NEX & SNA File Viewer (ZX Spectrum Next)

A read-only VS Code viewer for ZX Spectrum Next .nex files and ZX Spectrum .sna / .snx snapshots.

NEX files

Open any *.nex file and the viewer shows it in five tabs:

• Overview: every header field, decoded (V1.0–V1.2, plus the unofficial V1.3 fields written by sjasmplus). It also shows a file layout table with offsets and sizes, and validation results:
  • magic and version
  • truncated files
  • bank count
  • CRC-32C
• Loading Screen: the loading screen with its border colour, 1×–4× zoom and PNG export. Supported screen types:
  • Layer 2 256×192, 320×256 and 640×256
  • ULA
  • LoRes
  • Timex HiRes and HiColour
  • Tilemap
• Palette: all 256 colours of the 9-bit palette. If the file has no palette, the default Next palette is shown.
• Banks & Data: every 16K bank in file order, plus copper code, custom (appended) data, the header, palette and screen blocks. Clicking an entry opens a hex dump; offsets can be shown as file offset, block offset or CPU address. The copper block is decoded into its 16-bit instructions (WAIT / MOVE / NOOP / HALT) with Next register names; a hex dump of it is one click away.
• Disassembly: the first 100 Z80 instructions from the start address (PC). This covers undocumented opcodes and the ZX Next extended instructions (NEXTREG, MUL D,E, PUSH nn, LDIRX and others).

Snapshots (.sna / .snx)

Open any *.sna or *.snx file and the viewer shows it in four tabs:

• Overview: type (48K or 128K), start address, border, interrupt mode and IFF2, all Z80 registers including the shadow set and the decoded flag bits, the 128K paging state from port $7FFD (bank at $C000, normal or shadow screen, ROM, paging lock, TR-DOS), a screen preview and the file layout.
• Screen: the ULA screen from bank 5 and, for 128K snapshots, the shadow screen from bank 7. The screen that was on display is shown first. Zoom and PNG export work as for NEX files.
• Memory & Data: hex dump of every stored bank, the register header and any appended data. Banks that are paged in can be shown with their CPU address.
• Disassembly: the first 100 Z80 instructions from the start address.

48K snapshots (49179 bytes) do not store PC: it is pushed onto the stack, so the viewer reads it from memory at SP. 128K snapshots (131103 bytes, or 147487 when bank 5 or 2 is the paged one) keep PC, port $7FFD and the TR-DOS flag in a short block after the first three banks.

.snx files have the same layout as a 128K .sna; NextZXOS only keeps file handle 0 open for them, so a program can read private data appended to the file. The viewer lists such data as "Appended data".

NEX versions

Copper code

The copper block is always 2048 bytes, i.e. 1024 instructions of 16 bit each (big endian): %1hhhhhhv %vvvvvvvv is a WAIT for a raster position, %0rrrrrrr %vvvvvvvv writes a value to a Next register, $0000 is NOOP and $FFFF halts. Header byte 153 only says whether the block is present; there is no separate switch for using it. When it is present, the loader starts the copper in mode %01 (counter reset, then run) after the loading screen is shown, so the program repeats from instruction 0.