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My 8-bit processor (not completely done)

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makorihi

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So for my semester project in my 11th grade electronics course, I decided to build a processor! It is a single core processor built up from logic gates, decoders, flip-flops, ROM, RAM, etc. It has 8-bit capability, but it isn't completely finished. As now now, I've only programmed 1 opcode (stor #,reg), but thats because once the hardware is down, programming that is easy . . The real problem I have (besides my MC6845 not working for the CRT display) is that I don't have any arithmetic at all. No ALU. And I can't find any 8 bit ALU's, only the 4-bit LS181. I was thinking of putting two of those together, but I don't know much about combining them and there aren't resources that show that specifically. (I also need to get this done by the end of the school year -_-).

Regarding the processor, here are some pics.
https://i26.tinypic.com/6707xd.jpg
https://i28.tinypic.com/2hxr47l.jpg


This is how I've been testing what vals are in the registers . .
https://i31.tinypic.com/14kbj36.jpg

Stor 2,0100;
Stor 1,0200;
https://i27.tinypic.com/14j3vc1.jpg

And here is a picture of my register bank (synonymous to a cache?)
https://i28.tinypic.com/2zfo3zn.jpg

Sorry for not labeling the chips in the last picture. I'll make sure to make a better diagram later on. But basically, COLUMN of chips on the far right are LS377 octal registers. The next two over are LS244 octal controlled buffers (line drivers?). And in front of those are two decoders, another octal latch, some open collector inverters so I can clear things, etc. The two chips on the lower left are both LS244 so that I can use the register bank's bus as a bidirectional bus. I'll be putting the ALU right under the 3x3 of chips.

Soooooo anyways. Info on the ALU(s)?
 
hi,
Have you considered using an old style 8bit maths co-processor for the ALU.?

EDIT:

I dont see any decoupling capacitors anywhere on your pcb's, you must have good decoupling or else you will get erratic operation within the logic....
 
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Using a co-processor would somewhat defeat the goal of making my own processer, even though its only for math. At that point I would way why didn't I just use an FPGA, and then everything becomes pointless.
 
"only the 4-bit LS181. I was thinking of putting two of those together, but I don't know much about combining them and there aren't resources that show that specifically. (I also need to get this done by the end of the school year -_-)."

The 4 bit 74181 design was specifically designed to be tied together to whatever word length you wished. Almost all the 16 bit minicomputers from the major companies in the 70s used the x-181 as their ALU. The Texas Instrument data sheet for this chip showed the necessary connection for extending the word length ( I think it was just a two pin daisy chain hook up between chips) and you should be able to find it somewhere on the web.

Good luck
Lefty
 
ericgibbs, thank you for he pdfs, though they are on general ALU design. And yes, I'll keep the decoupling in mind.

Leftyretro, the two pins that probably should be together are the Cn and the Cn+4. Does this seem right?
https://i29.tinypic.com/2lsy0px.jpg

Without finding the data sheet I can't validate. However there was also a companion chip the 74182 that was designed to augment multi 74181 as a look ahead carry generator to cut cycle time. Possible a data sheet on the '182 would show multi 181 applications clearer.

Lefty
 
Seems like a useful way, though it would require quite a bit of board space. Unless I absolutely HAVE to, I'd rather not use EPROM -_-
 
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