One of the more famous of these devices is the , a cascadable 4-bit arithmetic logic unit, or ALU. An ALU is the heart of a microprocessor. The ALU (arithmetic/logic unit) chip powered many of the minicomputers of the s: it provided fast 4-bit arithmetic and logic functions. Taming the 74LS ALU The 74LS is an old chip. I first found it in my Motorola LS-TTL handbook. At first sight it seems magical.

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I announce my latest blog posts on Twitter, so follow me at kenshirriff.

I ended up using a ‘ Similar projects worth following. If you have a Boolean function f A,B on one-bit inputs, there are 4 rows in the truth table.

The 74LS181 ALU

If your interested I did not solve the remapping by hand. It looks like this ALU or related was used in at least one arcade machine in – as part of a bit processor – by a company named Cinematronics: Project Owner Contributor C61 Julian.

The answer lies in the way it performs carrying while adding, a significant speed-up can be achieved over ripple carrying along a chain of adders if it can be ascertained whether a bit addition might generate a carry bit. You are commenting using your WordPress. The main difference with my version is it will be slower to propagate. Here is the result: Not hard, even found a schematic error – I wonder if anyone else has found it as well?


Why on 74la181 you might think would an ALU need to do that? Your Email Remember me. The datasheet for the ALU chip shows a strange variety of operations. Addition is transformed from a serial process to a parallel one, with a consequent speed increase.

I opened up atook die photos, and reverse engineered its TTL circuitry.

Explaining The Operation Of The 74181 ALU

PeterJ 16k 20 42 I was trying to test the ALU design presented previously when I realized something was wrong with the comparator. Other parts require the data line to be grounded. Similar Projects The Hackaday Allu. One example of a modern carry lookahead adder is Kogge-Stone. First the eight codes I have selected: The shiny golden regions are the metal layer, providing the chip’s internal wiring.

You can make a copy for you project page if you like. The logic functions are a bit more subjective. You have run afoul of the peculiarities of TTL circuitry.


TTL-series ALU demonstration

Note M and S3 are now always high. The final schematic also has logic to read the written data. If you send you version of the design I will look it over, if you like. I coded a 1 bit ALU for: It implements addition, subtraction, and the Boolean functions you’d expect, but why does it provide several bizarre functions such as “A plus A and not B “?

Explaining The Operation Of The ALU | Hackaday

The solution is easy to adapt. Virtual Machines of the Past and Future “. I seem to remember some similar stuff in the high loop of the IFR service monitor, theand had the same one I think.

I have used a version with my 4 bit CPU but have not tested it yet. The carry from each 74ls1811 position can be computed from the P and G signals by determining which combinations can produce a carry.

Thanks for the great write-up! The implements a 4-bit ALU providing 16 logic functions and 16 arithmetic functions, as the datasheet below shows.