检查 Since this information is unavailable to the compiler,the machine code it generates for the expression a[j]just takes the address of a (that is, the first cell of the array),adds j to it,and retrieves the value stored in that address.(In fact,the same expression can also be written in C as *(a +j), which more closely reflects its implementation.)In contrast,when the PL/I compiler encounters the corresponding expression,it will also generate code to check that the index is indeed within the legal bounds.检查 Since this information is unavailable to the compiler, the machine code it generates for the expression a[ j ] just takes the address of a (that is, the first cell of the array), adds j to it, and retrieves the value stored in that address. (In fact, the same expression can also be written in C as ∗(a + j ), which more closely reflects its implementation.) In contrast, when the PL/I compiler encounters the corresponding expression, it will also generate code to check that the index is indeed within the legal bounds