HCS12 datasheet(22/452 Pages) FREESCALE

22 / 452 page

S12CPUV2 Reference Manual, Rev. 4.0

Freescale Semiconductor

2.2.1 Accumulators

General-purpose 8-bit accumulators A and B are used to hold operands and

results of operations. Some instructions treat the combination of these two

8-bit accumulators (A : B) as a 16-bit double accumulator (D).

Most operations can use accumulator A or B interchangeably. However,

there are a few exceptions. Add, subtract, and compare instructions

involving both A and B (ABA, SBA, and CBA) only operate in one direction,

so it is important to make certain the correct operand is in the correct

accumulator. The decimal adjust accumulator A (DAA) instruction is used

after binary-coded decimal (BCD) arithmetic operations. There is no

equivalent instruction to adjust accumulator B.

2.2.2 Index Registers

16-bit index registers X and Y are used for indexed addressing. In the

indexed addressing modes, the contents of an index register are added to

5-bit, 9-bit, or 16-bit constants or to the content of an accumulator to form

the effective address of the instruction operand. The second index register

is especially useful for moves and in cases where operands from two

separate tables are used in a calculation.

2.2.3 Stack Pointer

The CPU12 supports an automatic program stack. The stack is used to save

system context during subroutine calls and interrupts and can also be used

for temporary data storage. The stack can be located anywhere in the

standard 64-Kbyte address space and can grow to any size up to the total

amount of memory available in the system.

The stack pointer (SP) holds the 16-bit address of the last stack location

used. Normally, the SP is initialized by one of the first instructions in an

application program. The stack grows downward from the address pointed

to by the SP. Each time a byte is pushed onto the stack, the stack pointer is

automatically decremented, and each time a byte is pulled from the stack,

the stack pointer is automatically incremented.

When a subroutine is called, the address of the instruction following the

calling instruction is automatically calculated and pushed onto the stack.

Normally, a return-from-subroutine (RTS) or a return-from-call (RTC)

instruction is executed at the end of a subroutine. The return instruction

HCS12 Datasheet(PDF) 22 Page - Freescale Semiconductor, Inc