Why FPGA

A typical potential user will eventually ask, "Why do you need to use FPGA while other proven solutions exist, such as Raspberry Pi?"

There are debates about how to refer to the process of simulating real hardware using FPGA. Some people insist it's not emulation but rather true hardware replication, while any simulation using a traditional CPU should be referred to as emulation. I have my own opinion here. :) From my point of view, if the FPGA code is based on the circuitry of real hardware (along with the usual tweaks for FPGA compatibility), then it should be called replication. Anything else is emulation, since it uses different kinds of approximation to meet the same objectives. Currently, it's hard to find a core that can truly be called a replica – most cores are based on more-or-less functional recreations rather than true circuit recreation. The most widely used CPU cores – the Z80 (T80) and MC68000 (TG68K) – are pure functional emulations, not replications. So it's okay to call FPGA cores emulators, unless they are proven to be replicas.

To go back to the original question, then, why FPGA, if it's also just emulation? Well, FPGA emulation is fundamentally different than emulation on a CPU. Traditional emulators on CPUs execute code sequentially. This is a tricky method of emulation because real hardware has many chips and all of them work in parallel. The CPU, video chip/logic, audio chip, memory arbiter – all of them are working at the same time. So a traditional emulator has to take care of all these parts and try to emulate the whole orchestra at the same time by quickly "running" from one chip to another. This requires a lot of CPU power to emulate even an old and slow retro computer. Sometimes even a modern CPU working at 100 times the speed of the retro computer is not enough, so the emulator has to use approximation, skip emulation of some less important parts, or assume some standard work of the emulated system without extraordinary usage. Let's take a well-known emulator, UAE, emulating an Amiga. On a Raspberry Pi 3, you can run some Amiga CPU benchmarks and get crazy numbers like 100 times the original 68000 processor. So you may assume you have an emulated Amiga that is 100 times faster than real one. No, you don't. If you run different kinds of demos or games, you will see the video stutters sometimes. For example, if you play the well-known "State of The Art" demo by Spaceballs, you will notice video stuttering at some points, while a real Amiga 600 with 1x CPU speed plays the whole demo very smoothly. This is how traditional emulators on Raspberry Pi work.

FPGA emulation works very differently from traditional emulation on CPU. An FPGA is a large array of simple triggers and other logic – just like any other chip/CPU. The only difference is that specific chips/CPUs have these triggers and logic permanently connected, while FPGA allows you to connect them however you want. A special HDL (hardware description language) describes how to connect all these triggers/logic cells. Everything in FPGA works in parallel like in the original chips/devices. Thus, FPGA is pretty close to the original hardware. FPGA doesn't need high frequencies to emulate retro computers; it works at much lower frequencies than traditional emulators require. Since everything in FPGA works in parallel, it is no problem to handle any possible usage of the emulated system. Developers using FPGA usually concentrate on the specific part to make it work correctly – and it will work as it should in any possible scenario. In the same reference demo, "State Of the Art," using FPGA emulation, you can see smooth video through the whole playback, as on the original hardware.

You may want to ask, "So why not make all emulators on FPGA then?" The answer: FPGA programming is not so trivial. Every bit in FPGA works in parallel, so the developer needs to think in parallel as well :). What is trivial on CPU is not trivial on FPGA – although some parts that are trivial on FPGA cost a lot in CPU code.