Skip the next half-words if the source register's MSB (in SRC1) is set (the 2-complement value is negative).
The 4-bit SRC2 field contains the Skip Length, a signed integer (between -8 and 7) that indicates how many half-words must be skipped (from the instruction's opcode address).
The Imm16 field is not used because the negation field already provides with a XOR.
This is one of the instructions of the SKIP group, see also : SKIP, SZ/SNZ, SO/SNE/SNO/SE, SNS.