NEX file format

The NEX file format was designed as very simple and straightforward format to load self-contained applications into memory and start them. Through various extensions it did reach v1.3 at this moment, which allows for even more complex use cases.

The official distribution sports some goodies for NEX files (currently supporting V1.2 NEX files):

• nexload.asm
• NexCreator.c

Additional resources:

• SevenFFF's fork of NexCreator and nexload
• Varmfskii's ZX Next tools
• Ped's nexload2.asm (rewritten from scratch, supports V1.3 NEX files)
• Z00m's sjasmplus has support for NEX saving since v1.13.0, supports V1.3 NEX files since v1.14.4

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The basic structure of the file is:

block size	optional	description
512		"Next" string followed by file header, containing also map of memory banks stored in the file
512	*	optional palette (for Layer2, LoRes or Tilemap screen)
49152	*	Layer2 loading screen
6912	*	classic ULA loading screen
12288	*	LoRes loading screen
12288	*	Timex HiRes (512x192) loading screen
12288	*	Timex HiCol (8x1) loading screen
81920	*	Layer 2 320x256x8 or 640x256x4 loading screen
2048	*	Copper code for copper loading-effects
n * 16384	*	16kiB raw memory bank data in predefined order: 5,2,0,1,3,4,6,7,8,9,10,...,111 (particular bank may be omitted completely)
m	*	Optional binary data appended after "regular" NEX file content - custom format

Structure of the header block:

