Re: Apple II Tape Question?

[mjmahon@aol.com (Michael J. Mahon)]

In article <3E6368D7.8DB5E125@swbell.net>, Rubywand <rubywand@swbell.net>
writes:

>David Empson writes ...
>> 
>> Bryan Parkoff <BParkoff@satx.rr.com> wrote:
>> 
>> >     Do Apple II Tape use GCR?
>> 
>> No.  It has nothing in common with the format of data on floppy disks.
>> 
>> The data is stored as raw tones (probably modulated so that a tone
>> change represents a particular bit value, such as zero).
>
>     It uses two 500 uS half cycles for a "1" and two 250 uS half cycles
>for a "0". Seems like this is basically the scheme that KIM users ended
>up with-- i.e. the 1 and 0 tones got shorter and shorter until we got
>down to a single cycle for each.

Which is why the "average" speed is usually quoted as 1500bps,
assuming that 1's and 0's are equally probable.  In fact, 0's usually
predominate in non-compressed (virtually all cassette) files, so the
actual speed is usually faster than 1500bps.  This is no doubt why
the assignment of the shorter period to the 0.

>> The data written to the tape starts with a header, which
>> is used on playback for synchronization, followed by raw
>> data.  I'd have to look at the monitor source code to
>> refresh my memory on the details, but my vague recollection
>> is that it records the length of the data written but
>> nothing else about it (such as what type of file it might be).  There is
>> no concept of filenames.  I don't think it uses any kind of sectoring,
>> but might have a simple checksum for the entire set of data.
>>
>
>     The routine saves a checksum but not length. Remember, for binary
>stuff, you always had to write down start and end address for any binary
>file because those numbers are required for both the W (Write) and 
>R (Read) monitor commands.
>
>     When you do a SAVE of a BASIC program, I think BASIC writes a prefix
>which has the the start and ending addresses.

Yes, it does.  The address and length info are actually a separate tape
"record" prior to the program itself.

-michael

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