Talk:Lookup Table - Revision history

GHXX: /* Improve further */ Reply

2025-09-07T22:52:33Z

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	:::::::The article needs to be more direct at showing possible applications, including fast computation of the formula, storing data (effectively ROM) etc.		:::::::The article needs to be more direct at showing possible applications, including fast computation of the formula, storing data (effectively ROM) etc.
	:::::::I can write it tomorrow. [[User:DjSapsan\|DjSapsan]] ([[User talk:DjSapsan\|talk]]) 22:50, 7 September 2025 (UTC)		:::::::I can write it tomorrow. [[User:DjSapsan\|DjSapsan]] ([[User talk:DjSapsan\|talk]]) 22:50, 7 September 2025 (UTC)
			::::::::kk, yeah im curious to see/read what the general approach for constructing a general LUT will look like [[User:GHXX\|GHXX]] ([[User talk:GHXX\|talk]]) 22:52, 7 September 2025 (UTC)

DjSapsan: /* Improve further */ Reply

2025-09-07T22:50:04Z

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	::::::hmmmmmmmmmmm i see, in a sense the LUT i am /have-been describing is a special case of that then, as 0 outputs are unnecessary, and that then also avoids needing a circuit after the chains that actually selects the output values (i.e relays or something like that), and thus you get away with just ORing the chains.		::::::hmmmmmmmmmmm i see, in a sense the LUT i am /have-been describing is a special case of that then, as 0 outputs are unnecessary, and that then also avoids needing a circuit after the chains that actually selects the output values (i.e relays or something like that), and thus you get away with just ORing the chains.
	::::::Do you think it might work out to present a more general LUT further down that extends the circuit? (To my experience this specific case is the most common one, because usually a single bit is enough where speed is this necessary (if you can allow for 2 ticks instead of one you can replace the chains with a set of xors and a nand for example, which is a lot more configurable) [[User:GHXX\|GHXX]] ([[User talk:GHXX\|talk]]) 22:28, 7 September 2025 (UTC)		::::::Do you think it might work out to present a more general LUT further down that extends the circuit? (To my experience this specific case is the most common one, because usually a single bit is enough where speed is this necessary (if you can allow for 2 ticks instead of one you can replace the chains with a set of xors and a nand for example, which is a lot more configurable) [[User:GHXX\|GHXX]] ([[User talk:GHXX\|talk]]) 22:28, 7 September 2025 (UTC)
			:::::::I suggest to present a general LUT at first and then show the optimized variant and maybe other variants.
			:::::::The article needs to be more direct at showing possible applications, including fast computation of the formula, storing data (effectively ROM) etc.
			:::::::I can write it tomorrow. [[User:DjSapsan\|DjSapsan]] ([[User talk:DjSapsan\|talk]]) 22:50, 7 September 2025 (UTC)

GHXX: /* Improve further */ Reply

2025-09-07T22:28:10Z

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	:::::And in LW, I store characters for multi-segmented displays in LUTs.		:::::And in LW, I store characters for multi-segmented displays in LUTs.
	:::::Changing it should be simpler, because it doesn't require adding/removing parts, only changing the output bits. [[User:DjSapsan\|DjSapsan]] ([[User talk:DjSapsan\|talk]]) 22:19, 7 September 2025 (UTC)		:::::Changing it should be simpler, because it doesn't require adding/removing parts, only changing the output bits. [[User:DjSapsan\|DjSapsan]] ([[User talk:DjSapsan\|talk]]) 22:19, 7 September 2025 (UTC)
			::::::hmmmmmmmmmmm i see, in a sense the LUT i am /have-been describing is a special case of that then, as 0 outputs are unnecessary, and that then also avoids needing a circuit after the chains that actually selects the output values (i.e relays or something like that), and thus you get away with just ORing the chains.
			::::::Do you think it might work out to present a more general LUT further down that extends the circuit? (To my experience this specific case is the most common one, because usually a single bit is enough where speed is this necessary (if you can allow for 2 ticks instead of one you can replace the chains with a set of xors and a nand for example, which is a lot more configurable) [[User:GHXX\|GHXX]] ([[User talk:GHXX\|talk]]) 22:28, 7 September 2025 (UTC)

