to the FIDO Echo CBM. This file is copyright by
Jim Brain. It may be reposted on other networks, provided the file is not
changed (other than typos). This file is public domain. It may be used in
newsletters, magazines, etc., but I always appreciate knowing where it
goes. If you are planning on reposting or otherwise retransmitting this
file, it is best to wait a few days after the date of this post for any
errors to be shaken out.
Please mail any new questions for upcoming trivia (with answers) to my
address.
This edition and previous editions the trivia can be obtained from my
mailserver. Just mail my Internet address with the following in the
subject line (exactly):
FGET: trivia1
This will retrieve the first edition of the trivia. Replace the
number with the edition you want.
Here are the answers to Commodore Trivia Edition #15 for February, 1995
Q $0E0) What is the difference(s) between the Newtronics 1541 and the 1541C?
(only one difference is needed)
A $0E0) (George Page, a noted authority on CBM Drives, indicated that Commodore
made this a tough question to answer.) by the time the 1541C was
introduced, Commodore threw a number of drives together and called
them 1541Cs. The theoretical 1541C exhibited the following
features:
No head banging, and other problems fixed by modified ROMs.
Case color matches C64C and C128 computers.
Q $0E1) What happens when you type 35072121 in direct mode on the C64 and
hit return?
A $0E1) Simple answer: Most likely, the screen clears and the word READY.
is printed at screen top. This is the behavior seen when pressing
RUN-STOP/RESTORE. Alternately, nothing could happen, or the computer
could lock up.
Involved answer: There is a bug in BASIC 2.0. Easily fixed, but
destined to live life immortal. (long)
The bug is in the PETSCII number to binary conversion routine at
$a69b (LINGET). The routine basically reads in a character from the
line, multiplies a partial result by 10 and adds the new character
to the partial result. Here is a code snippet:
a96a rts
a96b ldx #$00 ; zero out partial result
a96d stx $14
a96f stx $15
a971 bcs $a96a ; not a number, return
a973 sbc #$2f ; PETSCII to binary
a975 sta $07
a977 lda $15 ; get hi byte or partial result
a979 sta $22
a97b cmp #$19 ; partial > 6399
a97d bcs $a953 ; yes, goto error
a97f lda $14 ; load lo byte of result
a981 asl ; lo*2
a982 rol $22 ; hi*2 + c
a984 asl ; lo*2
a985 rol $22 ; hi*2 + c
a987 adc $14 ; complete lo*5
a989 sta $14
a98b lda $22
a98d adc $15 ; complete hi*5
a98f sta $15
a991 asl $14 ; lo*2 complete lo*10
a993 rol $15 ; hi*2 complete hi*10
a995 lda $14
a997 adc $07 ; add new char
a999 sta $14
a99b bcc $a99f ; did lo overflow?
a99d inc $15 ; yes, inc hi
a99f jsr $0073 ; get next char
a9a2 jmp $a971 ; go through it again.
The problem is at $a97d. when the partial result is greater than 6399,
(if partial > 6399, then new partial result will be over 63999)
the routine needs to get to $af08 to print an error, but can't due to
branch restrictions. However, a branch that will get there is in the
preceding function, which handles the ON GOTO/GOSUB keywords ($a94b,
ONGOTO).
So, the BASIC writers just branched to the code in ONGOTO; specifically
$a953:
a94b jsr $b79e
a94e pha
a94f cmp #$8d ; is the keyword GOSUB ($8d)
a951 beq $a957 ; yes
a953 cmp #$89 ; is the keyword GOTO ($89)
a955 bne $a8e8 ; no, print SYNTAX ERROR.
a957 ... ; handle ON GOTO/GOSUB
This code is checking to make sure the ON (var) is followed with a
GOTO or GOSUB keyword.
The LINGET error handler branches to $a953, which compares
.A (which holds hi byte of partial result) to $89. Normally, this
fails, and the normal SYNTAX ERROR code is reached through the branch
to $a8e8. However, for partial results of the form $89XX, the check
succeeds, and BASIC tries to execute an ON GOTO/GOSUB call.
By the way, it is no coincidence that this error occurs on 35072121,
since one of the partial results is $8900 (hi byte is $89). In fact,
350721 will achieve the same result.
If the check succeeds, the code limps along until $a96a:
a969 pla ; complement to $a94e
a96a rts ; return
But we never executed $a94e, the push, so the stack is now
messed up. Since the stack held $9e, $79, $a5 before the PLA,
(The stack could hold other values, but I always saw these)
the RTS gets address $a579 to return to, which usually holds a BRK
opcode. The break handler is invoked, and the screen clears with the
READY. at the top.
