Neo Geo Architecture: A practical analysis

Great article! It actually makes an interesting point about the Z80 interrupt system: the Z80 does have an extremely powerful interrupt mode IM2 which supports an interrupt vector table with up to 128 entry points anywhere in memory and (theoretically) any number of prioritized interrupt sources, but this capability was not properly supported in western home computers (at least as far as I'm aware from the Speccy, CPC and MSX), in East German home computers (Z1013, Z9001, KC85/2..4) this mode was fully supported - which is interesting because those designs were much inferior to western home computers (at least when it comes to features required for video games).

The reason is that IM2 requires interrupt sources to implement a fairly complex 'daisy chain protocol'. Support chips are supposed to place the low byte of a 16-bit interrupt vector address on the data bus when the Z80 starts handling an interrupt, the Z80 will read that byte, and combine it with the special I register as high byte to build a 16-bit address which points into the interrupt vector table. Next the Z80 will read the 16-bit value from that address - which is the entry point of the interrupt service routine - and jump there.

Interrupt priorities are hardwired via the above mentioned 'daisy chain', and this prioritization is what makes the interrupt support logic complicated because pending interrupt requests should not get lost while a higher priority interrupt is serviced (it's all described here in detail: http://www.z80.info/1653.htm).

The standard Z80 family chips (CTC, PIO, SIO) all supported this complex protocol, but not any of the custom chips in western home computers. East German home computers used TTL logic and Z80 family chips instead of custom chips and thus could make full use of the Z80 interrupt capabilties, while western designs typically didn't (I would have taken sprites and hardware-scrolling any day over powerful IM2 interrupts though).

Thought I'd point out that this system has a really good MiSTerFPGA core that supports the Neo Geo AES, MVS and CD systems [0] if you'd like to give it a go and software emulators aren't your thing. MiSTerFPGA, for those unaware, is a project that uses a Terasic DE10 Nano FPGA evaluation board to reproduce older consoles, computers, and arcade systems in hardware [1]. And as a heads up, the cost for entry into MiSTerFPGA will soon drop sharply as there are clone boards for the DE10 and expansion memory boards that will cost well under half of current offerings. At under $99 for the DE10 clone and $20 for the 128K SRAM memory expansion that is basically a must have [2], you'll be able to run consoles up to PS1 / Saturn / N64 level. Above that you'll have to go to software emulation or hang on to see what the next round of FPGA-based retro-consoles (Replay2, MarsFPGA) will be able to offer.

[0] https://www.youtube.com/watch?v=Eog8solD9dc

[1] https://github.com/MiSTer-devel/Wiki_MiSTer/wiki

[2] https://x.com/TakiUdon_/status/1801578381613613111

One thing I didn't see mentioned is that there's a function for automatic sprite animation loops based on a global timer. This is how some Neo Geo games have huge environmental background animations like waterfalls, pulsing/flickering lights, etc. without bogging down the CPU.

Years ago, a computer magazine (I think in the UK) did an amusing look into the flip lids of the current CD consoles. Playstation, Saturn, Jaguar (I think) and the Neo Geo.

They placed a penny on the closed lid, and pressed the button to open it.

I forget which moved the penny furthest, but the Neo Geo came last.

Just last week google CTF featured a reverse engineering challenge of a neo geo game!

https://capturetheflag.withgoogle.com/challenges/rev-arcade

Also, the neo geo development wiki is an absolute goldmine of technical information about the console.

https://wiki.neogeodev.org

It's too bad that these consoles with their amazing graphics were so expensive at the time.

Fond memories… a game shop had one you could play on as a guest, there was always a queue. Unimaginable to young me that some people actually owned them.

I remember playing and enjoying a side scrolling shooter with weird penis-shaped space ships (Last Resort).

I love „Neo Turf Masters“ — which always has been Neo Geo exclusive AFAIK and possibly the best golf simulation if you don’t actually play Golf. There is a Switch „port“ which really is just emulating the console but a fun diversion from Zelda and Mario.

https://www.twitch.tv/videos/1211920539

Feels baller as fuck to own an AES, not gonna lie. The system we all lied about owning in middle school!

Love these articles so much and I just love learning about all these systems that obsessed me as a child.

Very well written.

If you have any nostalgia for (or just interest in) old consoles / arcade machines / game system, this article is well worth your time.

If this article sparks your interest, a good first game to try out is one from the Metal Slug series. I don't know any other that is so beautifully drawn and animated, like a cartoon come alive.