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Old 19 January 2021, 16:06   #1
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So what did they do before FPGAs?

I know the 68K can be emulated/run on an FPGA chip but this question is not about that. I bought a simple 8MB memory upgrade for my A500 and I can see this too has an FPGA chip (Xilinx something).

So my question is; what does this FPGA chip do? And how would hardware designers have done this before FPGA?

I am just curious how things where done before FPGA and what actually FPGA replaces/simplifies on such boards.

Last edited by Sim085; 19 January 2021 at 16:12.
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Old 19 January 2021, 16:40   #2
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Simply put, it's a box of switches and wires that can be can be connected together by a schematic that is loaded when it's powered up.

The alternative way is to have a chip made permanently to the same schematic, which is a lot more expensive to get manufacturing started but costs a less in the long run if you're making thousands of chips.
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Old 19 January 2021, 17:12   #3
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Before FPGAs you'd use a PAL (one-time programmable chip with simple logic functions between pins, set by blowing fuses within the chip), GAL (similar but more capable, and re-programmable a limited-number of times) or more recently, a CPLD. (That Xilinx chip might actually be a CPLD.)

As for what the Xilinx chip does, I'd imagine it's responsible for address decoding and generating the signals necessary to read, write and refresh the RAM.

(Fun fact: the first version of Turbo Chameleon 64 cartridge had both an Altera FPGA and a Xilinx CPLD in it - the latter used to talk to the C64 cartridge port because it's 5v tolerant, whereas the FPGA isn't.)
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Old 19 January 2021, 21:16   #4
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And before even that, you built it with good ol' TTL gates, which is why so many older systems and peripherals are chock full of small logic chips. Even if PALs and GALs were used, they were expensive and only used where really necessary. In most cases, using a half dozen logic chips would be cheaper so they'd go with that.

Compare your RAM board to an early Zorro board, or the A590, for example. Lots of small RAM chips, lots of standard logic chips and a couple of PALs.
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Old 19 January 2021, 21:33   #5
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Before the FPGA?


Discrete logic! Check out that amazing circuit board! The Lear-Siegler ADM3A is one of the best examples of the problem FPGAs solve. (and I have one, complete with the dip switch cover)

Apart from the obvious size and complexity issues imagine the power consumption... take a 1970s TTL logic component with current requirement of several mA, and multiply that by over one hundred. No wonder those linear power supply components have big chunk heatsinks on them...
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Old 19 January 2021, 21:36   #6
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There is a website with pictures of the various stages of the development of the Amiga 1000.

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Old 28 January 2021, 13:33   #7
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ASICs were used for larger designs that had large volumes.

Making an ASIC was expensive, so not many 3rd party Amiga expansions had them, instead opting for PAL, GAL, CPLD or later FPGA.
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