Programming for Microprocessors

Appendix 3 185

(c) The subscript operators ! and !!. These are both mapped into a

sequence of other operations. The indirection operator outside the

brackets is dependent on the mode of the operand on the left-hand

side of the ! or !!. The

!

maps as follows:

DEC (X, Y) ADDRESS, (I, J) BYTE;

X!I is equivalent to *(@X + I)

I!J is equivalent to %(@I + J)

XÜIis equivalent to *(X + I)

I!!J is equivalent to %(I + J)

Hence the first element of a table has subscript zero and subse-

quent bytes go up in steps of 1. In the above example, Y equals

X!2,

J equals X!5, and J also equals I!l. Obviously, I equals I!0.

The subscript operator can take any sub-expressio ...