RISC OS DCI4 statistics

Use this skill when the task is about the generic DCI4 statistics interface, not just one module's private counters.

This includes:

• Implementing a statistics supplier in a module or driver.
• Writing or reviewing a statistics gatherer.
• Understanding Service_StatisticEnumerate.
• Working with the supplier SWI used for SA_DESCRIBE and SA_READ.
• Documenting or debugging dci4_spctl and dci4_stdesc structures.
• Checking atomicity, volatility, or dynamic-supplier behaviour.

Core model

The interface has two roles:

• supplier: software that owns and exports statistics
• gatherer: software that enumerates suppliers and presents or consumes them

The protocol has two phases:

1. enumerate suppliers through Service_StatisticEnumerate
2. use each supplier's SWI to describe statistics with SA_DESCRIBE and read values with SA_READ

The interface is polled, not event driven, and is not re-entrant.

Primary source

The authoritative public specification is:

• https://gerph.github.io/riscos-prm-dci4/prm/html/networking/dci4/dcistatistics.html

This skill captures the operational details you usually need so it can be used on its own, but prefer the public specification if you need to verify an edge case.

Working rules

When using this skill:

• distinguish clearly between supplier-side and gatherer-side behaviour
• separate the enumeration phase from the per-supplier SWI phase
• be explicit about buffer sizing, alignment, and partial-range returns
• call out which fields must be static, zero, or RMA-allocated
• mention the non-re-entrant and polled nature of the interface
• do not assume any particular local gatherer or supplier implementation

Supplier enumeration

Suppliers are discovered with Service_StatisticEnumerate (&A1).

The gatherer issues the service call and receives a linked list of dci4_spctl structures in R0. Each supplier allocates its own structure in the RMA, fills it in, links it to the list, and returns the updated head pointer. The gatherer is responsible for freeing those returned structures after use.

Important dci4_spctl fields:

• next: linked-list pointer
• i_version: statistics interface version implemented by the supplier
• features: feature bits; if no defined feature contract is in use, this should be zero
• swinumber: supplier SWI used for later SA_DESCRIBE and SA_READ calls
• max_stat: highest statistic number, inclusive
• type: classification such as general supplier, network protocol, network driver, or mbuf manager
• s_version: supplier's own version number
• module: supplier module title
• title: short user-facing title
• description: longer user-facing description
• reset[8]: unique per supplier initialisation so a gatherer can detect that the supplier has restarted

Do not cache supplier SWI numbers across separate enumerations. The interface allows dynamic SWI assignment, especially when one module exposes multiple effective suppliers.

Supplier SWI contract

After enumeration, all communication uses the supplier SWI from dci4_spctl:

• R0 = SA_DESCRIBE (0) or SA_READ (1)
• R1 = first statistic number, inclusive
• R2 = last statistic number, inclusive
• R3 = output buffer pointer
• R4 = output buffer size in bytes
• R5 = number of statistics processed on exit
• R6 = number of bytes written on exit

The buffer must be word aligned.

If the buffer is too small for the whole requested range, the supplier may return only a prefix of the range. R5 and R6 tell the gatherer how much was actually processed.

Descriptions before values

A gatherer should normally describe statistics before reading them.

SA_DESCRIBE returns one fixed-size dci4_stdesc for each statistic:

• type: ST_...
• format: SxF_...
• presentation: SxP_...
• size: bytes reserved for the value on SA_READ; always a multiple of 4
• volatility: SV_STATIC, SV_VARIABLE, or SV_VOLATILE
• name: static string pointer
• name_tag: currently reserved; should normally be zero
• spare: reserved; should be zero

ST_UNUSED is special:

• it reserves a statistic number
• it occupies zero bytes when read
• only the type and size fields are meaningful
• gatherers should skip it when presenting results

Types, formats, and presentation

The main value types are:

• ST_BOOLEAN
• ST_STRING
• ST_INTEGER8
• ST_INTEGER16
• ST_INTEGER32
• ST_INTEGER64
• ST_ADDRESS
• ST_TIME

Key format and presentation families:

• booleans: normal or inverted values, with presentations such as on/off, yes/no, true/false
• strings: zero-terminated string values
• integers: signed or unsigned, optional big-endian layout, with hexadecimal/decimal/dotted presentation
• addresses: address-family-specific formats such as Ethernet, IP, or Econet
• times: absolute or relative values in one of the supported time encodings

When reviewing a supplier, check that the declared size, type, format, and presentation match what SA_READ really writes.

Atomicity and volatility

A supplier must perform each requested read atomically for the requested range, so the gatherer sees a consistent set of related values from that call.

Avoid long interrupt-disabled regions. A common design is:

• keep accumulated totals in one buffer
• keep live deltas in another buffer
• atomically swap the live and merge pointers
• merge outside the critical section
• clear the reused delta buffer afterwards

Volatility is part of the contract:

• SV_STATIC: constant for the supplier invocation
• SV_VARIABLE: unlikely to change within about five minutes
• SV_VOLATILE: may change rapidly

Suppliers should group statistics of similar volatility together where possible.

Typical gatherer workflow

For a gatherer, the normal sequence is:

1. issue Service_StatisticEnumerate
2. walk the returned dci4_spctl list
3. for each supplier, call the SWI with SA_DESCRIBE
4. size read buffers from the returned dci4_stdesc values
5. call the SWI again with SA_READ
6. interpret the values using the previously returned descriptions
7. free the enumeration structures from the RMA

If a supplier only partially satisfies a request because the buffer is too small, continue from the next unread statistic number.

Common review points

When reviewing a DCI4 statistics implementation, check for these issues:

• enumeration block not allocated from the RMA
• features set without a defined feature contract
• module, title, description, or name pointing at transient storage
• reset not changing across supplier reinitialisation
• size not a multiple of 4
• SA_READ writing a different layout from what SA_DESCRIBE promises
• name_tag or spare not zero when no extension defines them
• ST_UNUSED treated like an ordinary readable statistic
• gatherer caching SWI numbers across separate enumeration passes
• read path not atomic for logically related counters

Minimal supplier pattern

A supplier normally does two things:

1. On Service_StatisticEnumerate, allocate and link a dci4_spctl in the RMA.
2. In its supplier SWI, implement SA_DESCRIBE and SA_READ over a numbered statistic range.

The supplier SWI usually follows this pattern:

• validate R0
• initialise R5 and R6 to 0
• iterate from R1 to R2
• for SA_DESCRIBE, write one dci4_stdesc per statistic
• for SA_READ, write the value layout promised by the matching description
• stop early if the caller's buffer in R3/R4 is exhausted
• leave R0 to R4 unchanged and report progress in R5 and R6

Output expectations

Answers should usually cover:

1. how suppliers are discovered
2. how descriptions and values are fetched
3. what the structures and constants mean
4. what atomicity and volatility require in practice
5. any generic implementation or review guidance that helps the current task