Page 298 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

9.4 Loop Statements, Arrays, and Structs 275

In Figure 9.10 a for loop has an initialization expression, shown first in the for
list of expressions; a test expression, shown in the middle; and a loop termination
expression, shown last:

* 3: for(j = k = 0; j != i; j++) k += j;
CLRB ; j in D
CLRA
TFR D, X ; k in X
BRA LI ; do the test before the loop is done once
LO : LEAX D, X ; this is the statement that is executed in the loop
ADDD #1 ; this is the expression done after each loop is done
LI: CPD $0800 ; this is the loop test
BNE LO
A while loop has a test expression that is executed before the loop is executed once:
* 4: while(j != 0) k += —j;
BRA L3 ; do the test first
L2 : SUED #1 ; these are the two statements
LEAX D, X ; that are executed in the loop
L3 : TBNE D,L2 ; this is the loop test
A do while loop has a test expression that is executed after the loop is executed:
* 5: do k += j—; while (j != i);
L4 : LEAX D, X ; these are the two statements
DEX ; that are executed in the loop
CPD $0800 ; this is the loop test
BNE L4

Figure 9.11 illustrates nested for loops and the two-dimesional array index
addressing mechanism. This example shows how loop statements can themselves be
loops, in a nested loop construction, and how optimizing compilers make loops more
efficient. The outer for loop, for (i = sum =0 ; i < 10; i++) is encoded as an
initialization:

CLRA ; generate 0
CLRB ; in high and low bytes
STD $08 IE ; store to clear sum
STAB 1, SP ; store to clear i

and by the outer loop termination:
INC 1, SP ; count up
LDAA 1, SP ; get the variable to be tested
CMP A #10 ; compare against 10
BCS *-40 ; loop as long as i is less than 10
CMPA #3 ; check if another iteration is to be done
BCS *-30 ; if so, branch to the instruction following the initialization