L9803 datasheet(16/126 Pages) STMICROELECTRONICS

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Central Processing Unit

L9803

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Stack Pointer (SP) The Stack Pointer is a 16-bit register. Since the stack is 64 bytes deep,

the most significant bits are forced as indicated in Figure 3 in order to address the stack as it

is mapped in memory.

Following an MCU Reset, or after a Reset Stack Pointer instruction (RSP), the Stack Pointer

is set to point to the next free location in the stack. It is then decremented after data has

been pushed onto the stack and incremented before data is popped from the stack.

Note: When the lower limit is exceeded, the Stack Pointer wraps around to the stack upper

limit, without indicating the stack overflow. The previously stored information is then

overwritten and therefore lost.

The upper and lower limits of the stack area are shown in the Memory Map.

The stack is used to save the CPU context during subroutine calls or interrupts. The user

may also directly manipulate the stack by means of the PUSH and POP instructions. In the

case of an interrupt (refer to Figure 4), the PCL is stored at the first location pointed to by the

SP. Then the other registers are stored in the next locations.

When an interrupt is received, the SP is decremented and the context is pushed on the

stack.

On return from interrupt, the SP is incremented and the context is popped from the stack.

A subroutine call occupies two locations and an interrupt five locations in the stack area.

Condition Code Register (CC) The Condition Code register is a 5-bit register which

indicates the result of the instruction just executed as well as the state of the processor.

These bits can be individually tested by a program and specified action taken as a result of

their state. The following paragraphs describe each bit of the CC register in turn.

Half carry bit (H) The H bit is set to 1 when a carry occurs between bits 3 and 4 of the ALU

during an ADD or ADC instruction. The H bit is useful in BCD arithmetic subroutines.

Interrupt mask (I) When the I bit is set to 1, all interrupts except the TRAP software

interrupt are disabled. Clearing this bit enables interrupts to be passed to the processor

core. Interrupts requested while I is set are latched and can be processed when I is cleared

(only one interrupt request per interrupt enable flag can be latched).

Negative (N) When set to 1, this bit indicates that the result of the last arithmetic, logical or

data manipulation is negative (i.e. the most significant bit is a logic 1).

Zero (Z) When set to 1, this bit indicates that the result of the last arithmetic, logical or data

manipulation is zero.

Carry/Borrow (C) When set, C indicates that a carry or borrow out of the ALU occured

during the last arithmetic operation. This bit is also affected during execution of bit test,

branch, shift, rotate and store instructions.

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