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SI FORMAT:
There is another instruction
format that is two halfwords in length.
It is called the 51 Format.
This format
is used when one operand is in main
storage and the other operand (called
the immediate operand) is carried in the
instruction itself.
The SI Format looks like this:
r--------~------T------T------'
J
Op Code I
12
I
B1
I
D1
I
l _________
~
_____
~
______
~
______ J
In the SI Format the sto.rage operand
is the first operand.
Its effective
address does not include an indexing
factor.
In the SI Format the immediate
operand is fixed in length and is one
byte long.
An example of an SI Format is an
instruction called Move Immediate.
This
instruction moves the immediate operand
(12)
in the instruction to the storage
location.
.
r---------T------T------T------'
I
Op Code
I
12
I
0
I
1000 I
l _________
~
_____
~
____
~
______ J
Move Immediate
In the above instruction, the con-
tents of the 12 Field will be placed in
storage location 1000.
The 51 Format is
also identified by a 10 in bits 0 and 1
of the Op Code, just like the RS Format.
5ince bits 0 and 1 of the Op Code are
the same for both the RS and SI formats,
the remaining bits of the Op Code have
to tell the computer whether it is the
R5 or SI Format.
SS FORMAT:
In the four previous
formats, the operands were of fixed
length.
Now let's take a look at the
instruction format for variable length
operations.
Variable length operation use a stor-
age to storage concept.
The instruction
format is called the SS Format and looks
like this:
1':'"42
r--------~-----T--~-----~----T----'
I
Op Code
L
I
B1
I
D1
I
B2
I
D2 I
l---------~-'~r---~-----,~r---~----J
I
I I
I I
I
L-___
J l _________ J l ________ J
Length
Location
Location
Code
of 1st
of 2nd
Operand
Operand
(Destina-
(Source)
tion)
The S5 Format, because it must
address two storage operands, is 3 half-
words in length.
Because both operands are in storage
and the instruction is
3
halfwords in
length, the S5 Format is identified when
bits 0 and 1 of the Op Code contain
11.
In the S5 Format, an indexing factor
is not included in the generation of
storage addresses.
The 2nd byte of the
SS Format is the length code which con-
sists of 8 binary bits.
The maximum
value that can be expressed with
8
binary bits is 255.
Because all operands are at least one
byte long, the length code is used to
tell how many additional bytes are need-
ed.
For instance, a length code of 15
would tell us that the operand is 16
bytes long.
If an operand is to
be
one
byte long, the length code would be
zero.
So far we have been treating the
length code as one
8
bit binary number.
However, we are dealing with two oper-
ands.
Do they both have to be of the
same length?
The answer is not always.
It depends on the particular operation.
If we are concerned with moving a data
field from one area of storage to anoth-
er, we only need one length code.
If,
however, we are adding one storage field
to another, then we need to know the
length of both operands.
For arithmetic
type S5 operations, the length code is
split in two:
r-------T---~----T----T----T----~---,
lOp Codel Ll
I
L2 IBl
I D1
I
B2
I
D2 I
l _______
~
____
~
____
~
__
~
___
~
___
~
___ J
t
t
I
I
I
I
Length of--J
Length of
1st Oper-
2nd Oper-
and
and
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