Machine Problem 2 Supplement

22C:18, Spring 1996

Due Tuesday Feb. 27, 1995, in discussion

Notice: Extra credit will be given for support of full 32 bit signed operands. Full credit will be given for those who limit the stack to 16 bits.

Notice: The MAS system conforms to the sidenotes in the textbook on the Macintosh programming environment; see particularly the note on page 235. This means:

Your program consists of two segments, the code segment and the data segment. These are not necessarily contiguous in memory.
References to labels in the code segment are assembled using PC relative addressing.
Register A5 points to the base of the data segment. References to labels in the data segment are automatically converted to indexed form using A5 as the index register.
DC directives referencing labels in either the code or data segment do not contain the absolute addresses of the addressed items. Rather, they contain the displacement of the addressed item from the start of the indicated segment, and it is up to the programmer to use indexing or other forms of address arithmetic to convert these relative addresses to absolute addresses.

As a consequence of this, the following two statements that would be equivalent under the "normative" view of how assemblers should function are not equivalent under MAS!

		LEA	X,A0
		MOVE.L	#X,A0

If X is a label in the data segment, the following code is equivalent to the above LEA statement:

		MOVE.L	#X,A0
		ADDA	A5,A0

If X is a label in the code segment, and if A4 is a pointer to the base of the code segment, the following code is equivalent to the above LEA statement:

		MOVE.L	#X,A0
		ADDA	A4,A0

To make A4 point to the start of the code segment, make your code segment begin as follows:

	START:	LEA	START,A4

Note that, in all of the above, if XP is defined as XP: DC.L X, then MOVE.L #X,A0 is equivalent to MOVE.L XP,A0.