Session status bits (0x564d)

The session status byte gates every phase of the link. Bit names below are behavioral (recovered from code), not original SNK symbols.

Bit	Mask	Behavior
0	01	accepted an FC 01 <catalog> identity
1	02	accepted an FD 01 <catalog> identity; later selects the gameplay ordering branch
2	04	per-tick timeout/error event; cleared on tick entry, set by expired countdowns
3	08	enables the F8 gameplay branch
4	10	normal-transport busy/progress marker
5	20	session initialized / handshake-complete
6	40	identity frame validated
7	80	active-session enable — gates entry into normal (F0) transport

The bit 7 / bit 3 gate

The active handler (0x206f68) chooses transport sub-mode from bit 7:

206f6d  bit 7,(0x564d)
206f72  jrl Z,0x207447      ; bit7 CLEAR → F8 gameplay branch (0x207447, gated by bit 3)
206f75  set 4,(0x564d)      ; bit7 SET   → normal F0 transport continues here

So:

• bit 7 set → normal F0/bulk transport
• bit 7 clear + bit 3 set → F8 gameplay exchange

The transition from transport to gameplay (routine 0x20da5a) therefore clears bit 7 and sets bit 3 after the phase barrier completes (0x5651 == 0x5652 == 1), restores base control to F0, and resets the phase snapshots.

Observed vs. intuitive

"Active" (bit 7) meaning normal transport, not gameplay, is the single most counter-intuitive fact here. Executing the ROM was what pinned it down.

Values seen in a real run

From the dynamic validation trace:

Point	0x564d	Bits
after accepting FC	41	0, 6
active, normal transport	E1 / F1	0,5,6,7 (F1 also sets bit 4)
after F8 transition	69	0,3,5,6