| Gap | Status | Source |
|---|
| Copper palette effects sample once per 7MHz pixel: the copper is cycle-paced (RunToCycle: MOVE=2 / NOOP=1 cycles at 28MHz) and interleaves per pixel with the live-palette ULA row render, but half-pixel colour detail (two MOVEs inside one pixel) collapses to one sample, top/bottom border rows resolve once per raster line (end-of-line palette state — colours are live, sub-line copper detail is not), and NON-ULA layers (L2/sprites/tilemap) still see copper state at whole-line granularity | precision limit (per-pixel sampling; surface pinned by TestNexttestsCopper) | `pkg/next/copper/copper.go`, `pkg/ula/ula.go` (applyNextCompositor) |
| Sprite per-scanline bandwidth limit not modelled; $303B bit 1 (max per line) always reads 0 | deferred | `pkg/next/sprite/doc.go` |
| Compositor blend modes 6/7 approximated as SLU in the scanline painter (the faithful mixer exists in `mixer.go` but the painter has not migrated onto it); tilemap `tm_below` per-pixel bit approximated | deferred | `pkg/next/compositor/compositor.go` |
| Hi-res Layer 2 shows visible rows 0..239; the bottom 16 of the 256-line modes are cropped (off-window overscan) | documented simplification | `pkg/ula/ula.go` (renderHiResLayer2), ROADMAP |
| ULA inner screen + border now render through the LIVE Next ULA palette per pixel — colour redefinition AND transparency, ULANext + standard decode per zxula.vhd:483-558 (closed by the base/Copper work). Residue: Timex hi-res and the ULA-output-disabled fill keep the classic pre-render (live decode falls back to it), where transparency still relies on the legacy RGBA value-matching set | closed for the standard screen mode; Timex residue deferred to the Graphics/ULA groups | `pkg/ula/ula.go` (renderNextULARow), `pkg/next/compositor/compositor.go` (ULARGBA) |
| Sprites composite 8 rows lower than the hardware paper position (the 256-line sprite frame is aligned to the 240-line canvas origin; the canvas draws 24 border lines where hardware shows 32) | canvas simplification, constant offset | `pkg/next/compositor/compositor.go`; observed via the ported Level2Order test |
| Turbo-speed video timing: pkg/ula scanline/border tracking ignores the speed multiplier, so border effects are wrong above 3.5 MHz | deferred | `pkg/memory/memory.go` comment |
| Gap | Status |
|---|
| NR$C0 programmable IM2 vector bits 7:5 unmodelled; IM status bits stored as 0 | deferred |
| NR$C4 reset default: expbus enable bit needs the composed read-back mux (flagged ❌ in the matrix) | deferred |
| ~30 composed read-backs ($68, $69, $C0, $C4, $C6, $CC-$CE, $A9, $0B...) not individually pinned to the VHDL mux | audit gap |
| Port $FF bit 6 (Timex/SCLD ULA-frame-INT disable) not implemented in WritePort | confirmed gap (matrix axis 4) |
| No exhaustive port-by-port VHDL decode conformance test; no single test enumerates all 256 write masks | audit gap |
| NR$02 iotrap read bit 4; NR$8C soft-reset latched low nibble; NR$0A DPI bits; NR$69 display-control bits stored but inert | deferred |
| Frame-origin offset (CPU tstate 0 vs ULA hc0/vc0) unvalidated; line-INT at turbo; IM2 vector table gates not all wired | audit gap (matrix axis 5) |
| Gap | Status | Source |
|---|
| zxnDMA: interrupt/match logic and DMA-vs-CPU bus contention not modelled; descriptor mode (port $DB) deferred. Read/write cycle-length costs are charged in CPU T-states (the FPGA FSM ticks at 28MHz) — a documented model convention; the prescaler delay IS turbo-exact (prescaler*4^turbo/2 T-states, dma.vhd:250-255/424) | deferred | `pkg/next/dma/dma.go` |
| UART/ESP: AT responder only, no real networking or socket emulation | out of scope | `pkg/next/uart/doc.go` |
