9 track tapes and block sizes

Chuck Guzis cclist at sydex.com
Fri Oct 2 12:46:42 CDT 2020

On 10/1/20 11:40 PM, Warner Losh via cctalk wrote:
> On Fri, Oct 2, 2020, 12:05 AM Tom Hunter via cctalk <cctalk at classiccmp.org>
> wrote:
>> I have never figured out why Bob Supnik defined the magnetic tape
>> containers (TAP files) with the one byte padding for odd length records.
>> This seems very odd (pun intended).   :-)
>> Even on a machine which couldn't write 32 bit numbers (the record lenght)
>> on odd boundaries you could write the 32 bit number as 4 individual bytes.
>> Does anyone know the reason?

On the .TAP files that I provide to customers, I ignore the 16-bit
granularity and supply odd-length records as appropriate.  My own take
is that the original was intended for DEC 16/32 bit hardware.  There are
other systems that make it impossible to write an odd-length record.

More interesting are the 36-bit systems writing 7 track tapes (e.g.
Univac 1100), where a tape record has to be a multiple of 18 bits/3
"bytes" long.  An ANSI label record, for example, is 81 "bytes".
Similar situations exist for 9 track tapes written on nominally 6-bit


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