Inside Macintosh: Mac OS Runtime Architectures /

Chapter 10 - Classic 68K Runtime Architecture

The classic 68K runtime architecture is the original Macintosh runtime architecture, designed for computers running a Motorola 68000-series microprocessor. Applications are stored as segments that can be loaded into the application heap as necessary. The application space contains the application heap, the application stack, and the A5 world.

This chapter gives an overview of the classic 68K runtime architecture, with information about the following topics:

• the A5 world
• program segmentation
• the jump table
• addressing limitations of the original classic 68K architecture and MPW solutions for bypassing these limitations

The first three sections, which discuss the A5 world, program segmentation, and the jump table, assume the near model classic 68K architecture. Programs built using the near model rely on 16-bit addressing for code and data. The sections that follow introduce the far model, which relies on 32-bit addressing for code and data. Note that you have the option of incorporating only some of the far model characteristics when building your application.

For additional information you should consult the various volumes of the Inside Macintosh series.

Note

Classic 68K runtime code cannot use shared libraries. However, classic 68K runtime code can run transparently under emulation on PowerPC-based computers.

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Chapter Contents

The A5 World

Program Segmentation

The Jump Table

Bypassing MC68000 Addressing Limitations

Increasing Global Data Size

Increasing Segment Size

Increasing the Size of the Jump Table

32-Bit Everything

How 32-Bit Everything Is Implemented

Expanding Global Data and the Jump Table

Intrasegment References

The Far Model Jump Table

The Far Model Segment Header Structure

Relocation Information Format