ZX Spectrum emulator and Z80 IDE

Choose the machine in the toolbar; it restarts with that ROM.

The 2068 ROM moved most of the Spectrum ROM’s routines (CHAN-OPEN is at $1230, not $1601), so much 48K software that calls the ROM doesn’t run on it, as on the real machine — use the 48K for Spectrum games.

Bits 0–2 of port $FF (the DEC register) choose the screen:

Bit 6 of $FF masks the frame interrupt, and bit 7 says where paged memory comes from: 0 the DOCK (a cartridge), 1 the EXROM. Bit n of port $F4 (the HSR) replaces 8K chunk n ($n×2000) of the HOME bank with that chunk of the DOCK or EXROM; $4000–$7FFF is contended whatever is there. The Debugger shows the DEC and HSR, the screen mode, and a map of what each 8K chunk holds now (HOME ROM, HOME RAM, EXROM, DOCK ROM/RAM or an empty DOCK) — and the paging on the 128K.

The 2068’s BASIC keywords are on the Keyboard tab: SOUND (E mode, SYMBOL SHIFT + G), STICK (+ S), FREE (+ A), ON ERR (+ F), RESET (+ P) — E mode is CAPS SHIFT and SYMBOL SHIFT together (Shift + Ctrl) — and DELETE, which is CAPS SHIFT + 0 at the start of a line. Try SOUND 7,62;8,15;0,165;1,1 and then SOUND 8,0. With Arrow keys: joystick 1 (or a gamepad) the arrows move joystick 1 for STICK(1,1).

Drop a file on the screen, or use Open file…:

Save .SNA writes the machine’s state as a snapshot you can load again. Save .SCR saves the screen in the same .SCR sizes, so a hi-res or hi-colour picture keeps its mode, and Screenshot saves a PNG — on a 2068 at 704 × 592, every hi-res pixel square. On the 2068 the fast loader traps LD-BYTES in the EXROM ($00FC), where its tape routines live.

The Assembler tab has examples — hello world with RST 16, border colours, plotting into screen memory, attribute colour cycling, reading the keyboard, a beeper tune, and a sprite moved with Q A O P — and four for the Timex 2068 (choosing one switches to the TS2068): hi-res (64-column text and lines in 512 × 192, stepping through the eight colour pairs), hi-colour (a colour for every pixel row of a character), SOUND (a scale on the AY through ports $F5/$F6, the registers BASIC’s SOUND writes) and joysticks (a cell moved with STICK’s port). On a 2068 the Spectrum examples that call the ROM use the 2068’s addresses. Screen 2 is where the 2068 keeps BASIC’s stack, channels and program, so the hi-res and hi-colour examples copy that memory away, move the stack, and put both back before returning. Assemble & run (Ctrl/⌘+Enter) pokes the code into memory and calls it from BASIC, as `RANDOMIZE USR would; RET comes back to BASIC. Build .TAP makes a tape with a BASIC loader (CLEAR, LOAD "" CODE, RANDOMIZE USR) that runs on a real Spectrum, and Load as tape loads it in the emulator the same way.

The assembler takes the usual syntax: labels (with or without a colon), local labels starting with ., ORG, EQU (or =), DEFB/DB, DEFW/DW, DEFS/DS, DEFM strings, $ for the current address, and expressions with + - * / % & | ^ ~ << >>, LOW() and HIGH(). Numbers can be $FF, 0xFF, FFh, %1010 or 'A'. Every undocumented instruction is accepted too (SLL, IXH, OUT ©,0). The program starts at the label start if there is one.

The Code & debug tab is the debugger: beside the editor (below it on a phone) are the machine’s screen, the controls — Run/Continue (F5), Pause (F6), Step (F11), Step over (F10), Step out (Shift+F11), Run to cursor (Ctrl+F10) and Reset — the registers (type a new value to change one), the flags (click to flip), the code around PC, the stack, and watches (an address or a label from your program). As you step, ▶ follows the current line in the editor.

Click a line number beside the editor to set a breakpoint (a red ●); click it again to remove it. Right-click (or Alt-click) a line number to run to that line. Breakpoints set in the Debugger’s disassembly or typed as an address show in the editor too, every breakpoint is listed under the editor with its own × and Clear all, and they stay on their lines as you edit and re-assemble. When the program stops, an arrow and a highlight mark the line in the editor (scrolled into view) and the Debugger — or, if the CPU is inside the ROM, the IDE says so and the Debugger shows the ROM code. The Debugger shows the registers (with the alternate set, IX/IY, I and R, interrupt state and MEMPTR), the flags, the stack, a disassembly with your labels, and memory you can edit. Step (F7) runs one instruction, Step over (F8) runs a CALL or RST as one step, Step out runs to the end of the routine, and clicking an address runs to it. The timing line shows the T-state in the frame and the beam position. Point at the screen to see which screen and attribute bytes hold a pixel.

The Software panel (on the Software tab) has two parts:

There are no links to "complete ROM set" archives: most are copies of commercial software that is still under copyright, whatever the upload’s licence field says. Files you own you can always load — drop them onto the emulator or paste a link.

The Z80 is emulated instruction by instruction with every undocumented flag and MEMPTR behaviour, and timed per machine cycle so the ULA’s memory and I/O contention delay the CPU exactly as on the real machine: it matches all 1,356 of the FUSE emulator’s Z80 test cases, including the timing of every memory access. The screen is drawn in 8-pixel steps in beam order, so border stripes and most multicolour effects show. The 128K adds memory paging and the AY-3-8912 sound chip. The Timex 2068 follows the Fuse emulator’s SCLD: its memory is HOME (16K ROM and 48K RAM), the 8K EXROM and the DOCK, paged in 8K chunks; the video draws 16 pixels per 4 T-states so hi-res is exact; the frame timings are libspectrum’s (timings.c).

Add parameters to the page’s address to open it on a machine or an example:

Presets:

The Sinclair ROMs are © Amstrad. Amstrad have kindly given their permission for the redistribution of their copyrighted material but retain that copyright (comp.sys.sinclair FAQ; Cliff Lawson’s statement for Amstrad). The ROM images come from the FUSE emulator. The Timex 2068 ROMs (Fuse’s tc2068-0.rom, the 16K HOME ROM, and tc2068-1.rom, the 8K EXROM) are, in Fuse’s words, "derived from Amstrad code, so the above statement applies. The modifications made by Timex are in the public domain" (roms/README.copyright). The emulator, assembler and examples are this site’s own code.

Related: ZX81 emulator, retro image converter.