Operand 1 specifies the even register of an even-odd consecutive
pair of general purpose registers. For
instance R4 would represent registers 4 and 5, while R8 would represent
registers 8 and 9. SRDA is used to shift the 64 bits in the even-odd pair as if they
comprised a single register. The shift
is to the right. The number of bits
that are shifted is indicated by Operand 2.
The second operand address is not
used to address data; instead, the base/displacement address is computed and
the rightmost 6 bits are treated as a binary integer which represents the
number of bits to be shifted. We will
call this value the “shift factor”.
This leads to two distinct ways of coding the shift factor:
1) Directly - The shift factor
is coded as a displacement. Consider
the example below.
SRDA R8,5
In the above shift, the
second operand, 5, is treated as a base/displacement address where 5 is the
displacement and the base register is omitted.
The effective address is 5. (See Explicit
Addressing.) When represented as an
address the rightmost 6 bits still represent the number 5, and so the bits in
registers 8 and 9 are shifted to the right by 5 bits.
2) Indirectly - The shift
factor is placed in a register and the register is mentioned as the base
register in the base/displacement address.
L R5,FACTOR PUT SHIFT FACTOR IN REG
SRDA R8,0(R5) NOW SHIFT INDIRECTLY
...
FACTOR DC F’8’ SHIFT FACTOR IS 8 BITS
In this case, the effective
address is computed by adding the contents of base register 5 (which is 8),
with the displacement of 0. The
effective address is again 8, and the rightmost 6 bits of this address indicate
that the shift factor is 8.
Each method has its uses.
The direct method is useful in situations where the number of bits you
want to shift is fixed. Coding directly
allows you to look at the instruction to determine the shift factor. On the other hand, the indirect method
allows the shift factor to be determined while the program is executing. If the shift factor cannot be determined
until the program is running, the indirect method must be used.
When shifting algebraically, bits shifted out on the right are
lost, while bits equal to the sign bit replace vacated bits on the left. The sign bit in Operand 1 remains fixed,
preserving the sign of the integer.
The condition code is set by this instruction in all cases:
Condition Code Meaning Test With
0 Result = 0 BZ, BE
1 Result < 0 BL, BM
2 Result >
0 BH, BP
Consider the following instruction.
SRDA R8,3
This instruction represents
a right algebraic shift of registers 8 and 9 using a shift factor of 3. The shift factor has been coded
directly. As a result, 3 bits, 000, are
shifted out of the register on the right.
Vacated bit positions on the left are replace by 1’s (the sign bit is
negative). This is illustrated in the
diagram below. The condition code is
set to 1, indicating that the resulting binary integer is negative.
This instruction has an RS
format but the 4 low-order bits of the second byte are unused.
Some Unrelated SRDA’s
R4 =
B’11111111111111111111111111110000’
R5 =
B’11110000111100001111000011110000’
R6 =
B’00000000000000000000000000001111’
R7 =
B’00000000000000000000000000000000’
SRDA
R4,1 R4 =
B’11111111111111111111111111111000’
Cond.Code = 1
R5 =
B’01111000011110000111100001111000’
SRDA
R4,2 R4 =
B’11111111111111111111111111111100’
Cond.Code = 1
R5 =
B’00111100001111000011110000111100’
SRDA
R4,32 R4 =
B’11111111111111111111111111111111’
Cond.Code = 1
R5 =
B’11111111111111111111111111110000’
SRDA
R6,32 R6 =
B’00000000000000000000000000000000’
Cond.Code = 2
R7 =
B’00000000000000000000000000001111’
1) SRDA is a helpful
instruction to use when dividing binary numbers. Remember that the divide instruction requires that the even-odd
pair or registers be initialized with the dividend. In other words, the even-odd pair contains a 64-bit 2’s
complement integer. One way to
initialize the pair is to load the even register with a fullword that
represents the dividend, and then shift it using SRDA to the odd register.
This operation propagates the sign bit throughout the even
register. For example, to divide the
fullword X by the fullword Y, the following code could be used.
L R4,X PUT DIVIDEND
IN THE EVEN REGISTER
SRDA R4,32 SHIFT DIVIDEND
TO THE ODD REGISTER
D R4,Y READY TO
DIVIDE