Re: The use of temp segments in array assigments.

Posted by cadet1620 on
URL: http://nand2tetris-questions-and-answers-forum.52.s1.nabble.com/The-use-of-temp-segments-in-array-assigments-tp4029054p4030140.html

Mat wrote

How about evaluating the right side of the let statement *before* calculating the address on the left side?

Reusing the same example as above:

// Evaluate the right side of the let statement
push constant 8 

// Evaluate the array offset
push local 2    // i

// Pop the previously-evaluated right-side into array[offset]
push local 0    // a
add            
pop pointer 1   // that -> a[i]
pop that 0      // a[i] = expression value

This is a functional way to generate code for array assignment.

Basically, it comes down to trade-offs between complexity of parsing versus complexity of code generation.

a[index-expression] = source-expression;

If you are using the simple recursive descent design presented in the book, then you have parsed and compiled the code for index-expression before you have encountered source-expression. The simple code generator has already written index-expression's code to the VM file.

One way to handle this is to compile index-expression to a memory buffer instead of the VM file. Then you can copy index-expression's code into the VM file at the point where you need it after parsing/compiling source-expression.

Another choice is to separate parsing and compiling so that your parser can return index-expression (or pointers to it in the source stream) so that your code generator can compile index-expression when it gets to the point where it needs to be evaluated.

And thanks for this amazing book, I've been completely hooked since I started!

Yes, it can be quite addicting; I've been hooked since I first encountered the course 6 years ago!

--Mark