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Usefull stuff made with TTL logic

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per

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I recently made an 4 bit adder and a 4 bit subtracter out of TTL logics. They can be extended to as many bits you would like, and what I would like to know is if it will work (I haven't too much experience with TTL gates, but it looks like it is straightforward in one of my books).

I hope to some day make a whole processor and (a very primitive) computer system out of TTL logics one day...

Any comments?
 

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per said:
I recently made an 4 bit adder and a 4 bit subtracter out of TTL logics. They can be extended to as many bits you would like, and what I would like to know is if it will work (I haven't too much experience with TTL gates, but it looks like it is straightforward in one of my books).

I hope to some day make a whole processor and (a very primitive) computer system out of TTL logics one day...

Any comments?
Click to expand...
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Why not try it out on one of the free logic simulators out there?

m
 
A friend of mine did an entire TMS9900 system using TTL to replace all of the specialized logic chips many years ago. He started the project when he accidentally blew up a 99/4A and decided to rebuild it using whatever parts survived and replaced the rest with his homebrew TTL substitutes for the 9900 microprocessor and the 9901 interrupt controller. He lost the OS chips too, so he built a homebrew OS for it. He brought the huge board with his system on it to computer faires in Germany for many years. Not sure what happened to him or it in the interim, as I haven't seen him in about 10 years.
 
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Have a look at homebrewcpu.org

Also you can turn an adder into a subtracter by inverting the input you want to subtract and setting the carry flag. You can do the inversion selectively by using XOR gates. This is two's complement arithmetic. (fingers crossed that piece of information is correct...)
 
Thrashbarg said:
Have a look at homebrewcpu.org

Also you can turn an adder into a subtracter by inverting the input you want to subtract and setting the carry flag. You can do the inversion selectively by using XOR gates. This is two's complement arithmetic. (fingers crossed that piece of information is correct...)
Click to expand...

yep, in 2s compliment, to get a negative number, invert the number and add 1 (set the first carry-in)
 
Thrashbarg said:
Have a look at homebrewcpu.org

Also you can turn an adder into a subtracter by inverting the input you want to subtract and setting the carry flag. You can do the inversion selectively by using XOR gates. This is two's complement arithmetic. (fingers crossed that piece of information is correct...)
Click to expand...

Hi
To make that work, he'd need to add a carry in.
Dwight
 
I've actually fgured out a lot more how the processor s gona be. I can't post too much here snce my Networking card has some problems (I get it workng 1 out of 20 tries :( ).

The processor will be a 4-bt processor with 16 bits/byte. The reason why I have 16 bit/byte is because don't want to do multipile reads for one Op code (hence, each opcode is 16 bit). Input is buffered and latched on clock high, whle output is processed on clock low.

There will be 2 flags, a carry flag and a zero flagg, one 4*4 bit regster for general purpose use, one 4*4 bit register to buffer data on memory write/reads (can also be used as general purpose register if no mem R/W is done), one 4 bit Accumulator, and a 12 bit Instruction Pointer.

I'll be postng more soon. BTW, do anybody know any good TTL logic simulator?

Input/Output will first of all be swches and leds on a front panel, memory will be on seperate cards on a I/O bus.

*edit*
I'm only 16years old, so I don't have too many 74## chips avalible, but I would really enjoy if I could actually make the project work (at least in a simulator)
 
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