HCS12 Assembly Programming - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 66

2.6.4 Loop Primitive Instructions
A lot of the program loops are implemented by incrementing or decrementing a loop count.
The branch is taken when either the loop count is equal to zero or not equal to zero, depending
on the applications. The HCS12 provides a set of loop primitive instructions for implementing
this type of looping mechanism. These instructions test a counter value in a register or accu-
mulator (A, B, D, X, Y, or SP) for zero or nonzero value as a branch condition. There are predec-
rement, preincrement, and test-only versions of these instructions.
The range of the branch is from $80 (2128) to $7F (127) from the instruction immediately follow-
ing the loop primitive instruction. Table 2.4 shows a summary of the loop primitive instructions.

Mnemonic Function Equation or Operation

Decrement counter and branch if = 0
(counter = A, B, D, X, Y, or SP)

counter (counter) − 1

If (counter) = 0, then branch;
else continue to next instruction.
counter

←

dbeq cntr, rel

Decrement counter and branch if ≠ 0 (counter) − 1

If (counter) ≠ 0, then branch;
else continue to next instruction.
counter ← (counter) + 1
If (counter) = 0, then branch;
else continue to next instruction.
counter

←

dbne cntr, rel (counter = A, B, D, X, Y, or SP)

Increment counter and branch if = 0
(counter = A, B, D, X, Y, or SP) ibeq cntr, rel

Increment counter and branch if 0 (counter) + 1

If (counter) ≠ 0, then branch;
else continue to next instruction.
If (counter) = 0, then branch;
else continue to next instruction.

≠ ←

ibne cntr, rel (counter = A, B, D, X, Y, or SP)

Test counter and branch if = 0
(counter = A, B, D, X, Y, or SP)

tbeq cntr, rel

Test counter and branch if 0

(counter = A, B, D, X, Y, or SP)

≠ If (counter) 0, then branch;

else continue to next instruction.

≠

tbne cntr, rel

Note: 1. cntr is the loop counter and can be accumulator A, B, or D and register X, Y, or SP.

2. rel is the relative branch offset and is usually a label.

Table 2.4 ■ Summary of loop primitive instructions

2.6.5 Implementation of Looping Constructs
The statement “for i 5 n1 to n2 do S” can be implemented as follows:
n1 equ xx ; start index (a nonnegative number)

n2 equ yy ; end index (a positive number)

…

i ds.b 1

…

movb #n1,i ; initialize loop index i

loop ldaa i ; check loop index

cmpa #n2 ; “

bgt next ; if all iterations have been performed, then exit

… ; perform the loop operation

… ; “

inc i ; increment loop index