Motorola M68000 Guida Utente Scaricare il pdf (Pagina 103)

CHAPTER 7

THE STACK

In this chapter we will examine the hardware stack implementation

on the 68000. A stack is a type of data structure. Computer scientists refer

to data structures like stacks as abstract data types. This is because we

manipulate the data items contained on the stack by indirect means. In

other words, we do not have to know the exact details of the particular

abstract data structure in order to use it. The 68000 provides a number of

addressing modes and instructions that can be used to manipulate data

on a stack. Additionally, a number of other instructions use the stack to

support their main functions. The JSR instruction that we have already

used is a good example.

A stack is just about what its name implies: a stack of data elements. As

with a stack of dishes or books, items are normally added to or removed

from the top of the stack. While it is possible to add or remove an element

from the middle of a stack, it is not a proper use. The data elements that

form a 68000 stack may be bytes, words, or longwords. A stack may be

placed anywhere in memory. It can be of any size. Normally, one of the

eight address registers, A0 through A7, is used to reference the contents

of a stack. However, register A7 is the register that is used by many

instructions, such as JSR, to implicitly reference a stack. This register

also has the symbolic representation SP (stack pointer). The diagram on the

following page shows what an empty stack looks like. Each position in the

stack is capable of holding a different value. The initial contents of the stack

are of no significance. Whatever values are in the respective memory

addresses are indicated by the question marks in the diagram. The actual

length of the stack is up to the programmer as long as the total length fits

within an area the programmer has reserved.

Stack Instructions

While it might seem a bit confusing, the 68000 hardware stack is upside

down. That is, when we add something to the stack, the elements are

added to progressively lower memory addresses. For each data element

1 2 ... 98 99 100 101 102 103 104 105 106 107 108 ... 255 256

Nessun commento

Motorola M68000 Guida Utente Pagina 103