Variables in memory and constants

Two numbers are written into memory by the assembler and a third one is written into an instruction. The program reads the two out of memory, adds the third, and writes the total back into memory, which is where you read the answer.

There are sixteen registers, and a program with more than sixteen things to remember keeps the rest in memory. dc is what puts a value there before the first instruction of the program runs.

TAX equ 20

    move.l price, d0        ; total = price
    add.l shipping, d0      ; total = total + shipping
    add.l #TAX, d0          ; total = total + TAX
    move.l d0, total        ; write the answer back into memory

    org $2000
price:    dc.l 250
shipping: dc.l 35
total:    ds.l 1

The org $2000 puts the data at a fixed address, so the memory panel shows the same three longs wherever the code above them ends. Type 2000 in its address box:

label	address	before	after
price	$2000	000000FA	unchanged
shipping	$2004	00000023	unchanged
total	$2008	FFFFFFFF	00000131

dc.l writes a value; ds.l 1 reserves four bytes and writes nothing, which is why total reads FFFFFFFF before the program runs. That is what unwritten memory reads here, and it is worth seeing once: the room existed all along and nobody had put anything in it.