DjSapsan: /* Improve further */ Reply

2025-09-07T22:19:11Z

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	::::From my experience a lookup table is always simply used to compute one bit from a set of input bits, encoding an arbitrary logical formula by just listing all inputs that lead to a true output.		::::From my experience a lookup table is always simply used to compute one bit from a set of input bits, encoding an arbitrary logical formula by just listing all inputs that lead to a true output.
	::::I think the architecture of a "full" LUT as you are describing it would be fundamentally different as changing outputs in a normal LUT tends to be really annoying, as you would need to physically add/remove chains and couldnt simply flip a switch. [[User:GHXX\|GHXX]] ([[User talk:GHXX\|talk]]) 22:09, 7 September 2025 (UTC)		::::I think the architecture of a "full" LUT as you are describing it would be fundamentally different as changing outputs in a normal LUT tends to be really annoying, as you would need to physically add/remove chains and couldnt simply flip a switch. [[User:GHXX\|GHXX]] ([[User talk:GHXX\|talk]]) 22:09, 7 September 2025 (UTC)
			:::::@[[User:GHXX\|GHXX]] Maybe we misunderstand each other a little bit.
			::::: The "full" LUT is 100% equivalent to the "1-bit" LUT. It only adds a possibility to output a pattern of bits (as well as the same 1 bit).
			::::: In my experience, LUTs are used in programming, for example in Lua:
			:::::<code>t = {}<br>t[1] = "logic"<br>t[2] = "world"</code>
			:::::And in LW, I store characters for multi-segmented displays in LUTs.
			:::::Changing it should be simpler, because it doesn't require adding/removing parts, only changing the output bits. [[User:DjSapsan\|DjSapsan]] ([[User talk:DjSapsan\|talk]]) 22:19, 7 September 2025 (UTC)

GHXX: /* Improve further */ Reply

2025-09-07T22:09:00Z

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	::: <br>		::: <br>
	::: So... should we make two versions? [[User:DjSapsan\|DjSapsan]] ([[User talk:DjSapsan\|talk]]) 22:02, 7 September 2025 (UTC)		::: So... should we make two versions? [[User:DjSapsan\|DjSapsan]] ([[User talk:DjSapsan\|talk]]) 22:02, 7 September 2025 (UTC)
			::::@[[User:DjSapsan\|DjSapsan]] i think making a separate decoder page, if that name is appropriate ,would make sense; but, wouldnt that in a way be a multiplexer with fixed inputs (possibly using switches) that you are choosing from using address lines?
			::::From my experience a lookup table is always simply used to compute one bit from a set of input bits, encoding an arbitrary logical formula by just listing all inputs that lead to a true output.
			::::I think the architecture of a "full" LUT as you are describing it would be fundamentally different as changing outputs in a normal LUT tends to be really annoying, as you would need to physically add/remove chains and couldnt simply flip a switch. [[User:GHXX\|GHXX]] ([[User talk:GHXX\|talk]]) 22:09, 7 September 2025 (UTC)

DjSapsan: /* Improve further */ Reply

2025-09-07T22:02:49Z

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	::@[[User:DjSapsan\|DjSapsan]], No, usually it only produces one output bit (<math>Output1</math>), because all those chains get OR ed together, and unless the other output (<math>Output2</math>)) can be computed by <math>Output2 = Output1 \quad OR \quad (other chains)</math>, there is no way of computing that other output, as far as i am aware.		::@[[User:DjSapsan\|DjSapsan]], No, usually it only produces one output bit (<math>Output1</math>), because all those chains get OR ed together, and unless the other output (<math>Output2</math>)) can be computed by <math>Output2 = Output1 \quad OR \quad (other chains)</math>, there is no way of computing that other output, as far as i am aware.
	::I suppose you could maybe re-use the intermediate NOT outputs, but that is far from the general case from my experience. [[User:GHXX\|GHXX]] ([[User talk:GHXX\|talk]]) 20:55, 7 September 2025 (UTC)		::I suppose you could maybe re-use the intermediate NOT outputs, but that is far from the general case from my experience. [[User:GHXX\|GHXX]] ([[User talk:GHXX\|talk]]) 20:55, 7 September 2025 (UTC)
			:::@[[User:GHXX\|GHXX]] I'm so sorry, I keep merging this "optimized" LUT with a "full" LUT in mind.
			::: By "full" LUT, I mean essentially a decoder.
			::: We need to sort this out.
			::: The article talks about building a LUT only for true outputs. Thanks for reminding me, that the logic for non-true outputs is absent.
			::: But the "full" LUT has a logic for every single input combination (even if the corresponding output is off).
			::: <br>
			::: So... should we make two versions? [[User:DjSapsan\|DjSapsan]] ([[User talk:DjSapsan\|talk]]) 22:02, 7 September 2025 (UTC)

