The offset is counted in CRU bits. Since the TMS 9900 can only physically address 32768 words, but is logically byte addressable, it uses only hardware adresses for all even bytes. The odd bytes are handled internally. As a consequence, it can't create addresses to CRU bits in any other way than by two. Since the least significant bit doesn't exist electrically, incrementing the CRU hardware bit address requires the logical address to be incremented by two.
That's why LI R12,>1300 TB 1 and LI R12,>1302 TB 0 are the same thing.
This leaves 15 bits to the address. Since the three most significant address bits on the TMS 9900 are used to decode external instructions, we get twelve bits left. That's why the TMS 9900 can't handle more than 4096 CRU bits.
Edited by apersson850, Wed Mar 14, 2018 11:52 AM.