Changes from NFS 3.34 to NFS 3.34B

NFS 3.34B is a minor revision of Acorn NFS 3.34 for the BBC Micro. The two 8 KB ROMs are 98.7% identical at the opcode level.

Summary statistics

The low identical-byte count (36.2%) is misleading: most of the byte differences are address operands shifted by the 1-byte version string insertion. The opcode-level similarity (98.7%) reflects the true structural similarity.

ROM header

The ROM headers are identical:

There is no version string field in the header. The version number appears as an inline string printed by the *HELP handler.

Changes

1. Version string: "3.34" to "3.34B"

The ROM identification string printed by *HELP (at &81BF in 3.34) changed from "3.34" to "3.34B". This inserts one byte ("B") into the ROM image, shifting all subsequent addresses by +1.

2. Deleted RTS at end of page 6 relocated block

An RTS instruction at &964B (the final byte of the page 6 Tube host code block, runtime address &06FF) was removed. This 1-byte deletion compensates for the 1-byte version string insertion, keeping the ROM at exactly 8192 bytes and leaving all addresses from &964C onwards unchanged.

3. Tube WRCH register: R1 to R4

The most significant functional change. Five instructions in the Tube host code were modified to use Tube register R4 (&FEE6/&FEE7) instead of R1 (&FEE0/&FEE1) for WRCH character I/O:

The routine at &0527 is labeled tube_poll_r1_wrch in 3.34 (polling R1) and tube_poll_r4_wrch in 3.34B (polling R4), reflecting the register change.

These are all in the relocated Tube host code (BRK handler block and page 5 block), which is copied from ROM to RAM during initialisation.

Why this matters: The BBC Micro Tube ULA has four register pairs:

• R1 (&FEE0/&FEE1): events and escape signalling
• R2 (&FEE2/&FEE3): command bytes and general data transfer
• R3 (&FEE4/&FEE5): multi-byte data transfers
• R4 (&FEE6/&FEE7): one-byte transfers

In NFS 3.34, R1 was overloaded: it handled both WRCH character data and escape/event signalling. NFS 3.34B moves WRCH polling and data reads to R4 (the one-byte transfer register), which is a better semantic fit for single-character transfers. The escape check routine (&06E2), event handler (&06E8), and the R1 send routine (&06F7) remain on R1, preserving R1's role as the escape/event channel.

This is a protocol refinement that separates character I/O from escape/event signalling, reducing the potential for contention on R1.

4. Shifted address operands

All code from &81C8 to &964A in 3.34 appears at &81C9 to &964B in 3.34B, with absolute address operands adjusted accordingly. This accounts for the bulk of the byte-level differences but represents no functional change.

Address map

Relocated code block ROM sources (all +1):