| NR$0B joystick I/O mode: register exact, pin-repurposing behaviour (GPIO/UART on joystick pins) not modelled | out of scope | `pkg/next/wire.go` |
| RTC: clock-register writes discarded (host time is truth); only NVRAM persists; 1 Hz output disabled | pragmatic model | `pkg/next/rtc/rtc.go` |
| divMMC $2009 FRAMES-bump stub and bank-1 stub write-protect emulate firmware-installed handlers non-literally | pragmatic model | `pkg/next/divmmc/divmmc.go` |
| esxDOS F_READDIR entry layout simplified; F_FSTAT fills size + dir bit only; no success-with-carry contract | deferred | `pkg/next/esxdos/file_handlers.go` |
| .NEX loader: `Copper` field exists but is never populated (code states standard V1.2 carries no copper section, package doc says otherwise) — doc/implementation inconsistency to resolve | needs resolution | `pkg/next/nex/nex.go` vs `doc.go` |
| ROM SHA-256 digests reported but not enforced as a boot gate | planned | `pkg/next/install/install.go` |
| Audio event placement above 3.5 MHz is approximate (sample-exact placement is a known limit); Next DAC granularity notes in docs/spectrum-next.md | deferred | docs/spectrum-next.md |
| Item | Status | Source |
|---|
| IF1 RS-232 and SinclairNET: stubbed as "no peripheral connected"; CTR WAT CPU-stall not modelled | out of scope | `pkg/if1/ula.go` |
| Floppy controllers are I/O-advanced: no rotational/seek timing (weak sectors and Speedlock are modelled on the +3) | pragmatic model | fdc package docs |
| Beta density bit not modelled (TR-DOS always MFM) | pragmatic model | `pkg/betadisk/interface.go` |
| Pentagon-1024 mapping mode ($EFF7 reg 2) latched but not modelled | deferred | `pkg/memory/memory.go` |
| TZX blocks 0x12/0x13/0x15 (pure tone / pulse sequence / direct recording) parsed but skipped | deferred | `pkg/ula/tzx.go` |
| Floating bus returns $FF on +2A/+3/Next (correct) — noted here because it surprises people | correct behaviour | `pkg/ula/ula.go` |
| Multiface paging readback models $7F3F/$1F3F only | deferred | `pkg/ula/ula.go` |
| MEMPTR implemented to the depth zexall observes; some exotic update sites may be missing (only visible via F3/F5) — passes z80test v1.2a's memptr variant outright | documented depth | `pkg/z80/z80.go`; `pkg/testharness/z80test_test.go` |
| Per-access memory contention (`MemContend`) off machine-wide; lump T-state totals are the shipping model | deferred, gated on turbo contention work | `pkg/z80/z80.go` |
| SAM: MIDI, clock port, SD/IDE ports ignored | out of scope | `pkg/sam/io.go` |
| SAA1099 is datasheet-modelled (no hardware-verified reference core exists) | best available | `pkg/saa1099/saa1099.go` |
| Item | Status | Source |
|---|
| Time-travel ring captures CPU + visible 64K + ports + border only; upper RAM banks, divMMC RAM, NextRegs, MMU slots not captured/restored (phases 2a/2b catalogued) | deferred | `cmd/zx_go/timetravel.go` |
| wasm binary ~31 MB (Fyne linked as dead code); shrinking requires splitting the core out of `package main` | later optimisation | `wasm/STATUS.md` |
| Desktop run loop does not use the fastboot fast-forward (browser only) | deferred | `cmd/zx_go/fastboot.go` |
| The Next cannot tape-load in the browser player; .tap on the Next falls back to the 128K | deferred | `GoEmulator.js` |
| Browser direct-boot seed table and the nexload menu-index are coupled to the SD distro version; re-verification procedure documented | maintenance coupling | `packages/emulator-core/README.md` |
| Warm-boot path (`--warm-boot`) uses captured reference dumps; non-faithful by design, default off | dev tool | `cmd/zx_go/next.go` |