3.5 Ports and NextREG

Everything on a Spectrum that is not memory is reached through the Z80's I/O space, with IN and OUT naming a 16-bit port address. A ZX Spectrum Next inherits every port the machine ever had — the ULA, the 128K paging latches, the AY chips, a decade of third-party peripherals — and then adds a great deal of hardware of its own, far more than the remaining port addresses could sensibly express. Its answer is the NextREG: a 256-entry register file that holds essentially all of the Next-specific control state, reached through just two ports, 0x243B to select a register and 0x253B to read or write the selected one.

NextREG is therefore not a peripheral. It is the machine's control plane, and almost every other subsystem in this guide is configured through it — machine type and video timing, CPU speed, MMU slot mapping, layer priority and the palettes, clip windows, interrupt setup, the DivMMC and Multiface gates. This subsystem sits between the CPU and all of them, which has a practical consequence for debugging: a mistake in a port decode or a register handler almost never shows up here. It shows up as a wrong picture, a silent AY or a machine that will not boot.

Two files do the work. src/port/port_dispatch.{h,cpp} routes IN and OUT to whoever owns the address, and src/port/nextreg.{h,cpp} is the register file behind the port pair. Both are small; almost all of the content — which port belongs to whom, under what gate — lives in Emulator::init(), where every handler is registered.

Why masking, not comparison

A ZX Spectrum peripheral does not decode a 16-bit port number. It watches a few address lines and responds when they match, ignoring the rest. The ULA is the canonical case: zxnext.vhd:2582 decodes it as port_fe <= '1' when cpu_a(0) = '0' — that is, any even port, which is why the OUT (0xFC),A border trick works on real hardware. A dispatcher built on port == 0xFE gets that wrong, and jnext's did until it was fixed.

PortHandler is therefore a (mask, value) pair plus optional read and write callbacks, and register_handler(mask, value, rd, wr) is the whole registration API. The ULA is (0x0001, 0x0000); Timex screen mode is (0x00FF, 0x00FF) — low byte only, so OUT (0x12FF),A reaches it; the Kempston mouse is (0x0FFF, 0x0ADF), twelve lines; and NextREG select is (0xFFFF, 0x243B), a genuine full-address decode.

Dispatch

There is one path, not a fast path and a slow path: a linear scan over a std::vector<PortHandler> holding roughly fifty entries. Overlapping decodes are resolved by most-specific-wins — the handler whose mask has the most set bits is the most constrained decode, so it takes the port. That models the VHDL, where decodes are exclusive one-hot equations, without anyone having to hand-maintain a priority list.

Three details are worth knowing before adding a handler:

• Reads fall through on a missing read side. If the most specific match has no read callback, the scan continues to the next match that does. This models write-only ports, whose reads land on an overlapping decode instead.
• Writes can decline. A handler may call PortDispatch::decline_write() as its last action, and dispatch then retries with the next-most-specific match. The reason is that VHDL port decodes run in parallel: a decode ANDed with a cleared internal_port_enable bit simply drops out of the equation set, and an overlapping decode fires instead. So with NR 0x84 bit 2 clear, OUT (0x7FF1),A stops being a Soundrive write and becomes a plain 0x7FFD paging write.
• Observers run before dispatch, unconditionally. add_io_observer() installs a side-effect-only callback invoked on every IN and OUT regardless of what matches. The Multiface needs this: its enable and disable strobes share low bytes (0x1F, 0x3F, 0x9F, 0xBF) with the Kempston joystick and the DACs, which win the single-handler dispatch, yet the Multiface state machine must still see the strobe go past.

An unmatched read returns 0xFF via the default-read callback, matching zxnext.vhd:1868-1878. The floating bus is not the default: it is the registered read handler of port 0xFF alone, because the VHDL mux that produces it is scoped to the port_ff_rd decode. PortDispatch::in() and out() also carry the RZX hooks — a playback override that substitutes recorded IN values, and a recording tap.

How a subsystem gets a port

It does not register itself. Every handler is a lambda in Emulator::init() capturing this, and that is deliberate rather than lazy: nearly every real port is gated, and the gate is emulator-tier state. The gate is the internal_port_enable vector in NR 0x82-0x85, ANDed with the bus-port mask NR 0x86-0x89 when NR 0x80 bit 7 enables the expansion bus (zxnext.vhd:2392-2393). Emulator::effective_internal_port_enable() computes one byte of it, and propagate_effective_port_enables() re-pushes the derived bits into the subsystems that keep their own shadow copy — contention, DivMMC and the Multiface. A handler that skips its gate check is a bug, not a shortcut.

init() clears the handler and observer lists before registering anything, because a soft reset re-runs it and duplicate handlers would double-fire the auto-incrementing ports.

The NextREG file

NextReg holds std::array<uint8_t,256> regs_ plus three parallel arrays of per-register handlers. Port 0x243B writes the select latch; port 0x253B reads or writes the selected register.

Writes go through write_handlers_[reg] if one is installed, and the handler returns the canonical byte to be cached. That return value is the contract: a handler that masks reserved bits, or refuses a gated write, returns the masked or unchanged byte, so the readback matches hardware. Without a handler the raw byte is stored. NR 0x01, 0x0E and 0x0F are guarded read-only, being board-generic constants with no write strobe in the VHDL decoder.

Reads go through read_handlers_[reg], which normally pull live state from the owning subsystem, and otherwise fall back to the cache. Two further shapes exist:

• set_destructive_read_handler() — for registers whose read mutates state. NR 0x2C and 0x2E latch the Pi-I2S low bits into the NR 0x2D shadow, for instance. Such a handler may only be installed together with a side-effect-free twin, because peek() — the debugger's read — refuses to call a destructive handler. The API is shaped so that it is impossible to declare one without also saying how to observe it safely.
• cached(reg) is the last byte written and nothing more, not the register's value. Several registers are not owned by their cache: NR 0x69 bit 7 is also latched by port 0x123B, and NR 0x15 is recomposed from the renderer. Reading those through cached() yields a stale byte, which is exactly how the debugger once reported Layer 2 as off while displaying its graphics. Use peek().

The Z80N NEXTREG rr,nn opcode has its own write path, PortDispatch::nextreg_opcode_write_cb, which writes the named register directly and leaves the 0x243B select latch alone, per zxnext.vhd:4739-4744. Routing it through the port pair instead clobbered the latch and broke raster waits in real games. Both paths defer during the per-instruction tick window (Emulator::enqueue_cpu_nr_write); see A frame, end to end.

Registers that are cached only

A few registers are stored and read back faithfully but drive nothing, because the hardware behind them is out of scope for a software emulator. The expansion-bus control bits of NR 0x81 are latched in nr_81_ with no consumer — though bit 5, the NMI debounce, is wired — as is the expansion-bus/ESP reset line in NR 0x02 bit 7. The Pi GPIO output enables NR 0x90 and 0x93 are write-masked per the VHDL but drive no pins. The Pi and ESP GPIO inputs, NR 0x98-0x9B and 0xA9, get read handlers that return 0x00, meaning "nothing attached" — deliberately, rather than letting the output shadow leak into the input read.

The authoritative register and port lists are not reproduced here. They live in the FPGA source tree as cores/zxnext/nextreg.txt and cores/zxnext/ports.txt, with the Next wiki linked from doc/REFERENCES.md. What jnext adds on top of them is the per-row VHDL citation in test/nextreg/ and test/port/.