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storage. vacuum tubes or transistorized
registers.
In the storage to
accumulator concept one of the data
fields would be in main storage and the
other would be in an accumulator.
Both
fields would be brought out to the ALU,
operated upon, and the result would go
back into the accumulator (Figure 1-12).
Main
Storoge
1
ALU
J
1
Accumulator
Storage to Accumulator Concept
Figure 1-12.
Storage to Accumulator
Concept
Fo.r its variable length operations
the System/360 uses the storage-to-
storage storage concept (Figure 1-13).
Main
Storage
1
f
Variable
Field
Length
ALU
Operations
Figure 1-13.
System/360 Storage to
Storage Operations
As you have previously learned.
variable length fields can start at any
byte location in main storage.
They are
not restricted by storage boundaries as
are fixed length operands.
However.
1-14
there fixed length operands.
(Data
fields are sometimes referred to as
operands.)
However. there must be some
way of indicating to the system the
length of the fields.
In computers of
the past, this was done several ways.
The 1401 used a special word mark bit
over the high-order position of the
data.
The IBM 70S-II used zone bits.
In the System/360 variable length opera-
tions use binary and decimal operands.
In order to be code independent,
System/360 specifies the length of these
fields by a length code in the instruc-
tion.
The length code can be either 4 or 8
bits long, depending on the instruction.
The length code is in binary.
As a
result the maximum length can be either
16 or 256 bytes.
The values of the code
is one less than the total number of
bytes.
Length code of 0000 =
1
Byte
Length code of 1111
=
16 Bytes
Length code of 11111111
=
256
Bytes
Fixed-Length Operations:
When operat-
ing on fixed-length fields (such as half
words. words, or double words). the
System/360 uses the
storage-to-accumulator concept.
These
fixed-length operations use binary
operands.
For use as accumulators, the
system/360 has
16
registers available to
the programmer.
As these registers can
be used for purposes other than accumu-
lating. they are called general reg-
iste.rs (Figure 1-14).
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