Now, the BASIC 2.0 authors were justified in reusing the error
handler code in ONGOTO for LINGET, but they calculated the branch
offset wrong, according to my tests. If you have the LINGET error
handler branch to $a955, all these troubles disappear. You can
verify this procedure with the following BASIC program on a 64:
10 for t=57344 to 65535:poke t,peek(t):next
20 for t=40960 to 49151:poke t,peek(t):next
30 poke 43390, 214
40 poke 1, peek(1) and 254
Just to be complete, this error occurs when a 6 digit or greater line
number is entered and the first 6 digits indicate a number in the
range 35072-35327 ($8900-$89ff). Also, it appears the error occurs
on the VIC-20, but I didn't completely verify it. It would be
interesting to note if the error is found on all version of CBM BASIC.
Whew, what a mouthful.
Q $0E2) If a SID chip is producing a "sawtooth waveform", does the waveform look
like:
a) "/|/|/|/|" or
b) "|\|\|\|\" ?
A $0E2) a is the correcty answer.
Q $0E3) On BASIC 2.0, what special precaution(s) must one take when working with
relative files? (only one is needed)
A $0E3) Because BASIC 2.0 doesn't handle positioning in relative files quite
right, one must position the relative file pointer before AND AFTER
a read or write to a relative file.
Q $0E4) What incompatibility existed between C128 Rev. 0 ROMS and the REU?
A $0E4) OK, I admit it. I placed this answer and its discussion somewhere
in my store of information, and it must have fallen behind the
cabinet, because I cannot find it. I will post an answer to this
as soon as I can find it, but the answers really must go out, as
they have been held up long enough.
Q $0E5) What can trigger an NMI interrupt? (count all sources on one chip as
one)
A $0E5) The following sources can trigger an NMI interrupt:
1) The expansion port.
2) CIA #2.
3) The RESTORE key.
Q $0E6) What can trigger an IRQ interrupt? (count all sources on one chip as
one)
A $0E6) The following sources can trigger an IRQ interrupt:
1) The VIC-II chip.
2) CIA #1.
3) The expansion port.
Q $0E7) Where is the ROM in a 1541 located in the 64K memory map?
A $0E7) It is located from $C100 to $FFFF.
Q $0E8) Which VIA on the 1541 is hooked to the read/write head?
A $0E8) VIA #2, located in memory from $1C00 to $1C0E.
Q $0E9) In the Commodore DOS, what bit in the file type byte denotes a "locked"
file?
A $0E9) bit 6.
Q $0EA) If files are "locked" under Commodore DOS, under what condition(s) may
the file be changed?
A $0EA) Depending on the file, the following operations can be done on a
locked file:
1) Rename will change file name, although not contents of file.
2) Random access can be used to alter file.
3) Formatting the disk will alter the file. (duh!)
4) Save-with-replace (@0:) will replace file and unlock it.
5) Opening file in append mode will allow it to be changed, and
unlock it.
6) Opening a relative file and adding or changing a record will
succeed and unlock file.
Q $0EB) How big can a program file be on a 1541 or similar?
A $0EB) The file can be as large as a sequential file, since both are stored
in the same way: 168656 bytes. However, since a program contains its
load address as bytes 0 and 1, the largest program size is 168654
bytes.
Q $0EC) Under BASIC 2.0, how does one open a random access file on a disk
drive?
A $0EC) Random access (or direct access) files are a misnomer. What you
really doing is opening the disk for reading and writing. You need
two open command to access a random file: (assume drive 8)
open 15,8,15 and
open 1,8,4,"#1" will open a random access file using buffer 1.
open 1,8,4,"#" will open a random access file using the first
available buffer
Now, by using B-R, B-W, B-A or their replacements, you can write
data to sectors on the disk.
Note that Random access files are different from relative files.
Q $0ED) A file that has a '*' immediately before the filetype is called
a _________ file.
A $0ED) a splat file. This is its correct term, believe it or not.
Q $0EE) We know the 1541 and similar drives have 5 internal buffer areas, but
how many does an 8050 drive have?
A $0EE) Since the 8050 has twice the on-board RAM (4kB), it has 16 buffers, but
only 13 are available. (All CBM drives use one buffer for zero-page
memory, one for stack memory, and one for temporary variables.)
Q $0EF) On a "save-with-replace", where is the location of the first track and
sector of the new copy of the program saved in the directory entry for
the old copy?
A $0EF) The new first track is stored at location 28, and the new first sector
is stored at location 29. These values are copied to their
correct locations after the save is completed.
End of Commodore Trivia Edition #15!
Jim Brain
brain@mail.msen.com
602 North Lemen
Fenton, MI 48430
(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
