Original
Differences between Object Pascal and assembler expressions
The most important difference
between Object Pascal expressions and built-in assembler expressions is that
assembler expressions must resolve to a constant value -- a value that can be
computed at compile time. For example, given the declarations
const
X = 10;
Y = 20;
var
Z: Integer;
the following is a valid statement.
asm
MOV Z,X+Y
end;
Because both X and Y are constants, the expression X + Y is a convenient way of writing the constant 30, and the resulting instruction simply moves of the value 30 into the variable Z. But if X and Y are variables
var
X, Y: Integer;
-- the built-in assembler cannot compute
the value of X + Y at compile time. In this case, to move the sum of X and Y
into Z you would use
asm
MOV EAX,X
ADD EAX,Y
MOV Z,EAX
end;
In an Object Pascal expression, a
variable reference denotes the contents of the variable. But in an
assembler expression, a variable reference denotes the address of the variable.
In Object Pascal the expression X + 4 (where X is a variable) means the
contents of X plus 4, while to the built-in assembler it means the contents of
the word at the address four bytes higher than the address of X. So, even
though you’re allowed to write
asm
MOV
EAX,X+4
end;
this code doesn’t load the value of X plus
4 into AX; instead, it loads the value of a word stored four bytes beyond X.
The correct way to add 4 to the contents of X is
asm
MOV EAX,X
ADD EAX,4
end;
See also
译文
Object Pascal和汇编表达式间的不同
在Object Pascal表达式和内建汇编表达式之间最重要的不同之处在于，汇编表达式必须分解为一个常量值，该值在编译时可以被计算。例如，给出如下声明
const
X = 10;
Y = 20;
var
Z: Integer;
下面的语句是合法的：
asm
MOV Z,X+Y
end;
因为X和Y都是常量，表达式X + Y是写常量30的便利方法，因此指令简单地把值30传送到变量Z中。而如果X和Y是变量，
var
X, Y: Integer;
-- 那么内建汇编程序不能计算X + Y的值。这时，传送X与Y的和到Z中就需要使用如下代码：
asm
MOV EAX,X
ADD EAX,Y
MOV Z,EAX
end;
在Object Pascal表达式中，变量引用表示变量的内容（contents）。而在汇编表达式中，变量引用表示变量的地址。在Object Pascal中，表达式X + 4（这里的X是一个变量）表示X加4的内容，而对于内建汇编程序中，它表示一个单字的内容，该单字位于比X的地址高4个字节的地址处。因此，即使可以正确书写如下代码（语法正确且编译通过）
asm
MOV
EAX,X+4
end;
上面的代码也不能将X加4的值加载到AX寄存器中；上面的代码的含义是，加载了一个单字的值，该单字存储在距离X的地址有四个字节远的位置。把4加到X的内容中的正确方法是
asm
MOV EAX,X
ADD EAX,4
end;