offset	size	description
0	4	"Next" string
4	4	string with NEX file version, currently "V1.0", "V1.1", "V1.2" or "V1.3"
8	1	RAM required: 0 = 768k, 1 = 1792k
9	1	Number of 16k Banks to Load: 0-112 (see also the byte array at offset 18, which must yield this count)
10	1	Loading-screen blocks in file (bit-flags): 128 = no palette block, 64 = "flags 2" in V1.3 part of header define screen, 16 = Hi-Colour, 8 = Hi-Res, 4 = Lo-Res, 2 = ULA, 1 = Layer2 The loader does use common banks to load the graphics into, and show it from, i.e. bank5 for all ULA related modes and banks 9,10 and 11 for Layer2 graphics (loading these banks afterwards as regular bank will thus replace the shown data on screen). Only Layer2, Tilemap and Lo-Res screens expect the palette block (unless +128 flag set). While one can include multiple screen data in single file (setting up all relevant bits), the recommended/expected usage is to have only one type of screen in NEX file.
11	1	Border Colour: 0-7
12	2	Stack pointer
14	2	Program counter (0 = don't run, just load)
16	2	"Number of extra files" (currently not used by anything/anyone?)
18	112	byte flag (0/1) of 16k banks included in the file - this array is in regular order 0..111, i.e. bank5 in file will set 1 to header byte at offset 18+5 = 23, but the 16kiB of data for bank 5 are first in the file (order of bank data in file is: 5,2,0,1,3,4,6,7,8,9,10,...,111)
130	1	Layer2 "loading bar" 0 = OFF, 1 = ON (works only in combination with Layer2 screen data)
131	1	"Loading bar" colour (0..255) (for 640x256x4 mode the byte defines pixels pair)
132	1	Loading delay per bank (0..255 amount of frames), 0 = no delay
133	1	Start delay (0..255 amount of frames), 0 = no delay
134	1	Preserve current Next-Registers values (0 = reset machine state, 1 = preserve)
135	3	Required core version, three bytes 0..15 "major", 0..15 "minor", 0..255 "subminor" version numbers. (core version is checked only when reported machine-ID is 10 = "Next", on other machine or emulator=8 the latest loaders will skip the check)
138	1	Timex HiRes 512x192 mode colour, encoded as for port 255 = bits 5-3. I.e. values 0, 8, 16, .., 56 (0..7 * 8) When screens 320x256x8 or 640x256x4 are used, this byte is re-used as palette offset for Layer 2 mode, values 0..15
139	1	Entry bank = bank to be mapped into slot 3 (0xC000..0xFFFF address space), the "Program Counter" (header offset +14) and "File handle address" (header offset +140) are used by NEX loader after the mapping is set (The default ZX128 has bank 0 mapped after reset, which makes zero value nice default).
140	2	File handle address: 0 = NEX file is closed by the loader, 1..0x3FFF values (1 recommended) = NEX loader keeps NEX file open and does pass the file handle in BC register, 0x4000..0xFFFF values (for 0xC000..0xFFFF see also "Entry bank") = NEX loader keeps NEX file open and the file handle is written into memory at the desired address.
		End of V1.2 header fields, following fields are V1.3 extensions
142	1	0 = disable Expansion Bus by setting top four bits of Expansion Bus Enable Register ($80) to 0, 1 = do nothing (does apply only to cores 3.0.5+)
143	1	1 = Has checksum value, checksum algorithm is CRC-32C (Castagnoli), value itself is at the very end of the header block
144	4	File offset of first bank data (when loader is parsing known version, it should know where the banks start without this value, but it may use it for extra check whether the parsing of optional blocks between header and first bank was done correctly for files V1.3+, or it may even try to partially-load unknown future versions of NEX files by skipping unknown blocks between header and banks data, although that may lead to unexpected state for the app)
148	2	CLI buffer address (after "Entry bank" is paged in), 0 = no buffer
150	2	CLI buffer size - when address and size are provided, the original argument line passed to NEX loader will be copied to defined buffer (and truncated to "size", shorter string may be zero/colon/enter terminated as any other BASIC line) and register DE is set to the buffer address. The maximum size is 2048 bytes (longer lines can be probably salvaged from Bank 5 memory if the NEX file is not loading that bank and the app code search for the original line on its own).
152	1	Loading screen flags 2 - when first flag has bit6 +64 set (+128 no-palette is valid for new cases too, other old bits should be NOT mixed with new modes): 1 = Layer 2 320x256x8bpp, blocks: [512B palette +] 81920B data 2 = Layer 2 640x256x4bpp, blocks: [512B palette +] 81920B data (HiRes color value 0..15 is used as L2 palette offset - does apply to two new Layer 2 modes) For Layer 2 banks 9,10,11,12,13 are used to display the loading screen. 3 = Tilemode screen, block: [512B palette] (plus four configuration bytes in header at offset 154) Tilemap data are stored in regular (!) bank 5 - no specialized data block is used.
153	1	1 = Has copper code block, extra 2048B block after last screen data block, which will be set to Copper and the copper will be started with %01 control code (reset CPC to 0, and start). This can be used for example as "loading screen animation" feature (be aware the timing of load may vary greatly).
154	4	When Tilemode screen: four bytes array, values to set NextRegs: $6B, $6C, $6E, $6F
158	1	When Layer 2 320x256 or 640x256 screen + loading bar: loading bar Y position (bar is 2px tall, so 254 is very bottom)
159	349	reserved for future extensions, set to zero in older versions of file format.
508	4	optional CRC-32C (Castagnoli) - is calculated by checksumming (initial value is zero) file content from offset 512 (after this field) till end of file, including the optional custom binary appended after regular NEX file. And then continuing with checksumming the first 508 bytes of the header (stopping ahead of this field). This is intended for PC tools working with NEX files, or extra check tool running on Next, but not for regular loading of NEX files (value is stored as "uint32_t" in little-endian way).

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Possible roadmap/ideas for future (this is just collection of ideas, no timeframe or guarantee that they will ever happen, feel free to comment on them):

• probably compatible with current V1.2/V1.3 file formats
  • checking machine memory and use the RAMREQ to report low memory
  • having entry bank dynamically allocated by NextZXOS (entrybank = 255?)
  • changing working directory if path with directories was provided (adjust that also when passing command line in V1.3 to make file-only like) (can be extra option of loader, not stored in the file itself)
• V1.4+
  • preserving original NextZXOS stack SP value (stack pointer = 2?) (0 is valid SP starting from very "top" of RAM) (preserving is risky, as there is always possibility of ramtop being set in such unfortunate manner, that preserved sp will cause destruction of the loaded code, so the SP should be set up by the header every time, but maybe there can be extra flag to put old value into the new stack by loader, so the code can eventually get to the content of old stack - if the NEX file doesn't overwrite the banks 5/2/0)
  • custom palettes also for ULA/HiRes/HiCol screens (maybe +64 flag?) (or invert the +128 in V1.3 for those three modes, i.e. 0 = no pal / 128 = has pal)
  • return multiple open file handles to the .nex file. Specify how many handles and an address in the header (you can now since V1.3 re-parse the original arguments in CLI buffer and re-open the NEX file yourself).
  • With a flag (bit 7 of number of handles?) to force close all existing open handles first.