Changes from ANFS 4.24 to ANFS 4.25

ANFS 4.25 is the next Master 128 ANFS after 4.24. Both are 65C02 ROMs that keep their filing-system workspace in HAZEL hidden RAM at &C000–&C2FF, so 4.25 inherits the whole Master-128 architecture unchanged. The two 16 KB ROMs share 98.2 % of their opcode structure — closer than any adjacent pair earlier in the ANFS chain. Only 10.3 % of bytes match at the same offset, because a single two-byte instruction inserted early in the ROM shifts almost every later address by +2.

The change is a one-instruction refinement, not a redesign, and does not alter externally-visible behaviour.

ROM header

The service entry moves up by two bytes, tracking the inserted instruction ahead of it.

The one functional change: svc_state cleared on the transmit-done path

The transmit-done exit tx_done_exit restores X and Y from the stack and loads A = 0 (success) before returning. 4.25 inserts a single STA svc_state at &85AC between the LDA #0 and the RTS, so the path now also writes the success code to the service-state byte instead of leaving whatever an earlier stage of the service call had put there:

        lda #0            ; A = 0 (success)
        sta svc_state     ; 4.25: also clear svc_state to success   <- new
        rts               ; return with A = 0

Those two bytes (85 A9) are the whole delta. Everything else below is a mechanical consequence of them.

Consequences of the +2 shift

• Every ROM address at or after &85AC moves up by two. All JSR / JMP targets, dispatch-table entries, command-table records and self-modifying-code operand bytes follow the shift; none is a genuine edit. The two trailing &FF padding bytes at the very top of the 4.24 ROM are dropped so the image stays exactly 16 KB.
• Boot-filename pointer. The auto-boot path loads the boot-filename address with LDX #&3E : LDY #&8D in 4.24; because the filename string moved with the shift, 4.25 loads LDX #&40 : LDY #&8D — the same pointer to the same (shifted) string.
• Credits string. The easter-egg text printed by print_inline reads Advanced NFS 4.25 (was Advanced NFS 4.24); the four authors (B Cockburn, J Dunn, B Robertson, J Wills) are unchanged.

Dispatch-table bases

Every PHA/PHA/RTS dispatch table and the extended-vector table shift with the code around them: