Directly from Siemens help on SLW:
SLW (shift left word) shifts only the contents of ACCU 1-L to the left bit by bit. The bit places that are vacated by the shift instruction are filled with zeros. The bit that is shifted out last is loaded into the status word bit CC 1. The number of bit positions to be shifted is specified either by the address <number> or by a value in ACCU 2-L-L.
SLW <number>: The number of shifts is specified by the address <number>. The permissible value range is from 0 to 15. The status word bits CC 0 and OV are reset to zero if <number> is greater than zero. If <number> is equal to zero, then the shift instruction is regarded as a NOP operation.
SLW: The number of shifts is specified by the value in ACCU 2-L-L. The possible value range is from 0 to 255. A shift number >16 always produces the same result: ACCU 1-L = 0, CC 1 = 0, CC 0 = 0, and OV = 0. If 0 < shift number <= 16, the status word bits CC 0 and OV are reset to 0. If the shift number is zero, then the shift instruction is regarded as a NOP operation.
Cheers
Borte