HC705J2 datasheet(21/78 Pages) MOTOROLA

21 / 78 page

MOTOROLA

CENTRAL PROCESSOR UNIT

MC68HC705J2

4-2

4.1.1 Accumulator

The accumulator is a general-purpose 8-bit register.

The

CPU uses the

accumulator to hold operands and results of arithmetic and nonarithmetic

operations.

4.1.2 Index Register

The 8-bit index register can perform two functions:

• Indexed addressing

• Temporary storage

In indexed addressing, the CPU uses the byte in the index register to determine

the conditional address of the operand.

See 4.3.5 Indexed, No Offset,

4.3.6 Indexed, 8-Bit Offset, and 4.3.7 Indexed, 16-Bit Offset.

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

storage.

4.1.3 Stack Pointer

The stack pointer is a 16-bit register that contains the address of the next free

location on the stack. During a reset or after the reset stack pointer (RSP)

instruction, the stack pointer contents are preset to $00FF. The address in the

stack pointer decrements as data is pushed onto the stack and increments as

data is pulled from the stack.

The ten most significant bits of the stack pointer are permanently fixed at

0000000011, so the stack pointer produces addresses from $00C0 to $00FF. If

subroutines and interrupts use more than 64 stack locations, the stack pointer

wraps around to address $00C0 and begins writing over the previously stored

data. A subroutine uses two stack locations; an interrupt uses five locations.

HC705J2 Datasheet(PDF) 21 Page - Motorola, Inc