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PROGRAMMING THE MAGNETIC TAPE UNITS
This section contains only information special to programming the magnetic tapes from the
DATANET-30.
More detailed information on magnetic tape and additional programming infor-
mation may be found in the manual for the magnetic tape units and other publications.
Magnetic tape units can be operated in two different modes: decimal and special binary. During
forward movement of the tape, information can be written on or read from tape in both modes.
During backward movement, information can be read from tape in both modes.
During the decimal mode of operation the zone bits -- the two most significant bits of each
six-bit binary-coded decimal (BCD) character -- are altered during transfer of information
between magnetic tape and memory.
This alteration of the zone bits takes place automatically
in the tape controller as follows:
BCD Character
in Memory
0 0
xxxx
0 1
xxxx
1 0
xxxx
1 1
xxxx
BCD Character
on Tape
0 0
xxxx
1 1
xxxx
1 0
xxxx
0 1
xxxx
The four least significant bits (XXXX) of each BCD character are the same in memory or on
tape with one exception: a BCD 0 in memory is 000000 but on magnetic tape it is 001010. The
alteration of. information during the decimal mode takes place for any configuration of bits (there
are no illegal bit configurations).
The alteration of information during the decimal mode of
operation makes the DATANET-30 magnetic tapes compatible with magnetic tape formats now
in use.
During binary operations of magnetic tapes, information is transferred between magnetic tapes
and memory without alteration of bits.
Decimal Mode
In the decimal mode of magnetic tape operations, 18 bits (18- 1) of a memory word correspond
to 3 BCD characters.
Each word from memory is checked for parity in the tape controller.
When information is read from tape, bits 0 and 1 are made 0 when three BCD characters enter
a memory cell.
The following illustration shows the relationship between a word in memory
and the three BCD characters on magnetic tape.
[ID/l\lr/l\~~lr
c:J
~@------------
V-44
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