I. File list
------------
tema1.asm contains the main function of the program that evaluates an AST
using the EvaluateAST subroutine
strtonum.asm contains util functions used in EvaluateAST, i.e.
StringToNumberSubroutine and StringLengthSubroutine
Makefile used to assemble and link the program resulting 'tema1' executable
II. Program flow
----------------
After the program calls the getAST subroutine to parse the string given at stdin
to an AST, EvaluateAST is called. It will try to evaluate the expression in the
tree using the following algorithm:
if(node is number) then
return node
else if(node.content is operator) then
return left_subtree `operator` right_subtree
endif
where operator can be +, -, * or /. To accomplish this, the CheckIfSubtreeIs
Operator will be called and the correct jmp will be performed. If the node
is a number then the program jumps to ReturnNumericValue and EvaluateAST
returns the corresponding number using StringToNumberSubroutine. If the node
is an operator then the program continues to execute instructions from
ReturnOperationResult. Here the subroutine is recursively called for right
subtree and left subtree. They can be found at offset 8, respectively 4 from
the node_ast pointer. In the end comparing the operator with +, -, * and /
will decide which operation to produce. After this the function returns the
result.
III. Utils from strtonum
------------------------
A. StringtoNumberSubroutine
This subroutine tries to compute a number represented as string to an int type
value. Firstly the subroutine checks if the number starts with a minus sign
so that at the end of the subroutine the number can be accordingly set. Then
it computes the length of the string so that it can iterate through each
byte until a NULL byte is encountered. The number is calculated using the
following formula: 'number = number * 10 + (digit - '0')' where '0' is the
ascii value of character '0'. After this the number is returned and multiplied
with -1 if it contained a '-' sign.
B. StringLengthSubroutine
The code in this subroutine simply increments <ECX> until a NULL byte is
encountered in the string from <ESI>, producing in the end the length
of the string. Of course there are more optimized methods to compute
the length of a string but this is the simplest and can be easly understood
when debugging the program.
IV. Flaws and corner cases
--------------------------
No optimisation was run on the present code so the size of the program and
the execution time is relatively high for such an algorithm. Also the program
does not check if the resulting values can be fit in a 4-byte register, causing
a possible overflow.
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