ABEL Designing a 4-to-1 Multiplexer

[EvLer]

I have this ABEL file but can't figure out where 0, 1, 2, 3 come from in "when-then" statements...

"The following ABEL code demonstrates how to implement a combinatorial multiplexer.
" 4 to 1 multiplexer design

SEL0..SEL1 pin;
A,B,C,D pin;
MUX_OUT pin istype 'com';
SEL = [SEL1,SEL0];

equations

when SEL==0 then MUX_OUT = A;
else when SEL==1 then MUX_OUT = B;
else when SEL==2 then MUX_OUT = C;
else when SEL==3 then MUX_OUT = D;

thanks beforehand.

 

[berkeman]

SEL = SEL[1:0] are the two selector pins. The 2-bit code on those pins goes from 0 to 3, and that MUXes one of the four inputs A-D to the single output pin. When SEL[1:0] = 0b01 = 0x1, for example, input B is routed to the output. Make sense?

 

[EvLer]

hmmm... sorta... so i have to look at the select lines' binary code to get the numbers...ok, makes sense.
But how do you know it's B? i missed that part...

 

[berkeman]

SEL0..SEL1 pin;
A,B,C,D pin;
MUX_OUT pin istype 'com';
SEL = [SEL1,SEL0];
equations
when SEL==0 then MUX_OUT = A;
else when SEL==1 then MUX_OUT = B;
else when SEL==2 then MUX_OUT = C;
else when SEL==3 then MUX_OUT = D;

(apologies for the spacing of the following table -- no font control...)
SEL[1:0] Input-->Output
====== ===========
00 A-->Output
01 B-->Output
10 C-->Output
11 D-->Output