Acorn NFS and Acorn ANFS Glossary

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MOS vectors

BRKV (Break Vector)

MOS vector at &0202. Called when a BRK instruction is executed.

NFS installs its own BRK handler to process network errors, replacing the default MOS error handler while NFS is the active filing system.

EVNTV (Event Vector)

MOS vector at &0220. Called when an enabled event occurs.

NFS uses this for Econet event handling, responding to network receive-complete and transmit-complete events.

FSCV (Filing System Control Vector)

MOS vector at &021E. Called for filing system control operations including *RUN, */, and unrecognised * commands.

NFS intercepts this to forward commands to the fileserver. FSCV code 3 passes raw command text; codes 2 and 4 (in 3.35D) parse the filename through GSINIT/GSREAD first.

FILEV (File Vector)

MOS vector at &0212. Called for whole-file operations (OSFILE).

NFS intercepts this to handle remote file loading and saving over Econet.

ARGSV (Arguments Vector)

MOS vector at &0214. Called for open-file argument operations (OSARGS).

NFS intercepts this to query and set attributes on remote file handles.

BGETV (Byte Get Vector)

MOS vector at &0216. Called for single-byte file reads (OSBGET).

NFS intercepts this to read bytes from remote files over Econet.

BPUTV (Byte Put Vector)

MOS vector at &0218. Called for single-byte file writes (OSBPUT).

NFS intercepts this to write bytes to remote files over Econet.

FINDV (Find Vector)

MOS vector at &021C. Called for file open/close operations (OSFIND).

NFS intercepts this to open and close remote file handles on the fileserver.

GBPBV (Get/Put Byte Vector)

MOS vector at &021A. Called for multi-byte file transfers (OSGBPB).

NFS intercepts this to perform block reads and writes on remote files.

NETV (Network Vector)

MOS vector at &0224. Called for Econet network operations.

Used by the Econet module for low-level network I/O including transmit, receive, and broadcast operations.

RDCHV (Read Character Vector)

MOS vector at &0210. Called when OSRDCH reads a character from input.

NFS intercepts this to support remote input during Tube operations, polling the Tube register for characters from the second processor.

WRCHV (Write Character Vector)

MOS vector at &020E. Called when OSWRCH writes a character to output.

NFS intercepts this to redirect character output to the second processor via the Tube during remote operations.

MOS API calls

GSINIT (General String Input Initialise)

MOS routine at &FFC2. Initialises the string input state for parsing a command-line argument.

Used with GSREAD to handle quoted filenames and |-escaped special characters. NFS 3.35D uses GSINIT/GSREAD for */ and *RUN filename parsing.

GSREAD (General String Read)

MOS routine at &FFC5. Reads the next character from a command-line argument previously set up by GSINIT.

Handles |-escaped special characters and quoted strings. Each call returns one character until the argument is exhausted.

OSBYTE (OS Byte)

MOS routine at &FFF4. Performs miscellaneous OS operations selected by the accumulator value.

NFS uses OSBYTE calls for keyboard control (&7C to clear escape), handle queries (&C6/&C7 for EXEC/SPOOL handles), and ROM management.

OSARGS (OS Arguments)

MOS routine at &FFDA. Reads or writes attributes of an open file handle, selected by the accumulator value, with the file handle in Y and a parameter block address in X.

NFS intercepts OSARGS via the ARGSV vector to handle remote file handle queries over Econet. A=0 with Y=0 returns the filing system number (5 for NFS).

OSCLI (OS Command Line Interpreter)

MOS routine at &FFF7. Executes a * command string.

NFS uses internal OSCLI calls to dispatch the *NET1-*NET4 sub-commands, which manage file handles for Econet remote operations.

OSFILE (OS File)

MOS routine at &FFDD. Performs whole-file operations (load, save, read/write attributes) using a parameter block pointed to by X and Y.

NFS intercepts OSFILE via the FILEV vector to load and save files over Econet. The parameter block format differs between host and fileserver: NFS translates between the MOS 4-byte attribute format and the fileserver's 2-byte format using a lookup table.

OSFIND (OS Find)

MOS routine at &FFCE. Opens or closes a file, returning a file handle in A. A=0 closes the handle in Y; non-zero A values open the file named at X,Y.

NFS intercepts OSFIND via the FINDV vector to manage remote file handles on the fileserver. 3.35K adds initialisation of the sequence number tracking byte when opening a file, fixing a potential protocol error.

