68HC705JB2 datasheet(28/128 Pages) NXP

28 / 128 page

GENERAL RELEASE SPECIFICATION

August 28, 1998

CENTRAL PROCESSING UNIT

MC68HC705JB2

3-2

REV 1.1

3.2

ACCUMULATOR (A)

The accumulator is a general purpose 8-bit register as shown in Figure 3-1. The

CPU uses the accumulator to hold operands and results of arithmetic calculations

or non-arithmetic operations. The accumulator is not affected by a reset of the

device.

3.3

INDEX REGISTER (X)

The index register shown in Figure 3-1 is an 8-bit register that can perform two

functions:

•

Indexed addressing

•

Temporary storage

In indexed addressing with no offset, the index register contains the low byte of

the operand address, and the high byte is assumed to be $00. In indexed

addressing with an 8-bit offset, the CPU ﬁnds the operand address by adding the

index register content to an 8-bit immediate value. In indexed addressing with a

16-bit offset, the CPU ﬁnds the operand address by adding the index register

content to a 16-bit immediate value.

The index register can also serve as an auxiliary accumulator for temporary

storage. The index register is not affected by a reset of the device.

3.4

STACK POINTER (SP)

The stack pointer shown in Figure 3-1 is a 16-bit register. In MCU devices with

memory space less than 64 Kbytes the unimplemented upper address lines are

ignored. The stack pointer contains the address of the next free location on the

stack. During a reset or the reset stack pointer (RSP) instruction, the stack pointer

is set to $00FF. The stack pointer is then decremented as data is pushed onto the

stack and incremented as data is pulled off the stack.

When accessing memory, the ten most signiﬁcant bits are permanently set to

0000000011. The six least signiﬁcant register bits are appended to these ten ﬁxed

bits to produce an address within the range of $00FF to $00C0. Subroutines and

interrupts may use up to 64($C0) locations. If 64 locations are exceeded, the

stack pointer wraps around and overwrites the previously stored information. A

subroutine call occupies two locations on the stack and an interrupt uses ﬁve

locations.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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