Freescale Semiconductor, Inc.
GENERAL RELEASE SPECIFICATION
April 30, 1998
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 nds the oper and address by adding the
index register content to an 8-bit immediate value. In indexed addressing with a
16-bit offset, the CPU nds 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 64k-bytes 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 signi cant bits are permanently set to
0000000011. The six least signi cant register bits are appended to these ten x ed
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 ve
locations.
3.5
PROGRAM COUNTER (PC)
The program counter shown in Figure 3-1 is a 16-bit register. In MCU devices
with memory space less than 64k-bytes the unimplemented upper address lines
are ignored. The program counter contains the address of the next instruction or
operand to be fetched.
CENTRAL PROCESSING UNIT
MC68HC05PL4
REV 2.0
3-2
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