OSGBPB (OS Get Bytes/Put Bytes)

MOS routine at &FFD1. Performs multi-byte file read or write operations using a 13-byte parameter block.

NFS intercepts OSGBPB via the GBPBV vector. The Tube host code for OSGBPB had a bug in 3.35D where the carry result was not sent to the co-processor, fixed in 3.35K.

OSRDCH (OS Read Character)

MOS routine at &FFE0. Reads a single character from the current input stream.

OSWRCH (OS Write Character)

MOS routine at &FFEE. Writes a single character to the current output stream.

OSWORD (OS Word)

MOS routine at &FFF1. Performs OS operations using a parameter block.

NFS uses OSWORD for Econet transmit (&10) and receive (&11) operations, passing control blocks that specify station numbers, buffer addresses, and byte counts.

Hardware

ADLC (Advanced Data Link Controller)

The MC6854 ADLC chip at &FEA0-&FEA3. Handles the Econet network hardware protocol.

Provides HDLC-framed serial communication over the Econet cable. NFS accesses the ADLC's status and control registers directly for low-level network operations.

CR1 (Control Register 1)

ADLC control register at &FEA0 (active when AC=0). Configures receiver and transmitter operating modes.

Key bits: b0=RIE (RX interrupt enable), b1=TIE (TX interrupt enable), b2=RX_DISCONTINUE (abort receive), b3=RX_RESET, b4=TX_RESET, b6=RIE complement, b7=AC (address control — selects CR1 vs CR3).

CR2 (Control Register 2)

ADLC control register at &FEA1 (active when AC=0). Controls frame transmission and prioritised status bits.

Key bits: b0=PSE (prioritised status enable), b1=2/1 byte mode, b2=flag/mark idle, b3=FC_TDRA (TDRA select: FIFO available vs any byte), b4=CLR_TX_ST (clear TX status), b5=RTS (request to send), b6-b7=not used. Typical values: &1E (RX idle), &82 (flag idle wait), &A7 (TX handshake with RTS+PSE).

CTS (Clear To Send)

ADLC status register 1 bit 4. Indicates that the Econet clock hardware is present and the network is physically connected.

NFS tests CTS in SR1 during the INACTIVE polling loop after detecting a quiet line. If CTS is not set, the station has no clock signal and cannot transmit. The "No Clock" error is generated when CTS is absent.

Econet

Acorn's low-cost local area network for BBC Micro and other Acorn computers.

Uses a four-wire bus with clock, data, and ground lines, managed by the ADLC chip. Each station has a unique station number (and optional network number for bridged networks).

FIFO (First In, First Out)

A hardware buffer in the MC6854 ADLC that queues bytes for transmission or reception. The ADLC has separate TX and RX FIFOs (3 bytes each).

NFS interrupt handlers feed outbound bytes into the TX FIFO and drain inbound bytes from the RX FIFO during Econet frame transmission and reception. FIFO status bits in the ADLC status registers drive the NMI handler's decision to read or write data.

FV (Frame Valid)

ADLC status register 2 bit 1. Set when a complete, error-free frame has been received (valid CRC, no overrun, no abort).

NFS NMI handlers test FV to confirm a received scout or data frame is complete before processing its contents. FV interacts with PSE: when PSE is enabled, FV masks RDA in the prioritised status.

INACTIVE

ADLC status register 2 bit 2. Set when the Econet line is idle — no carrier is detected and no frame transmission or reception is in progress.

NFS polls INACTIVE before attempting to transmit, spinning in a tight loop with a three-byte timeout counter. If INACTIVE is never detected, the "Line Jammed" error is generated. The line must be idle before a station can assert RTS and begin sending a scout frame.

INTOFF

Disable Econet NMIs by reading the station ID register at &FE18. The read has the side effect of clearing the NMI enable latch in the Econet hardware, preventing further NMIs until INTON is performed.

NFS 3.65 uses INTOFF in combination with a self-modifying NMI shim. ANFS replaces the self-modifying approach with double INTOFF (two consecutive reads of &FE18) for robustness.

INTON

Re-enable Econet NMIs by reading the video ULA control register at &FE20. The read has the side effect of setting the NMI enable latch, allowing the ADLC to trigger NMIs again.

Always paired with a preceding INTOFF. The NMI-disabled window between INTOFF and INTON protects critical ADLC register accesses from being interrupted by incoming network traffic.

