HCS08 datasheet(112/290 Pages) MOTOROLA

112 / 290 page

Central Processor Unit (CPU)

MC9S08GB/GT

112

MOTOROLA

7.3.3 Stack Pointer (SP)

This 16-bit address pointer register points at the next available location on the automatic last-in-first-out

(LIFO) stack. The stack may be located anywhere in the 64-Kbyte address space which has RAM and can

be any size up to the amount of available RAM. The stack is used to automatically save the return address

for subroutine calls, the return address and CPU registers during interrupts, and for local variables. The

AIS (add immediate to stack pointer) instruction adds an 8-bit signed immediate value to SP. This is most

often used to allocate or deallocate space for local variables on the stack.

SP is forced to $00FF at reset for compatibility with the earlier M68HC05 Family. HCS08 programs

normally change the value in SP to the address of the last location (highest address) in on-chip RAM

during reset initialization to free up direct page RAM (from the end of the on-chip registers to $00FF).

The RSP (reset stack pointer) instruction was included for compatibility with the M68HC05 Family and

is seldom used in new HCS08 programs because it only affects the low-order half of the stack pointer.

7.3.4 Program Counter (PC)

The program counter is a 16-bit register that contains the address of the next instruction or operand to be

fetched.

During normal program execution, the program counter automatically increments to the next sequential

memory location every time an instruction or operand is fetched. Jump, branch, interrupt, and return

operations load the program counter with an address other than that of the next sequential location. This

is called a change-of-flow.

During reset, the program counter is loaded with the reset vector which is located at $FFFE and $FFFF.

The vector stored there is the address of the first instruction that will be executed after exiting the reset

state.

7.3.5 Condition Code Register (CCR)

The 8-bit condition code register contains the interrupt mask (I) and five flags that indicate the results of

the instruction just executed. Bits 6 and 5 are set permanently to logic 1. The following paragraphs

describe the functions of the condition code bits in general terms. For a more detailed explanation of how

each instruction sets the CCR bits, refer to the HCS08 Family Reference Manual, volume 1, Motorola

document order number HCS08RMv1/D.

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