## IntroductionA little while back I found that it was getting hard to source Dil-based ALUs, for example the 74LS181, or 74LS381 integrated circuits. So, I decided to implement my own on the simplest PAL chip I possibly could, a GAL20V8. The GALU426 is thus a 4-bit ALU with carry in and carry out.GALU426 supports 2 arithmetic functions: Cout:Res = A+B+Cin and Cout:Res = A-B-Cin. GALU426 supports 6 logical functions: Cout:Res = 0; Cout:Res = ~B; Cout:Res = A XOR B; Cout:Res = A XNOR B; Cout:Res = A NOR B; Cout:Res = A AND B. GALU426 is suitable for hobby electronics projects, such as a home-brew TTL CPU. I'll quite happily sell you a pre-programmed GALU426 for £5 (ex P&P) as of 11/12/09! ## DescriptionGALU426 is implemented on a 20V8 GAL device using 12 inputs and 8 outputs.The 12 inputs are A3..A0, B3..B0, Cin, F2..F0. The 8-outputs are Cout3..Cout0, Sum3..Sum0.
Note, the subtract with carry is a Z80-style subtract with carry (a-b-cy) not a subtract with borrow (a+~b+cy). More details to follow! |