How to Program a CAD / G-code/Machine commands freesource code?

hello;

thanks to Dan Hudgins for referencing this forum for my project.
I building a micro CNC machine
I want to write SW for simple CAD front /to G-code / to Machine commands

the information regarding the programming is very limited to some
g-code description.

I think those steps are needed to complete a sw for a cnc?

1- data structure in the CAD part, I can build a simple CAD GUI, but
what is the concept for the data structure.
do we make all the input from the mouse as line and arc as was
suggested to me. then use these in a data list to create G-code
listing that can be reviewed by the use?

2- how does the milling of a surface is translated from a CAD data
structure to a G-code.

what is the data structure for a cad surface suitable for translation
to g-code? line and arc will not represent surface shape milling.
Extrusion will allow a line or arc to be shaped and we can define that
structure as a way to be translated into many lines and arcs to match
the shape.

what is the data structure for extrusion to can be translated to
G-code easily?

3- usually it is the format of the cad output that translated to
g-code. the only reason to have a cad data structure (or format
output to file) is to allow the automatic translation to G-code.
the reason of g-code translation step is to allow the user to review
and correct the automatic translation. or to allow the use of direct
g-code if needed for simpler shapes

4- examples of how to translate g-code to machines codes?

I have seen some old very elementary pascal simple incomplete sample
for the parallel port. the translation is for a simple single line or
arc motion. which was is straight forward with some thought to it for
efficiency.

since G-code is a line and arc operation then we can connect between
cad structures and g-code translation as each is a name for subroutine
that contains the machine code to perform the motion according to the
motor controller specifics needs as step and direction for parallel
port as specific rs232 serial port commands for a commercial
controllers needs.

5- how to work out the surface data structure into motion into tens of
lines or arcs motion to create a surface from a g-code?

g-code will give only line and arc. so we need to create tens of
g-code lines and arcs. is there is efficient way to do that or that
simple brute force with a very long list of lines showing the g-codes

do we put the g codes in a loop which represent the structure some how
to change the x,y,z as needed for each line.

or we use a straight line operations where the z change for every step
(within the line) to make a curved surface. how do we automate such
a loop = how do we have a formula to give the surface height for a line?

as I see now brute force is not suitable for surface. do we use Nurbs
operation to represent a line analytically then we use that equation
to give the z for the curve shape?

is that why Nurbs is used in surface application.

6- but simple engraving SW does not use Nurbs, what is the concept
they use in the SW? for engraving.

7- what available free codes are available in the community to use or
review?

those are lot of question, but your help is welcomed

elwolv