MC9RS08KA2 Datasheet, PDF (58/132 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 8 Central Processor Unit (RS08CPUV1)
7
0
ACCUMULATOR A
13
87
PROGRAM COUNTER
0
PC
13
0
SHADOW PROGRAM COUNTER
SPC
CONDITION CODE REGISTER Z C CCR
CARRY
ZERO
Figure 8-1. CPU Registers
In addition to the CPU registers, there are three memory mapped registers that are tightly coupled with the
core address generation during data read and write operations. They are the indexed data register (D[X]),
the index register (X), and the page select register (PAGESEL). These registers are located at $000E,
$000F, and $001F, respectively.
7
0
INDEXED DATA REGISTER D[X] (location $000E)
7
0
INDEX REGISTER
X (location $000F)
7
0
PAGE SELECT REG PAGESEL (location $001F)
Figure 8-2. Memory Mapped Registers
8.2.1 Accumulator (A)
This general-purpose 8-bit register is the primary data register for RS08 MCUs. Data can be read from
memory into A with a load accumulator (LDA) instruction. The data in A can be written into memory with
a store accumulator (STA) instruction. Various addressing mode variations allow a great deal of flexibility
in specifying the memory location involved in a load or store instruction. Exchange instructions allow
values to be exchanged between A and SPC high (SHA) and also between A and SPC low (SLA).
Arithmetic, shift, and logical operations can be performed on the value in A as in ADD, SUB, RORA,
INCA, DECA, AND, ORA, EOR, etc. In some of these instructions, such as INCA and LSLA, the value
in A is the only input operand and the result replaces the value in A. In other cases, such as ADD and AND,
there are two operands: the value in A and a second value from memory. The result of the arithmetic or
logical operation replaces the value in A.
Some instructions, such as memory-to-memory move instructions (MOV), do not use the accumulator.
DBNZ also relieves A because it allows a loop counter to be implemented in a memory variable rather than
the accumulator.
During reset, the accumulator is loaded with $00.
MC9RS08KA2 Series Data Sheet, Rev. 2
58
Freescale Semiconductor