APOLLO is a homemade didactic 4-bit RISC PROCESSOR about the ALU and made of discrete TTL logics and bipolar Memories. 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. Please show how you are interconnecting the two devices, particularly the carry from the LSB to the MSB device. It looks like that may not be.
|Published (Last):||28 August 2009|
|PDF File Size:||19.8 Mb|
|ePub File Size:||9.51 Mb|
|Price:||Free* [*Free Regsitration Required]|
The least significant nibble in the example represents the max positive number with 4 bits, so when incremented by one goes to zero Is there any way to 71481 work with only positive numbers? Or solve this issue in any other way.
It looks like 774181 may not be wired correctly. The 74LS doesn’t deal with signed numbers, just unsigned integers. It is up to you to interpret the values in the form you need. Second, combining and to get is very strange, but for instance, if you are applying to the A inputs, and to the B, and a Mode select ofyou’ll get A – 1, or So I’d recheck your select lines and your inputs.
I’d especially check things like a swap between pins 2 and 3, which would confuse A0 and S0. And the upper stage which should receive a carry-in looks like it is tied low instead.
Your problem is not related to signed vs.
The adder works the same on each bit stage regardless of your interpretation of what the binary represents. That’s the beauty of 2’s complements. The logic circuits for each bit stage is the same generally. You just interpret the values differently. All the answers helped me in finding the bug. There are 3 file versions of the chip, but the right one is strangelly the v2 not the v3.
Explaining The Operation Of The 74181 ALU
This works fine with most of addings I tried, but some don’t work. I’m frustrated because with this problem the ALU is completelly usesless.
Show a schematic 2. That book link isn’t working, at least for me. Link to data sheet so people don’t have to look it up.
Hosting image sites are blocked at job: Posted image at project page it’s the fourth image in gallery: Edited the question and added a link to datasheet pdf. Just occured me that this can be a problem on this specific implementation of for Logisim I downloaded circuit library add-on from this guy: It’s not official part of the Logisim. The two devices should be interconnected similar to this: First, get rid of the 2nd Concentrate on getting a 4-bit result and understanding it.
WhatRoughBeast 49k 2 28 Entrepreneur 2 5.
You are right about the two points. I misconnected the Carry input and output actually inverted them between Hi and Lo nibbles. And the two’s aalu binary operations are exactly the same as unsigned binary, it’s just the way to interpret the numbers that changes now I know, at the moment of the question i didn’t know.
Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password.
TTL-series ALU demonstration