From Intellivision Wiki
The Sign Flag (S), if set, indicates that the previous operation resulted in negative value, which is determined by a one (1) in bit 15 of the result. If clear, then the previous operation resulted in a positive value, determined by a zero (0) in bit 15 of the result. The behavior of the following opcodes is affected by the current value of the Sign Flag.
In addition to affecting the opcodes above, the Sign Flag is affected by the following opcodes.
- Indicates slight variation from normal Sign Flag behavior. See opcode documentation for more details.