Mnemonic
| SRA |
Description
The contents of the accumulator are shifted right, a bit at a time. Zeros (one for each right-shifted bit) are shifted into accumulator bit-position 0, regardless of the initial value of bit-position 0. Bits are shifted out of accumulator-position 15 and lost. An example of a right shift of two positions is:
The number of positions shifted is specified by a shift count. Location of the shift count is determined by the T bits in the instruction:
| T Bits (6 and 7) | Shift Count Location | |
|---|---|---|
| 00 | Low-order six bits of displacement (in the shift instruction) | |
| 01 | Low-order six bits in index-register 1 | |
| 10 | Low-order six bits in index-register 2 | |
| 11 | Low-order six bits in index-register 3 |
A value must be put into the shift-count location before the operation is started. The operation is ended when the specified number of shifts have been performed. The shift count, however, need be set up only once. After the operation is ended, the original shift count (for example, a count in index-register 1) is the same as its initial value. (Decrementing of the count is performed in separate circuits.)
The maximum shift count that can be specified is 63 (111111 in binary). A right-shift of sixteen, however, puts zeros in all positions of the accumulator.
A shift count of zero (000000 in binary) causes this instruction to perform as a no-operation: contents of the accumulator remain unchanged.
Core storage is not addressed during execution of the shift-right-accumulator instruction.
Indicators: The carry and overflow indicators are not affected during execution of the SRA operation.
Examples
Shift Left Accumulator/No Operation
| Assembler Language Coding | Hexadecimal Value | Description of Instruction | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Label | Operation | F | T | ||||||||
|
|
32 | 33 | 35..40.. | |||||||
| SRA | DISP | 18*X | Contents of A shift right the number of shift counts in DISP | ||||||||
| SRA | 1 | 1900 | Contents of A shift right the number of shift counts in XR1 | ||||||||
| SRA | 2 | 1A00 | Contents of A shift right the number of shift counts in XR2 | ||||||||
| SRA | 3 | 1B00 | Contents of A shift right the number of shift counts in XR3 | ||||||||
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