NAUG Ch7 — Events

Holmes & Dickens, The New Advanced User Guide, pp.126-130. The event system is MOS’s user-facing wrapper over a subset of IRQs — gets you a “callback when X happens” without writing a full IRQ handler.

Key facts captured

• EVNTV at &220/&221 — called by MOS’s IRQ routine for each enabled event.
• 10 event types (0-9), enumerated below.
• A = event number on entry to EVNTV. X/Y = event-specific data.
• Handler constraints: don’t enable IRQs, finish in <2 ms, preserve all registers, chain through old vector.
• OSBYTE &0E (*FX 14,n) — enable event n.
• OSBYTE &0D (*FX 13,n) — disable event n. Returns old enable state in X (0=disabled).
• OSEVEN (&FFBF) — generate a user event. Y = event number. Not available across Tube. C=0 on exit iff the event was enabled.
• Event enable flags stored at &2BD-onwards in page 2 (os-workspace). Reading directly is faster than OSBYTE.

Event table (NAUG §7.1 p119)

#	Cause	Entry data
0	Output buffer empty	X = buffer number
1	Input buffer full	X = buffer number, Y = char value that couldn’t be inserted
2	Character entering input buffer	Y = ASCII char (note: independent of input stream — fires for any incoming char)
3	ADC conversion complete	X = channel number
4	Start of vsync	— (50 Hz on UK BBC)
5	Interval timer crossed zero	— (timer set via OSWORD &04)
6	ESCAPE condition detected	—
7	RS423 error	X = 6850 status byte shifted right 1, Y = char received
8	Econet	— (or user use if no net hardware)
9	User event	Y = event number on call to OSEVEN

Event 4 — vsync

The cheapest “fire every frame” hook. Compared to hooking IRQ1V directly:

• Event 4 path: cheap to enable (*FX 14,4), runs inside MOS IRQ at ~50 Hz, your handler sits at EVNTV.
• IRQ1V/IRQ2V path: lower latency, you test System VIA CA1 bit yourself, can do raster-rate work. See interrupts.

Event 4 is the lazy/safe option; IRQ hooking is the performance option. The handler-runtime constraint is the same either way (< 2 ms).

Filed into

• events — Master events reference + handler patterns.
• Updates: interrupts cross-links to events as the “easier than IRQ hooking” alternative.

Open follow-ups

• Exact &2BD-onwards layout for event enable flags (Master differs from Model B) — os-workspace has the rough range; precise addresses would need MOS disassembly confirmation.

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