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.
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.
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