Motorola M68000 User Manual download pdf (Page 28)

Addressing Capabilities

2-28 M68000 FAMILY PROGRAMMER’S REFERENCE MANUAL MOTOROLA

2.6 OTHER DATA STRUCTURES

Stacks and queues are common data structures. The M68000 family implements a system

stack and instructions that support user stacks and queues.

2.6.1 System Stack

Address register seven (A7) is the system stack pointer. Either the user stack pointer (USP),

the interrupt stack pointer (ISP), or the master stack pointer (MSP) is active at any one time.

Refer to Section 1 Introduction for details on these stack pointers. To keep data on the

system stack aligned for maximum efﬁciency, the active stack pointer is automatically

decremented or incremented by two for all byte-size operands moved to or from the stack.

In long-word-organized memory, aligning the stack pointer on a long-word address

signiﬁcantly increases the efﬁciency of stacking exception frames, subroutine calls and

returns, and other stacking operations.

The user can implement stacks with the address register indirect with postincrement and

predecrement addressing modes. With an address register the user can implement a stack

that ﬁlls either from high memory to low memory or from low memory to high memory.

Important consideration are:

• Use the predecrement mode to decrement the register before using its contents as the

pointer to the stack.

• Use the postincrement mode to increment the register after using its contents as the

pointer to the stack.

• Maintain the stack pointer correctly when byte, word, and long-word items mix in these

stacks.

To implement stack growth from high memory to low memory, use -(An) to push data on the

stack and (An) + to pull data from the stack. For this type of stack, after either a push or a

pull operation, the address register points to the top item on the stack.

BOTTOM OF STACK

LOW MEMORY

(FREE)

TOP OF STACK

HIGH MEMORY

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Motorola M68000 User Manual Page 28