5.1 Choosing a machine

JNEXT emulates five machines. Pick one with --machine, or from Machine > Machine Type while running — switching restarts the machine, so load your program afterwards. (A .jns, .sna, .szx or .z80 picks its own machine: the one it was saved on.)

--machine	Machine	What you get
48k	ZX Spectrum 48K	48K RAM, beeper only, original timing and contention
128k	ZX Spectrum 128K	Bank switching, AY sound chip, 128K timing
plus3	ZX Spectrum +2A/+3	128K plus the +3's extended paging model
pentagon	Pentagon 128	The Next's Pentagon personality: the 128K ROM and paging, no contention, Pentagon's 448 x 320 frame at 50 Hz, a Covox on port 0xFB, and 512K/1024K paging through NextREG 0x8F
next	ZX Spectrum Next	Everything: Layer 2, sprites, tilemap, 3 AY chips, DMA, the copper, up to 28 MHz (default)

Three things actually change with the machine type:

• Available hardware. A 48K has no AY chip and no memory paging; a Next has extra video layers and a much faster CPU. Software written for one generally will not run on an earlier one. Like the Next firmware's own 48K, 128K, +3 and Pentagon personalities, the four non-Next machines start with the Next-only ports off (Timex port 0xFF, the DMA's port 0x6B, Layer 2 and sprites, the CTC, and 0xDFFD except on the Pentagon); software can turn them on through NextREG 0x82-0x85, as on the hardware.
• Timing. Each machine has its own frame length and screen timing, which is what makes music and raster effects run at the right speed.
• Contention. On real 48K/128K/+3 hardware the video circuitry steals memory cycles from the CPU, slowing it in a very specific pattern. JNEXT reproduces this, which is why timing-sensitive software behaves as it does on the real machine. Contention only applies at 3.5 MHz — raise the CPU speed and it disappears, exactly as on a real Next.

Choose the machine the software was written for. Running a 48K game on the Next usually works, because the Next is backwards compatible, but the original machine is the faithful choice.

Every machine type needs an SD-card image, including 48K: that is where JNEXT reads the BASIC ROMs from, exactly as real Next hardware does. See chapter 3.

A 48K start-up screen, and the Next booting NextZXOS.

The Pentagon is the Next's own Pentagon 128 personality, the one the Next firmware's machine menu starts: it uses the 128K ROM (128.rom on the SD card). It is not a model of the original Russian clone's hardware.

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