Maximum buffer size for read from magtape?

[Doug Gwyn <gwyn>]

The problem is that the tape controller only has a 16-bit register
for the byte count to be transferred (via DMA).  The number 64K
is indistinguishable from 0 using 16-bit arithmetic.  Most DMA
controllers I have seen would transfer 64K if their byte count
register were loaded with 0, although the device driver may decide
not to let you try this.

Be advised that 80Kb far exceeds the ANSI maximum tape block size.