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Jorge Nuno
Very interested

Joined: 11 Jun 2007
Posts: 329
Location: Azeitão, PT

 Posted: Mon Dec 24, 2007 10:00 pm    Post subject: I made the project for one 4bit cpu 2 years ago, but it never got away from the paper sheet... the alu had only nand, add and a comparator.
TmEE co.(TM)
Very interested

Joined: 05 Dec 2006
Posts: 1947
Location: Estonia, Mahtra village

 Posted: Mon Dec 24, 2007 10:13 pm    Post subject: I'd go crazy if I had to do a ALU (and I have no idea where to begin)..._________________Mida sa loed ? Nagunii aru ei saa
Jorge Nuno
Very interested

Joined: 11 Jun 2007
Posts: 329
Location: Azeitão, PT

 Posted: Mon Dec 24, 2007 11:51 pm    Post subject: Begin with the instruction decoding scheme, and the registers/acumulator layout, the alu will apear by itself as soon you decide and decode math instructions... Something really basic... Then update the design to make it more powerfull if you want (I didn't)
HardWareMan
Very interested

Joined: 15 Dec 2007
Posts: 542
Location: Kazakhstan, Pavlodar

Posted: Tue Dec 25, 2007 7:13 am    Post subject:

 TmEE co.(TM) wrote: I'd go crazy if I had to do a ALU (and I have no idea where to begin)...

Simple summator is (A XOR B) as Result and (A AND B) as Carry.
A B C F
0 0 0 0
0 1 0 1
1 0 0 1
1 1 1 0
Maybe you can use ALU chip. Even in USSR IC logic was one. К155ИП3 => 74xx181. It is 4 bits, but it can be expanded. And Carry flag supported.
Schematic diagram:

Opcodes table:

PS On pictures "B" = High (logic 1) and "H" = Low (logic 0)
Chilly Willy
Very interested

Joined: 17 Aug 2007
Posts: 1955

 Posted: Tue Dec 25, 2007 10:03 am    Post subject: I just used the TTL ALU chip (4 bits wide)... microcoding was enough work to satisfy my cravings. I was able to clock it up to the speed of the EPROMs. I used a two-stage pipe to make it easier to handle the output delay from the ALU. Also made the flow control of the microcode easier. One guy I knew did the ALU as discrete parts. His proto board was about three times the size mine was. A lot of stuff in an ALU. I checked with the professor before using the ALU to see what the limit was on parts we could use. A lot of folks in the class used a Xilinx part.
TmEE co.(TM)
Very interested

Joined: 05 Dec 2006
Posts: 1947
Location: Estonia, Mahtra village

 Posted: Tue Dec 25, 2007 12:35 pm    Post subject: Well, I'm not gonna make any CPUs, I'll rather spend a little on a 68K or Z80 (or whathever I want)... it will be lot cheaper (and lot less fun)._________________Mida sa loed ? Nagunii aru ei saa
Chilly Willy
Very interested

Joined: 17 Aug 2007
Posts: 1955

 Posted: Fri Dec 28, 2007 7:33 am    Post subject: If you want to do 68k stuff these days, you'd be better off using a ColdFire MPU. Super-fast 68k-ish core with DMA and all sorts of good stuff in one package.
TmEE co.(TM)
Very interested

Joined: 05 Dec 2006
Posts: 1947
Location: Estonia, Mahtra village

 Posted: Fri Dec 28, 2007 1:25 pm    Post subject: First I finish 12th grade, and then go to technology school and then I'll maybe start doing some more serious hardware stuff... now I just don't have enough knowledge..._________________Mida sa loed ? Nagunii aru ei saa
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