ASSIGH OPERATOR

This operator can assign a new value to the variable. By sign := it is naturally divided on two parts. Left part contains the destination variable name, and the right one - the expression, that must be calculated and put into variable. Being simple educational compiler, "COMPAS" doesn't allow to use brackets ( ). So right part can contain not more than one operation.

"COMPAS" always control the fact, that types of left and right parts must be identical.

In most common case assign operator consists from 3 steps:
get first operand (after :=)
calculation, if it's necessary: do operation or calculate function
save the result of previous actions.

Examples and operator structure

Examples of typical assign operators, available in "COMPAS", are exemplified below.

for INTEGER: a := 1; a := 1234; a := b; a := b + c; a := b - c; a := b * c; a := b DIV c; a := b MOD c; a := ABS(b); a := SQR(b);

for CHAR: d := 'w'; d := e; d := SUCC(e); d := PRED(e); a := ORD(d); d := CHR(a);

for BOOLEAN: f := FALSE; f := g; f := i > 100; f := g AND h; f := g OR h; f := g XOR h; f := NOT g;

Note, that right part of assignment can contain operation or function. All variables can contain index.

The typical structures of this operator are the following:

{variable} := {variable or constant}

{variable} := {variable or constant} {operation sign} {variable or constant}

{variable} := {function name} ( {variable or constant} )

Step 1: get first operand

First operand is always placed in the definite processor register. In "E97" case it is R1, in Intel - AX.

There are several variants of the first operator:

• (v1) simple variable (a)
• (v2) variable with concrete index (a[2])
• (v3) variable with another variable as index (a[i])
• (v4) local variable in procedure or function
• (v5) constant (23, 'y', TRUE)

(v3) is more complex variant, because compiler can't calculate array element address - it "doesn't know" the value of index variable. So it has to generate a little piece of code, that will do this calculation during run-time.

Variant (v4) will be discussed later.

(v5) is a simple case: it transforms to the only processor instruction, which put constant value into fix register. All constants for processor are presented in numeric form, of course.
"E97" uses different type of instruction for "long" (>15) and "short" (<=15) constants.

After this step in any case the first operand is placed in fixed register (R1 in "E97" and AX in Intel processor).

Step 2: do operation

This step is optional: when right part of the operator doesn't contain any operation or function it is just skip.

In "E97" and in Intel compilers this step is realized slightly different: in last case the value of the second operand is previously extracted to CX register and only then operation is made. I have to use this additional action because Intel's instruction set is more complex and not "symmetrical" for all operations. For example, it can add memory cell value to AX, but can't multiply them. By the way, multiplication and division have some other individualities in Intel.
"E97" as a virtual educational computer, has more simple and logical instruction set.

When the right part of the operator has operation, "COMPAS" generates code, which realize this operation with fix register (R1 in "E97" and AX in Intel) and the second operand. The result is always put to the same fix register. I want to remind You, that Intel code takes the second operand from CX register.

Step 3: save result