HCS12 datasheet(220/452 Pages) FREESCALE

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Instruction Glossary

S12CPUV2 Reference Manual, Rev. 4.0

220

Freescale Semiconductor

Operation:

Effective Address

⇒ SP

Description:

Loads the stack pointer with an effective address specified by the program.

The effective address can be any indexed addressing mode operand

address except an indirect address. Indexed addressing mode operand

addresses are formed by adding an optional constant supplied by the

program or an accumulator value to the current value in X, Y, SP, or PC.

See 3.9 Indexed Addressing Modes for more details.

LEAS does not alter condition code bits. This allows stack modification

without disturbing CCR bits changed by recent arithmetic operations.

Operation is a bit more complex when LEAS is used with auto-increment or

auto-decrement operand specifications and the SP is the referenced index

address bus in the case of a load index instruction. In the case of a

pre-increment or pre-decrement, the modification is made before the index

modification would have taken effect after the address went out on the

address bus, so post-modification does not affect the content of the index

In the unusual case where LEAS involves two different index registers and

post-increment or post-decrement, both index registers are modified as

demonstrated by the following example. Consider the instruction LEAS

4,Y+. First S is loaded with the value of Y, then Y is incremented by 4.

LEAS

Load Stack Pointer with Effective Address

LEAS

CCR Details:

HIN

––––––––

Source Form

Address

Mode

Object Code

Access Detail

HCS12

M68HC12

LEAS oprx0_xysp

LEAS oprx9,xysp

LEAS oprx16,xysp

IDX

IDX1

IDX2

1B xb

1B xb ff

1B xb ee ff

(1)

1. Due to internal M68HC12 CPU requirements, the program word fetch is performed twice to the same address during this

instruction.

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