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I decided to build a nine-bit computer

madned.substack.com

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Re: I decided to build a nine-bit computer

#31

Post by tyingq »

marktangotango wrote: post #30
tyingq wrote: post #28

Earlier quoted context omitted.

Driving LED matrix displays is a good example, since they require good adherance to timing on the output signal. Especially at high refresh rates. There's lots of hobby projects that get away with just using the CPU, but you're throwing a lot of horsepower at something a cheap CPLD could handle fine. There's also solutions like using the "PRU" in a Beaglebone to drive the display...the PRU is basically a microcontrol…

That's interesting, looks like the PRU is built into the AM3358 SOC, is that correct?

Yes, I believe it's in all (most?) of the products within the "Sitara" line, or at least AM33XX models.

Lowrisc.org also has a similar plan for what they call "minion cores" in their RISCV based product, whenever that happens. Some NXP processors also have something called an "eTPU" that seems similar.

Re: I decided to build a nine-bit computer

#32

Post by giovannibajo1 »

Nintendo 64 had a 9-bit RAM (Rambus RDRAM). Only 8 bits of each byte were accessible from the MIPS CPU for obvious reasons; the 9th bit was only used by the GPU (called "RDP") to store extra information while rendering (begin a UMA architecture, the CPU used the same RDRAM used by the CPU). Typically it contained a flag called "coverage" that was used to discriminate pixels on the edge of polygons, that were later subject to antialiasing. By reading back pixels using the CPU, you would be unable to see the coverage flag.

Re: I decided to build a nine-bit computer

#33

Post by wolfram74 »

giovannibajo1 wrote: post #32

Nintendo 64 had a 9-bit RAM (Rambus RDRAM). Only 8 bits of each byte were accessible from the MIPS CPU for obvious reasons; the 9th bit was only used by the GPU (called "RDP") to store extra information while rendering (begin a UMA architecture, the CPU used the same RDRAM used by the CPU). Typically it contained a flag called "coverage" that was used to discriminate pixels on the edge of polygons, that were later su…

Yet another weird bit of N64 lore, I'm amazed mupen64 works as well as it does.

Re: I decided to build a nine-bit computer

#35

Post by al2o3cr »

sillyquiet wrote: post #6

Regarding the interesting bit (to me) in there about the advantages of FPGAs over an SBC like the Pi (speed)- does anybody know of any blogs or projects where an FPGA's speed helped in a hobby project where software running on an SBC wasn't fast enough? I can imagine a few, mostly real-time projects involving expensive computations (image or pattern recognition maybe?), but I would love to see some concrete examples.

This is a commercial product so it's not _quite_ what you asked for, but the production volume is pretty low (100s) and the implementation is literally an FPGA dev board mounted to the back of the interface panel.

https://intellijel.com/shop/eurorack/cylonix-rainmaker/

In this module, the FPGA's ability to do LOTS of computations in parallel is used to produce 16 taps of pitch-shiftable delay along with a 64-tap comb filter.

Re: I decided to build a nine-bit computer

#36

Post by carlsonmark »

FredFS456 wrote: post #9
sillyquiet wrote: post #6

Regarding the interesting bit (to me) in there about the advantages of FPGAs over an SBC like the Pi (speed)- does anybody know of any blogs or projects where an FPGA's speed helped in a hobby project where software running on an SBC wasn't fast enough? I can imagine a few, mostly real-time projects involving expensive computations (image or pattern recognition maybe?), but I would love to see some concrete examples.

RF/radio is one solid application. Can't really do the signal processing fast enough on an SBC.

Not just the signal processing on the received data, but if you want to transmit something, you will probably be using one or more DDS channels to do so. Those may be in the FPGA, or external chips. Either way, if you are mixing the outputs of the DDS, being off by a single clock cycle can cause your transmitted data to be complete garbage.

With an FPGA and external DDS chips, this is difficult to do just because of mismatches in PCB trace lengths and/or small temperature fluctuations. With a microcontroller, it is nearly impossible to do even when using DMA because of memory bus contention.

Re: I decided to build a nine-bit computer

#37

Post by tyingq »

IshKebab wrote: post #11
sillyquiet wrote: post #6

Regarding the interesting bit (to me) in there about the advantages of FPGAs over an SBC like the Pi (speed)- does anybody know of any blogs or projects where an FPGA's speed helped in a hobby project where software running on an SBC wasn't fast enough? I can imagine a few, mostly real-time projects involving expensive computations (image or pattern recognition maybe?), but I would love to see some concrete examples.

I think you won't find many because an FPGA that is as fast at computation as a Raspberry Pi will be thousands of pounds. The real advantage is latency and low-level control.

There are cases where an FPGA is used to make a faster CPU. Old CPUs, of course, but it's still a pretty active niche. There are soft cores for Z80s, 6502, and other old CPUs that run circles around the real hardware.

Re: I decided to build a nine-bit computer

#38

Post by buescher »

mad_ned wrote: post #5
krallja wrote: post #2

I hope your address bus is three nonads wide (3^3 bits = 128MiB address space). It would be thematically even better if you used ternary logic, but I’m not sure that FPGA can handle more than two voltage levels.

haha yes, excellent suggestions! I did think about ternary logic actually but I don't know of an FPGA that supports it. I considered creating like a primitive that burns 2 register bits to approximate it even, and just throw away the 4th state and pretend I have 3-state logic on all the layers above. but i have enough on my hands just trying to get the stupid timing working on a simple CPU. Im not actually a CPU desi…

This is a fantastic hobby project. Have you thought about doing something with the "extra" bit along the lines of tagging bytes for type or garbage collection or whatever like the lisp machines?

Re: I decided to build a nine-bit computer

#40

Post by errcorrectcode »

mad_ned wrote: post #21
thehappypm wrote: post #13

Unbelievably tangential but your dog is very cute and I want to see more pictures!

Wish granted! @the.bessie.report on Instagram

Hey, we don't need any Aladdin djinns showing off their magical puppers here. Definitely against guidelines and regulations. ;)
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