FOSSi open-source replacement for Z80 classic 8-bit CPU
RZ80 · Retro computing · 1 × 1 slot
Rendered from the layout at 2.5 µm per pixel. Layers: NwellDiffusionPolysiliconMetal 1Metal 2Metal 3Metal 4Metal 5Top metalPad openings
Logic density
| Inside the core logic cells per mm² | <1k
(1% of buffer maximum) over the 12.92 mm² core: what the design itself packed |
|---|---|
| Inside the seal ring logic cells per mm² | <1k
over all 19.65 mm² of the slot, pad ring included |
| Densest mm² logic cells per mm² | 5k (8% of buffer maximum) |
| Core covered by logic | 1% |
| Transistor density transistors per mm² | 177k average, 189k in the mm² with the most logic cells |
Every figure here is measured from the public layout, the same way for every design on the run: see data and method, and how its designs used the process.
Silicon proven, pin compatible, open-source replacement for Zilog Z80 a classic 8-bit CPU.
Source repository Die photograph
| Die | 3.93 × 5.12 mm (20.1 mm²) |
|---|---|
| Core area | 12.92 mm² inside the pad ring
as set in the design's own layout settings |
| Inside the seal ring | 19.65 mm² (the die less its 26 µm seal ring) |
| Placements | 2 on the reticle, in slots A4, B4 |
| Bond pads | 74 |
| Transistors | 2,292,402 |
| Logic cells | 5,209 + 208,620 filler and tap cells |
| Libraries | 9t-5V (5k) |
| SRAM | No SRAM |
Submission
| Licence | Apache-2.0 declared
Repository: Apache-2.0 in LICENSE — agrees |
|---|---|
| Template | wafer-space/gf180mcu-project-template 1.3.5 |
| How it was checked | Commit and PDK pinThe licence and template were read from the repository at commit 56308db64709.The template release named above is the one whose Makefile pins the same PDK as that commit (PDK_TAG 1.6.6). |
| Chip-on-board | Not selected at submission |
| Precheck | Passed
the check it was accepted on, 2025-12-20, gf180mcu-precheck 1.5.5 (from its image)
Passed latest re-run, 2026-08-02, gf180mcu-precheck 1.7.3 step results and logs |
Process features
Which parts of the GF180MCU process this design uses: its transistor voltages and where they come from, its gate oxides, and any devices drawn by hand outside the standard cells, SRAM and pads.
| 5 V | 9t-5V standard cells |
|---|---|
| Gate oxides | Thick only (5 V/6 V) in the design itself |
| I/O library | gf180mcu_fd_io (1122 cells), gf180mcu_ws_io (18 cells) |
Where the area went
How this design's area splits between logic cells, filler and tap cells, other and pad ring.
- Logic cells 0.19 mm² (1%)
- Filler and tap cells 12.71 mm² (63%)
- Other 0.02 mm² (<1%)
- Pad ring 7.20 mm² (36%)
Most used cells
The ten standard cells this design uses most.
| Cell | Library | Instances | What it does |
|---|---|---|---|
| nand2_1 | 9t-5V | 699 | 2-input NAND gate |
| nor2_1 | 9t-5V | 347 | 2-input NOR gate |
| mux2_1 | 9t-5V | 309 | 2-input multiplexer |
| dlyd_1 | 9t-5V | 288 | Delay buffer |
| aoi21_1 | 9t-5V | 259 | AND-OR-Invert compound gate |
| dlyc_1 | 9t-5V | 257 | Delay buffer |
| oai21_1 | 9t-5V | 236 | OR-AND-Invert compound gate |
| clkinv_1 | 9t-5V | 171 | Clock inverter |
| dffq_1 | 9t-5V | 170 | D flip-flop (1 bit storage) |
| nand3_1 | 9t-5V | 164 | 3-input NAND gate |
Logic density map
Logic cells per mm² in each square millimetre of the die. Hatched outlines are SRAM macros; the red outline is the densest mm² that avoids them (5k logic cells per mm²), found by sliding a 1 mm square over the die in 0.1 mm steps, so it can straddle the fixed squares and find more than any one of them. Squares at the edges are partial, so they read low.
