STACK OVERFLOW MODULE

5.4PROGRAMMING THE STACK OVERFLOW MODULE

To use the stack overflow module, you must initialize the stack pointer and specify the stack boundaries. The stack overflow interrupt is nonmaskable, so you do not need to enable the interrupt in the INT_MASK1 register or enable interrupt servicing by executing the EI instruction.

5.4.1Initializing the Stack Pointer

Initialize the stack pointer (Figure 5-2) to an address that is two bytes (in 64-Kbyte mode) or four bytes (in 2-Mbyte mode) greater than the desired starting address (top) of the stack. Ensure that this value is within the boundaries you specify in STACK_BOTTOM and STACK_TOP.

SP	Address:	18H
	Reset State:	XXXXH

The system’s stack pointer (SP) can point anywhere in page 00H; it must be word aligned and must always be initialized before use. The stack pointer is decremented before a PUSH and incremented after a POP, so you should initialize the stack pointer to two bytes (in 64-Kbyte mode) or four bytes (in 2-Mbyte mode) above the highest stack location. If stack operations are not being performed, locations 18H and 19H may be used as standard registers.

15

0

Stack Pointer

	Bit	Function
	Number
	15:0	Stack Pointer
		This register makes up the system’s stack pointer.
		Figure 5-2. Stack Pointer (SP)

5.4.2Enabling the Stack Overflow Module and Specifying Stack Boundaries

Two registers, STACK_BOTTOM and STACK_TOP, control the stack overflow detection circuitry. Write to STACK_BOTTOM (Figure 5-3) to establish the lower boundary for the stack, then write to STACK_TOP (Figure 5-4) to establish the upper boundary and to enable the overflow detection circuitry.

If an overflow occurs, the stack overflow module sets the stack overflow interrupt pending bit in INT_PEND1. Your interrupt service routine must write to the STACK_TOP register to re-enable the overflow detection circuitry. Otherwise, the R/S flip-flop (Figure 5-1 on page 5-1) remains set, and the stack overflow module is unable to signal another overflow.

5-3

8XC196EA USER’S MANUAL

	STACK_BOTTOM		Address:	1FA0H
			Reset State:	0000H
	The value you write to this register specifies the lower address limit for the stack. The overflow
	detection circuitry compares the stack pointer (SP) value on the peripheral address bus to the
	STACK_TOP and STACK_BOTTOM values.
	15			8
		Lower Stack Limit (high byte)
	7			0
		Lower Stack Limit (low byte)
	Bit	Function
	Number
	15:0	Lower Stack Limit
		This register specifies the lower address limit for the stack. If the stack pointer value is
		equal to or less than this value, the stack overflow module sets the nonmaskable stack
		error interrupt (INT14) pending bit in INT_PEND1.
		Figure 5-3. Lower Stack Limit (STACK_BOTTOM) Register
	STACK_TOP		Address:	1FA2H
			Reset State:	FFFFH

The value you write to this register specifies the upper address limit for the stack. You must write to this register to enable the stack overflow module. The overflow detection circuitry compares the stack pointer (SP) value on the peripheral address bus to the STACK_TOP and STACK_BOTTOM values.

	15	8
		Upper Stack Limit (high byte)
	7	0
		Upper Stack Limit (low byte)
	Bit	Function
	Number
	15:0	Upper Stack Limit
		This register specifies the upper address limit for the stack. If the stack pointer value is
		equal to or greater than this value, the stack overflow module sets the nonmaskable stack
		error interrupt (INT14) pending bit in INT_PEND1.
		Figure 5-4. Upper Stack Limit (STACK_TOP) Register

5-4

6

Standard and PTS

Interrupts