IRQ (Interrupt Request)

A maskable hardware interrupt on the 6502 processor, triggered by peripheral devices pulling the IRQ line low.

In the BBC Micro, IRQs handle keyboard, timer, and VIA events. NFS's Econet interrupt handling uses NMIs rather than IRQs for time-critical network operations, since NMIs cannot be masked.

PSE (Prioritised Status Enable)

ADLC control register 2 bit 0. When set, enables prioritised status reporting in status register 2: FV, RX abort, and error conditions take priority over RDA.

NFS enables PSE during receive to detect frame-complete and error conditions via SR2. The interaction between PSE, FV, and RDA is central to the NMI handler's frame reception logic.

NMI (Non-Maskable Interrupt)

A hardware interrupt on the 6502 that cannot be disabled. Triggered by the ADLC when Econet network activity requires immediate attention.

NFS installs an NMI handler (via the NMI workspace at &0D00) that manages the ADLC during frame transmission and reception. The handler reads ADLC status registers to determine the interrupt cause and processes TX/RX data accordingly. The NMI shim at &0D00 dispatches to version-specific handlers in pages 4-6.

RDA (Receive Data Available)

ADLC status register 2 bit 7. Set when data is available to read from the RX FIFO.

NFS NMI handlers test RDA (via BIT SR2, checking the sign flag) to determine whether to read the next received byte from the ADLC data register. When PSE is enabled, FV can mask RDA.

RAM (Random Access Memory)

Main memory. NFS copies relocated code blocks from ROM into RAM pages during initialisation for execution at runtime addresses.

ROM (Read-Only Memory)

In Acorn context, typically refers to a sideways ROM — a 16 KB slot in the BBC Micro's paged ROM bank (&8000-&BFFF).

NFS occupies one 8 KB sideways ROM slot. Up to 16 sideways ROMs can be installed, bank-switched by the MOS.

RTS (Request To Send)

ADLC control register 2 bit 5. When set, asserts the RTS line to signal readiness to transmit on the Econet bus.

NFS sets RTS in CR2 when beginning a transmission (scout or data frame). The typical TX setup value &A7 includes RTS along with CLR_TX_ST, FC_TDRA, and PSE.

sideways ROM

One of up to 16 ROM slots in the BBC Micro, all mapped to the same address range (&8000-&BFFF) and bank-switched by the MOS. Only one sideways ROM is paged in at a time.

SR1 (Status Register 1)

ADLC status register at &FEA0 (read when AC=0). Reports transmitter and receiver status.

Key bits: b0=RDA (via 2/1 byte mode), b1=S2RQ (SR2 request), b2=loop/FD, b3=DCD, b4=CTS, b5=TXU (underrun), b6=TDRA, b7=IRQ. NFS reads SR1 to check CTS (clock present) and TDRA (TX FIFO ready), and to acknowledge pending interrupts.

SR2 (Status Register 2)

ADLC status register at &FEA1 (read when AC=0). Reports frame-level receive status and error conditions.

Key bits: b0=AP (address present), b1=FV (frame valid), b2=INACTIVE (idle line), b3=abort, b4=FCS error, b5=DCD, b6=RX overrun, b7=RDA. When PSE is enabled, FV and error bits take priority over RDA. NFS NMI handlers use BIT SR2 to test AP, INACTIVE, FV, and RDA during frame reception.

TDRA (Transmit Data Register Available)

ADLC status register 1 bit 6. Set when the TX FIFO has space for at least one more byte.

NFS NMI handlers test TDRA (via BIT SR1, checking the overflow flag) before writing each byte to the ADLC data register. If TDRA is not set, the handler branches to an error or retry path.

TIE (Transmit Interrupt Enable)

ADLC control register 1 bit 1. When set, enables NMI generation when TDRA becomes set (TX FIFO ready for data).

NFS enables TIE in CR1 when switching from receive to transmit mode, so the NMI handler is called each time the ADLC is ready to accept the next TX byte. Typical CR1 value &44 sets TIE with RX_RESET.

Tube

Acorn's second-processor interface, connecting the BBC Micro host to an external processor via four register pairs (R1-R4) at &FEE0-&FEE7.

Supported second processors include the 6502, Z80, 80186, and ARM. NFS contains relocated host code that manages Tube communication for remote filing operations, handling character I/O, data transfers, and escape signalling across the register pairs.

