Changes from NFS 3.62 to NFS 3.65

NFS 3.65 is the final release of Acorn NFS. Compared to 3.62, the two ROMs are 96.8% identical at the opcode level (34 change blocks). Most code shifts uniformly due to the shorter ROM title, but there are two significant functional changes to the Econet hardware handling.

ROM header

The title was shortened from 7 characters (4 leading spaces + "NET") to 3, saving 4 bytes and shifting all subsequent header data and code addresses.

NOP padding removed (14 bytes saved)

Service handler entry (9 NOPs at &80F7 in 3.62). The 9 NOP instructions described as "bus settling time for ADLC probe" were removed entirely. The service handler now begins immediately with PHA / CMP.

svc_4_star_command padding (5 NOPs at &81B0 in 3.62). Five NOP bytes between return_2 and svc_4_star_command were removed. The dispatch table was updated to target the handler directly.

*EX trampoline deleted (11 bytes saved)

In 3.62, the EX command table entry dispatched to ex_trampoline at &9FB5, which disambiguated EX from EXEC by checking whether the next character after "EX" was printable (>= &21). If so, the command was forwarded to the fileserver as EXEC; otherwise it was handled locally as *EX.

In 3.65, the trampoline is removed. Instead, the command table entry for "EX" uses a } (ASCII &7D) terminator byte. Since } cannot appear in a valid command, the MOS command parser itself rejects *EXEC before reaching the handler. The dispatch address points directly to ex_handler at &8C4F.

print_hex moved inline (~&9F9D to ~&8DCA)

The print_hex and print_hex_nibble subroutines were at &9F9D-&9FB4 near the end of the ROM in 3.62. In 3.65 they are inserted inline at &8DCA, closer to their callers.

The algorithm was also simplified: the nibble-to-hex conversion now uses ORA #&30 / CMP #&3A instead of CMP #&0A / ADC #&30, removing a carry dependency. The SEC return convention was dropped.

Self-modifying INTOFF mechanism (STA &0D1C)

This is the most significant functional change. In 3.62, NMIs were disabled by reading the Econet station ID register (BIT &FE18), which has the side-effect of clearing the NMI enable latch. In 3.65, a new RAM workspace byte at &0D1C is used as a self-modifying opcode within the NMI dispatch shim at &0D00. Writing different values to &0D1C switches between NMI-enabled and NMI-disabled states:

• LDA #&40 / STA &0D1C disables NMI processing
• LDA #&2C / STA &0D1C re-enables NMI processing

This pattern appears in five locations: 1. INACTIVE polling loop (test_inactive_retry) 2. After CTS not detected (inactive_retry) 3. TX prepare (after CTS detected) 4. wait_idle_and_reset

The change suggests Acorn found a timing or reliability issue with the hardware-based NMI disable. The self-modifying code approach atomically controls NMI processing in RAM, providing a more robust mechanism.

ADLC CR1 reset before TX (STX &FEA0)

A new CR1 write was added to the TX prepare path:

LDX #&C0     ; CR1 = &C0: AC | RX_RESET
STX &FEA0    ; Write to ADLC CR1

This resets the ADLC receiver before beginning transmission, preventing potential receive-during-transmit issues. In 3.62, TX prepare wrote only to CR2 (&E7).

NMI handler install changes

Several NMI handler install calls changed from the one-byte install_nmi_handler (which only sets the JMP low byte, assuming the high byte is already correct) to the two-byte set_nmi_vector (which sets both bytes). This was required because some handlers shifted across a page boundary (&97xx to &98xx), making the one-byte install insufficient.

Relocated code changes

The four relocated blocks shift +3 bytes in ROM source address:

Workspace, page 4 and page 5 are byte-identical. Page 6 has changes from runtime address &06CE: the trampoline JMPs shifted, and 29 bytes of &FF padding were inserted before them, pushing the trampolines from runtime &06CE to &06EB.

Free space

Total free space increased from 17 bytes (3.62) to approximately 43 bytes (3.65), distributed as 29 bytes of &FF padding at &9633-&964F and 14 bytes at the end of the ROM.