Datasheet

Central Processor Unit (CPU)

MC68HC711D3 Data Sheet, Rev. 2.1

34 Freescale Semiconductor

3.2.1 Accumulators A, B, and D

Accumulators A and B are general-purpose 8-bit registers that hold operands and results of arithmetic

calculations or data manipulations. For some instructions, these two accumulators are treated as a single

double-byte (16-bit) accumulator called accumulator D. Although most instructions can use accumulators

A or B interchangeably, these exceptions apply:

• The ABX and ABY instructions add the contents of 8-bit accumulator B to the contents of 16-bit

• The TAP and TPA instructions transfer data from accumulator A to the condition code register or

from the condition code register to accumulator A. However, there are no equivalent instructions

that use B rather than A.

• The decimal adjust accumulator A (DAA) instruction is used after binary-coded decimal (BCD)

arithmetic operations, but there is no equivalent BCD instruction to adjust accumulator B.

• The add, subtract, and compare instructions associated with both A and B (ABA, SBA, and CBA)

only operate in one direction, making it important to plan ahead to ensure that the correct operand

is in the correct accumulator.

3.2.2 Index Register X (IX)

The IX register provides a 16-bit indexing value that can be added to the 8-bit offset provided in an

instruction to create an effective address. The IX register can also be used as a counter or as a temporary

storage register.

3.2.3 Index Register Y (IY)

The 16-bit IY register performs an indexed mode function similar to that of the IX register. However, most

instructions using the IY register require an extra byte of machine code and an extra cycle of execution

time because of the way the opcode map is implemented. Refer to 3.4 Opcodes and Operands for further

information.

3.2.4 Stack Pointer (SP)

The M68HC11 CPU has an automatic program stack. This stack can be located anywhere in the address

space and can be any size up to the amount of memory available in the system. Normally, the SP is

initialized by one of the first instructions in an application program. The stack is configured as a data

structure that grows downward from high memory to low memory. Each time a new byte is pushed onto

the stack, the SP is decremented. Each time a byte is pulled from the stack, the SP is incremented. At

any given time, the SP holds the 16-bit address of the next free location in the stack. Figure 3-2 is a

summary of SP operations.

When a subroutine is called by a jump-to-subroutine (JSR) or branch-to- subroutine (BSR) instruction, the

address of the instruction after the JSR or BSR is automatically pushed onto the stack, least significant

byte first. When the subroutine is finished, a return-from-subroutine (RTS) instruction is executed. The

RTS pulls the previously stacked return address from the stack and loads it into the program counter.

Execution then continues at this recovered return address.

When an interrupt is recognized, the current instruction finishes normally, the return address (the current

value in the program counter) is pushed onto the stack, all of the CPU registers are pushed onto the stack,

and execution continues at the address specified by the vector for the interrupt.