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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You will all no doubt be familiar with the 74sl181 series logic integrated circuits, they provide the glue logic for countless projects. One of the more famous of these devices is thea cascadable 4-bit arithmetic logic unit, or ALU. An ALU is the heart of a microprocessor, performing its operations.

The appeared in many lates and earlys minicomputers, will be familiar to generations of EE and CS students as the device they were taught about ALUs on, and can now be found in some home-built retrocomputers. Why on earth you might think would an ALU need to do that?

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. He explains the function required to perform this operation, and suddenly the unusual extra function makes sense.


Addition is transformed from a serial process to a parallel one, with a consequent speed increase. We learned something from the article, and so may you.

TTL-series ALU demonstration

If you would like to see a in action, take a look at this 4-bit 74 logic single board computer. Allu board is still around here somewhere. Many of the early minicomputers had either a or an AMD bit-slice design. I suspect that these posts only attract old EE farts by sparking nostalgic feelings. Speaking from experience… these posts work just like fly paper for me. There is a decent-sized group of retrocomputing afficionados not fueled by nostalgia alone.

These circuits contain interesting, pretty elegant solutions for problems of the day that some see as apu. Hacked the root password and had a 16 terminal miniframe computer in the house where I started to learn a Unix.

Explaining The Operation Of The ALU | Hackaday

74lw181 I then played with the old database as well as the financial software that was still on it. Finally, the imaginary HDL I learned in college is real, and can be used to create actual hardware! Since the FPGA were eventually used in production hardware, there are now versions that are hybridized with built in flash and ARM cpus.

There are even free Verilog to gcc C compilers around that will auto-generate modular logic testing software source code. I looked at this and thought it would be a good way to learn schematic entry.


It would be interesting to scale this to 8 bits though, through in some registers and addressing and see what you get.

I think I even have a few bigger chips, but not many. There are some new players in the field as well. I along with 2 other techs built one in 74xx and 4xxx logic. Needless to say, it worked, and could do 4-bit math easily. I miss those days. You are commenting using your WordPress.

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The 74LS181 ALU

Find it, write it up, post it to the Hackaday tips line! Mr Benchoff, please keep these coming!

Not by an even bigger chance. Leave a Reply Cancel reply Enter your comment here Fill in your details below or click an icon to log in: Email required Address never made 7ls181.