Page 119 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

96 Chapter 4 Assembly Language Programming

1 1 0000 ORG $868
2 2 0868 * this program squares the number N between 0 and 15
3 3 0868 0001 N: EQU I
4 4 0868 NSQ: DS.B 1
5 5 0869 00010409 TABLE: DC.B 0,1,4,9,16,25,36,49,64,81
086D 10192431
0871 40516479
0875 90A9C4E1
6 6 0879 CE0869 LDX #TABLE ; POINT X TO TABLE
7 7 087C C601 LDAB #N ; PUT N INTO B
8 8 087E A6E5 LDAA B,X ; PUT N**2 INTO A
9 9 0880 7AO868 STAA NSQ ; STORE RESULT
12 12 088F 00 BOND
Figure 4.4. Assembler Listing for the Program Square

4.3 Mechanics of a Two-Pass Assembler

Some questions will soon arise about how symbolic addresses can be used without error.
These questions have to be answered in terms of forward references, and their answers
have to be understood in terms of how an assembler generates its output in two passes.
Although we do not study how to write an assembler program (except in problems at the
end of the chapter), we do want you to get a feeling for how it works so that you can
understand how forward references are limited by what a two-pass assembler can do.
How does an assembler work? We begin by reading down through the instructions,
called a pass. The first pass builds a symbol table, a list of all the symbolic addresses
for labels and their values. The second pass will generate both the listing that shows the
effects of each assembler line and the object code that is used to run the program.
We have earlier used the symbol "*" for the location counter. The location counter
keeps track of the address where the assembler is when it is reading the current assembly-
language statement, somewhat like the program counter does when the program runs.
The location counter symbol "*" is always the address of the first byte of the instruction.
In both passes, the location counter advances as code is generated.
The assembly-language program of Figure 4.5 finds all the odd, negative, 1-byte
integers in the array COLUMN and puts them into the array ODD. On the first pass, the
ORG statement sets the location counter to $800. Thus the label N has the value $800,
the label M has the value $801, the label COLUMN has the value $802, and the label ODD
has the value $834. The instruction CLR M will take three bytes (and we know what
they are), the instruction LDAB N will take three bytes (and we know what they are), and
so forth. Similarly, we see that the first byte of instruction
LOOP: LDAA 1,X+
will be at location $872. Thus the symbolic address (the container) LOOP has the value
$872. Continuing in this way, we come to
BPL JUMP