HCS12 datasheet(414/452 Pages) FREESCALE

414 / 452 page

S12CPUV2 Reference Manual, Rev. 4.0

414

Freescale Semiconductor

B.6.3 Accumulator Offset Indexing

This indexed addressing variation allows the programmer to use either an

8-bit accumulator (A or B) or the 16-bit D accumulator as the offset for

indexed addressing. This allows for a program-generated offset, which is

more difficult to achieve in the M68HC11. The following code compares the

M68HC11 and CPU12 operations.

The CPU12 object code is only one byte smaller, but the LDX # instruction

is outside the loop. It is not necessary to reload the base address in the

index register on each pass through the loop because the LDAA B,X

instruction does not alter the index register. This reduces the loop execution

time from 15 cycles to six cycles. This reduction, combined with the 25-MHz

bus speed of the HCS12 (M68HC12) Family, can have significant effects.

B.6.4 Indirect Indexing

The CPU12 allows some forms of indexed indirect addressing where the

instruction points to a location in memory where the address of the operand

is stored. This is an extra level of indirection compared to ordinary indexed

addressing. The two forms of indexed indirect addressing are 16-bit

constant offset indexed indirect and D accumulator indexed indirect. The

reference index register can be X, Y, SP, or PC as in other CPU12 indexed

addressing modes. PC-relative indirect addressing is one of the more

common uses of indexed indirect addressing. The indirect variations of

indexed addressing help in the implementation of pointers. D accumulator

indexed indirect addressing can be used to implement a runtime computed

GOTO function. Indirect addressing is also useful in high-level language

compilers. For instance, PC-relative indirect indexing can be used to

efficiently implement some C case statements.

C6 05

CE 10 00

A6 00

26 F7

LDAB

#$5

[2]

LOOP LDX #$1000 [3]

ABX

[3]

LDAA

0,X

[4]

DECB

[2]

BNE

LOOP

[3]

C6 05

CE 10 00

A6 E5

04 31 FB

LDAB

#$5

[1]

LDX

#$1000

[2]

LOOP LDAA

B,X

[3]

DBNE

B,LOOP

[3]

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