ULA (Uncommitted Logic Array)

A semi-custom chip. In the Tube interface, the Tube ULA manages the register pairs (R1-R4) and the handshaking protocol between host and second processor.

R1 handles events and escape signalling (and WRCH in 3.34/3.35D), R2 handles command bytes, R3 handles multi-byte transfers, and R4 handles one-byte transfers. NFS 3.34B temporarily moved WRCH from R1 to R4; 3.35D reverted this.

Filing systems

CFS (Cassette Filing System)

The BBC Micro's built-in filing system for cassette tape storage.

CSD (Current Selected Directory)

The currently active directory on the fileserver for file operations. Stored at &0F03 in the NFS workspace.

The CSD handle determines which directory is used for relative filename lookups. In 3.35D, *EX conditionally skips filenames based on the CSD handle state; this check was removed in 3.35K.

DFS (Disc Filing System)

Filing system ROM for floppy disc storage, typically provided by Acorn or third-party ROMs.

DNFS (Disc and Network Filing System)

A combined ROM containing both DFS and NFS, allowing disc and network access from a single ROM slot.

EOF (End Of File)

Indicates that all data in a file has been read. NFS maintains per-handle EOF hint flags at &0E07 to avoid unnecessary network round-trips when checking file position.

The EOF hint cache was broken in 3.35D: a bug in the BGETV handler always set the hint flag regardless of file position, forcing a network round-trip on every EOF check. Fixed in 3.35K.

NFS (Network Filing System)

The Econet filing system ROM that provides remote file access over the Econet network.

This project covers standalone NFS versions 3.34, 3.34B, 3.35D, and 3.35K. The related DNFS 3.00/3.60 versions combine NFS with DFS in a single ROM.

Networking

CR (Carriage Return)

ASCII character &0D. Used as the line/command terminator in MOS command parsing and Econet protocol messages.

RX (Receive)

Refers to data reception over Econet — the receive buffer, receive control block, or the act of receiving a network packet.

TX (Transmit)

Refers to data transmission over Econet — the transmit buffer, transmit control block, or the act of sending a network packet.

four-way handshake

The Econet reliable delivery protocol for data transfers between stations. Consists of four sequential frames: scout, scout ACK, data, and final ACK.

The transmitting station sends a scout frame containing the destination address and control byte. If the destination is listening, it replies with a scout ACK. The transmitter then sends the data frame, and the receiver confirms with a final ACK. Any failure at any stage causes a retry or error. NFS implements the full four-way handshake in its NMI handler chain.

scout

The initial short frame in an Econet four-way handshake. Contains the destination station address, source station address, and a control byte identifying the operation type (port number and function code).

The scout tests whether the remote station is listening before committing to a potentially large data transfer. The NMI handler nmi_rx_scout processes incoming scouts by checking the destination address against the local station ID. If the scout is for us (or a broadcast), the handler sets up to receive the data frame.

RXCB (Receive Control Block)

The 12-byte parameter block for OSWORD &11 (Econet receive). Specifies the port number, station filter, buffer address, and buffer size for an incoming data transfer.

NFS maintains an RXCB in its workspace for each active receive operation. The buffer address bytes at offsets 4-7 determine whether the transfer targets main memory or Tube address space.

TXCB (Transmit Control Block)

The 12-byte parameter block for OSWORD &10 (Econet transmit). Specifies the destination port, station address, control byte, buffer address, and byte count for an outgoing data transfer.

The low 3 bits of the TXCB status byte encode the error class when a transmission fails (0=Line Jammed, 2=Not listening, 3=No Clock, etc.). NFS masks these bits to select the appropriate error message.

Miscellaneous

MOS (Machine Operating System)

The BBC Micro's ROM-based operating system.

Provides the vector system, API calls (OSBYTE, OSWORD, etc.), service call dispatch, sideways ROM management, and the filing system framework that NFS plugs into.

relocated code

Code stored in the ROM image that is copied to RAM at a different address during initialisation.

NFS relocates blocks to zero page (&0016-&0076), page 4 (&0400-&04FF), page 5 (&0500-&05FF), and page 6 (&0600-&06FF). The BRK handler, Tube host code, and Econet interrupt handlers all run from these relocated blocks.

service call

A broadcast mechanism in the MOS where the OS calls each sideways ROM's service entry point with a service number in the accumulator.

ROMs inspect the number and either handle the call (claiming it by zeroing A) or pass it on. NFS handles service calls for filing system selection, command dispatch, and Econet events.