GHXX at 20:56, 7 September 2025

2025-09-07T20:56:10Z

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	:@[[User:GHXX\|GHXX]], yes to all, except one thing. Why LUTs should output only a single bit? I thought using it to produce a pattern of bits is a common application. [[User:DjSapsan\|DjSapsan]] ([[User talk:DjSapsan\|talk]]) 20:45, 7 September 2025 (UTC)		:@[[User:GHXX\|GHXX]], yes to all, except one thing. Why LUTs should output only a single bit? I thought using it to produce a pattern of bits is a common application. [[User:DjSapsan\|DjSapsan]] ([[User talk:DjSapsan\|talk]]) 20:45, 7 September 2025 (UTC)
	::No, usually it only produces one output bit (<math>Output1</math>), because all those chains get OR ed together, and unless the other output (<math>Output2</math>)) can be computed by <math>Output2 = Output1 \quad OR \quad (other chains)</math>, there is no way of computing that other output, as far as i am aware.		::@[[User:DjSapsan\|DjSapsan]], No, usually it only produces one output bit (<math>Output1</math>), because all those chains get OR ed together, and unless the other output (<math>Output2</math>)) can be computed by <math>Output2 = Output1 \quad OR \quad (other chains)</math>, there is no way of computing that other output, as far as i am aware.
	::I suppose you could maybe re-use the intermediate NOT outputs, but that is far from the general case from my experience. [[User:GHXX\|GHXX]] ([[User talk:GHXX\|talk]]) 20:55, 7 September 2025 (UTC)		::I suppose you could maybe re-use the intermediate NOT outputs, but that is far from the general case from my experience. [[User:GHXX\|GHXX]] ([[User talk:GHXX\|talk]]) 20:55, 7 September 2025 (UTC)

GHXX: /* Improve further */ Reply

2025-09-07T20:55:27Z

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	:@[[User:GHXX\|GHXX]], yes to all, except one thing. Why LUTs should output only a single bit? I thought using it to produce a pattern of bits is a common application. [[User:DjSapsan\|DjSapsan]] ([[User talk:DjSapsan\|talk]]) 20:45, 7 September 2025 (UTC)		:@[[User:GHXX\|GHXX]], yes to all, except one thing. Why LUTs should output only a single bit? I thought using it to produce a pattern of bits is a common application. [[User:DjSapsan\|DjSapsan]] ([[User talk:DjSapsan\|talk]]) 20:45, 7 September 2025 (UTC)
			::No, usually it only produces one output bit (<math>Output1</math>), because all those chains get OR ed together, and unless the other output (<math>Output2</math>)) can be computed by <math>Output2 = Output1 \quad OR \quad (other chains)</math>, there is no way of computing that other output, as far as i am aware.
			::I suppose you could maybe re-use the intermediate NOT outputs, but that is far from the general case from my experience. [[User:GHXX\|GHXX]] ([[User talk:GHXX\|talk]]) 20:55, 7 September 2025 (UTC)

DjSapsan: /* Improve further */ Reply

2025-09-07T20:45:03Z

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	Lmk what you think, please [[User:GHXX\|GHXX]] ([[User talk:GHXX\|talk]]) 20:31, 7 September 2025 (UTC)		Lmk what you think, please [[User:GHXX\|GHXX]] ([[User talk:GHXX\|talk]]) 20:31, 7 September 2025 (UTC)

			:@[[User:GHXX\|GHXX]], yes to all, except one thing. Why LUTs should output only a single bit? I thought using it to produce a pattern of bits is a common application. [[User:DjSapsan\|DjSapsan]] ([[User talk:DjSapsan\|talk]]) 20:45, 7 September 2025 (UTC)

GHXX at 20:31, 7 September 2025

2025-09-07T20:31:56Z

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	However, I suspect it needs some improvements.		However, I suspect it needs some improvements.
	How it can be improved in terms of phrasing and/or additional information ? [[User:DjSapsan\|DjSapsan]] ([[User talk:DjSapsan\|talk]]) 17:36, 7 September 2025 (UTC)		How it can be improved in terms of phrasing and/or additional information ? [[User:DjSapsan\|DjSapsan]] ([[User talk:DjSapsan\|talk]]) 17:36, 7 September 2025 (UTC)

			@[[User:DjSapsan\|DjSapsan]],

			Imo we should add that the AND-inputs may also be negated.

			And that a lut can only output a single bit, in the general case, and not a batch of outputs (instead of 'The output may be a single bit or a pattern of bits.')

			The "we can avoid delays by not using buffers" paragraph should imo be replaced with one that states that the not-inputs are generally fed through a fastbuffer first, so that no backpropagation happens.

			Lmk what you think, please [[User:GHXX\|GHXX]] ([[User talk:GHXX\|talk]]) 20:31, 7 September 2025 (UTC)