This is a work-in-progress disassembly of the ZX Spectrum version of Sim City.
This disassembly was started back in 2000 with the aim of porting the whole game to the z88. Sufficient information has been determined to allow porting of the engine to other z80 machines.
As can be seen with the NES version (and presumably others as well), the game consists of two processes:
- A UI process, responsible for handling user input and rendering the display
- A simulation process
On the Spectrum this is achieved by im2 based task swapping.
This clear separation has made it possible to retarget the engine to run on multiple z80 based computers.
The CPC also had a port of SimCity. Small changes were made to the simulation engine as a result of more memory being available:
- ZX had 4/4/4 evolution points for RIC zones, CPC had 8/8/10
- As a result ZX had 345 8x8 glyps, and CPC had 471
- Warning thresholds are different:
- Pollution 40 on the CPC against 60 on the ZX
- Crime 30 against 20
- Traffic 42 against 60.
- The "need" (more zones) message is sent 25 zone imbalance against 19.
- Pollution average: This is calculated differently
- Other changes in growth logic
The rebuild utilises the original simulation engine and assets. Instead of being interrupt based, the simulation and UI cooperatively multitask which allows porting to hardware where an interrupt source is not available.
The following targets are available:
- Amstrad PCW
- Aquarius+
- Cambridge z88
- Mitsubishi Multi8
- MSX
- NEC PC-88
- Sam Coupe
- Samsung SPC-1000
- Sharp MZ-2500
- Sharp X1
- Tatung Einstein 256
- ZX Spectrum Next
| Path | What |
|---|---|
disassembly/ |
The Spectrum disassembly. Kept as a faithful reference: comments and extra labels only; test/golden/run.py checks that nothing else changes. |
engine/ |
The simulation and tool logic extracted from it as a platform-independent engine (engine/PLATFORM_INTERFACE.md). |
ui/ |
The shared game user interface, in C, written against the small platform API in ui/platform.h. |
target/zx-original/ |
The original Spectrum build, assembled from the engine and original UI code; byte-identical to the disassembly. |
platform/common/ |
Settings and hooks the engine needs, for C programs. |
platform/console/ |
z88dk console and keyboard front end. |
target/aqplus/ |
Aquarius+-specific platform implementation: bitmap renderer, AY sound, capabilities and AQX CRT settings. |
target/common/ |
Code shared between targets: the AY/SAA sound-effect player and hooks, line editor, mini map, icon and chart drawing. |
target/multi8/ |
Mitsubishi Multi 8: planar bitmap renderer, PSG sound, tape (.cas) image. |
target/pc88/ |
NEC PC-8801: ALU/bank-switched renderer, D88 disk image. |
target/sam/ |
SAM Coupe: allram layout with paged data, SAA 1099 sound, MGT disk image with save/load. |
target/z88/ |
Z88-specific implementation: OZ integration (the menus and their <> shortcuts, a framed 7-row text window), tool icons (greyed out when unaffordable), demand bars, sound and ROM CRT settings. |
target/spc1000/ |
Samsung SPC-1000: MC6847 256x192 mono and 128x192 colour (mini maps) through I/O ports, AY sound, cassette save/load in the ROM's format (z88dk's tape_save/tape_load_to), a packed program with a bootstrap that unpacks it. |
target/msx2/ |
MSX2 under MSX-DOS 2: screen 5 with the Atari ST's art copied by the VDP out of an atlas in video RAM, the code in two mapper segments (UI and simulation) switched into page 2, PSG sound from H.TIMI (in page 3), a mouse in a joystick connector, DOS file save/load. |
target/mz2500/ |
Sharp MZ-2500: 320x200 in 16 colours on planar video RAM (the Atari ST's art in a planar atlas, tiles blitted through borrowed 8 KB windows), banked UI and simulation code, YM2203 sound, SIO mouse and keyboard pointer, boot disk image whose last cylinders hold 8 save slots (floppy controller driven directly by z88dk's mz2500_disk_rw). 96x96 map, no scenarios. |
target/x1/ |
Sharp X1: the 320x200 colour screen behind I/O ports, PSG sound, keyboard from the sub CPU's interrupt, 2D disk image with save/load (z88dk's x1_disk_read/x1_disk_write). |
target/zx/ |
Spectrum-specific platform implementation: bitmap renderer, capabilities and CRT settings. |
target/zxn/ |
ZX Spectrum Next: the program at $0000 with its own start-up and NEX packaging, Layer 2 graphics in the ST art's 16 colours, AY sound from the frame interrupt, esxDOS save/load. |
target/test/ |
Headless assembly platform used by the golden-master tests. |
target/test_ui/ |
Headless C platform used by the UI tests. |
assets/ |
Binary art and sound data used by the targets (tiles, icons, st_sounds.bin). |
tools/ |
extract_text.py (generate ui/text.h and ui/text.c), find_dead.py, make_map_art.py, make_z88_icons.py, the extract_* and make_*assets scripts that build assets/ from the Atari ST and CPC versions (the ST sound effects are packed by extract_st_sounds.py and played by target/common/ay_sfx.asm). |
Makefile |
Builds every platform (make zx, make aqplus, ...) and runs the tests (make check). |
test/ |
run_all.sh runs everything (the SAM MAME tests only when SAM_MAME is set, the Next's JNext tests only when JNEXT is). |
PORTING.md |
Strategy, status and plan. |
Needs z88dk.
make aqplus # Aquarius+ -> build/aqplus/simcity.aqx (the loader) and simcity.bin (the game; keep them together)
make z88rom # Z88 ROM application, 32 KB card image -> build/z88rom/simcity.epr
make zx # ZX Spectrum -> build/zx/simcity.tap (incomplete)
make multi8 # Multi 8 -> build/multi8/simcity.cas
make pc88 # PC-8801 -> build/pc88/simcity.d88
make x1 # Sharp X1 -> build/x1/simcity.d88
make spc1000 # Samsung SPC-1000 -> build/spc1000/simcity.spc
make msx2 # MSX2 (MSX-DOS 2) -> build/msx2/simcity.dsk and disk/ (SIMCITY.COM, .UI, .SIM, .HI and TILES.DAT: all five are needed)
make mz2500 # Sharp MZ-2500 -> build/mz2500/simcity.dsk (needs a z88dk with the mz2500 --low appmake and disk driver)
make sam # SAM Coupe -> build/sam/simcity.mgt
make zxn # ZX Spectrum Next -> build/zxn/simcity.nex
make check # golden master, tile drawing, UI tests, all builds
z88dk's tools (zcc, z88dk-z80asm, z88dk-ticks) must be on the PATH.