How to tell value of bit [a] (pg. 109) in the Architecture?

[AnalyticLunatic]

I am working on creating my Assembler using C#, and I can accurately specify the first 3-bits (left-most) by deciding if the command is an A/C-command. And on the right-side I can clearly determine if/what the [jump] command should convert to and the same for the [dest].

Where I am getting a little stuck is my 4th bit from the left, [a], as seen on page 109: http://nand2tetris.org/chapters/chapter%2006.pdf

a/c  |comp                    |dest      |jump
0 v v |                           |            |
1 1 1 |a c1 c2 c3 c4 c5 c6|d1 d2 d3|j1 j2 j3

Can anyone assist me with this in how I determine whether this [a] is 0/1? Once I determine A I can write appropriate switch statements for my [comp] command conversions.

Thanks in advance!

[cadet1620]

Look at the column headers in the table (c1-c6) at the bottom of page 109. that shows how the 'a' bit is affected by the computation mnemonic.

--Mark