Microprocessor Theory and Applications with 68000/68020 and Pentium

8

ASSEMBLY LANGUAGE PROGRAMMING WITH THE 68020

In this chapter we describe the fundamental concepts associated with assembly language programming of the Motorola 68020 microprocessor. The 68020 contains new addressing modes and several new instructions beyond those of the 68000. To present the 68020 instruction set in a simplified manner, some of the 68020 advanced instructions are not covered. All 68000 assembly language programs can be executed by the 68020 without modifications. Note that a background in the 68000 software described in Chapter 6 is required before understanding the topics contained in this chapter.

8.1 Introduction

The 68020 was Motorola's first 32-bit microprocessor. The design of the 68020 is based on the 68000. The 68020 can perform a normal read or write cycle in three clock cycles without wait states as compared to the 68000, which completes a read or write operation in four clock cycles without wait states. As far as the addressing modes are concerned, the 68020 includes new modes beyond those of the 68000. Some of these modes are scaled indexing, larger displacements, and memory indirection. Furthermore, several new instructions are added to the 68020 instruction set, including the following:

• Bit field instructions are provided for manipulating a string of consecutive bits with a variable length from 1 to 32 bits.
• Two new instructions are used to perform conversions between packed BCD and ASCII or EBCDIC digits. Note that a packed BCD is a byte containing ...