How this design grew
Open-source Z80 has been made on 7 shuttles since Jun 2024. Every version, oldest first, with this one in bold:
- Jun 2024 · Tiny Tapeout 7 Zilog Z80 — 2 × 2 tiles
- Nov 2024 · Tiny Tapeout IHP 0.2 Zilog Z80 — 2 × 2 tiles
- Nov 2024 · Tiny Tapeout 9 Zilog Z80 — 2 × 2 tiles
- Mar 2025 · Tiny Tapeout IHP 25a Zilog Z80 — 2 × 2 tiles
- Nov 2025 · Tiny Tapeout GF 0.2 Zilog Z80 — 2 × 2 tiles
- Dec 2025 · wafer.space Run 1 RZ80 FOSSi open-source replacement for Z80 classic 8-bit CPU — 1 × 1 slot HZ80 FOSSi open-source replacement for Z80 classic 8-bit CPU (0.5 x 1 slot) — ½ × 1 slot
- Jul 2026 · wafer.space Run 2 HZ80 FOSSi open-source replacement for Z80 classic 8-bit CPU (0.5 x 1 slot) — ½ × 1 slot QZ80 FOSSi open-source replacement for Z80 classic 8-bit CPU (0.5 x 0.5 slot) — ½ × ½ slot
Tiny Tapeout tile Google shuttle wafer.space die
Its dies compared
Every die of this design measured here, oldest first, with this one in bold.
| Made on | Slot | Library | Gate oxides | Logic cells | Per mm² of core | Densest | SRAM |
|---|---|---|---|---|---|---|---|
| wafer.space Run 1 · HZ80 | ½ × 1 | 9t-5V | 5 V/6 V only | 5,143 | 1k | 5k | none |
| wafer.space Run 1 · RZ80 | 1 × 1 | 9t-5V | 5 V/6 V only | 5,209 | <1k | 5k | none |
| wafer.space Run 2 · HZ80 the same logic cells and transistors as HZ80 on wafer.space Run 1 |
½ × 1 | 9t-5V | 5 V/6 V only | 5,143 | 1k | 5k | none |
| wafer.space Run 2 · QZ80 | ½ × ½ | 9t-5V | 5 V/6 V only | 5,096 | 3k | 4k | none |
From Tiny Tapeout tiles to a die of its own
Open-source Z80 began as tiles on Tiny Tapeout 7, and now has dies of its own on wafer.space Run 1 and wafer.space Run 2.
The fabricated die
A photograph of the finished chip, taken through a microscope after manufacture: optical micrograph © 2026 InfoSecDJ, CC BY-SA 4.0
Precheck
Before a design is made, the wafer.space platform checks it against the foundry's design rules. RZ80 was accepted on this check, and has been checked again since with newer rules.
All 15 steps passed, with 0 KLayout DRC errors, on 2025-12-20.
Every step, and the latest re-run
| Step | Accepted on check 204, 2025-12-20, initial, gf180mcu-precheck 1.5.5 (from its image) PDK wafer-space/gf180mcu 1.6.6, LibreLane e2f0b0e | Latest re-run check 714, 2026-08-02, drc_update, gf180mcu-precheck 1.7.3 PDK open_pdks 1.0.604, LibreLane 5fe3db1 |
|---|---|---|
| KLayout.ReadLayout | passed | passed |
| KLayout.CheckTopLevel | passed | passed |
| KLayout.CheckSize | passed | passed |
| KLayout.GenerateID | passed | passed |
| KLayout.Render | not in this flow | passed |
| KLayout.Density | passed | passed |
| Checker.KLayoutDensity | passed | passed |
| KLayout.ZeroAreaPolygons | passed | passed |
| Checker.KLayoutZeroAreaPolygons | passed | passed |
| KLayout.Antenna | passed | passed |
| Checker.KLayoutAntenna | passed | passed |
| Magic.DRC | passed | passed |
| Checker.MagicDRC | passed | passed |
| KLayout.DRC | passed | passed |
| Checker.KLayoutDRC | passed | passed |
| KLayout.WriteLayout | passed | passed |
| KLayout DRC errors | 0 | 0 |
| Magic DRC errors (advisory) | 0 | 0 |
| Files | steps · log (gzip) | steps · log (gzip) |
The steps of the gf180mcu-precheck LibreLane flow, as run by platform.wafer.space. The design was accepted on the newest finished check before it was submitted; later re-runs check the same file with updated rules. KLayout DRC is the sign-off check. Magic DRC is advisory: its count does not stop a design. In the logs, Magic's "Error while reading cell" lines about fill cells are messages from its reader, not check failures.