Apple 165c manual Hardware Overview, Memory Map

C H A P T E R 1

Hardware

Hardware Overview

This section discusses the hardware differences between the Macintosh PowerBook 165c and the Macintosh PowerBook 160/180 computers.

IMPORTANT

Memory sizes, addresses, and other data are specific to each type of Macintosh computer and are provided for informational purposes only. To ensure that your application software maintains compatibility across the Macintosh line and to allow for future hardware changes, you are strongly advised to use the Macintosh Toolbox and Operating System routines wherever provided. In particular, never use absolute addresses to access hardware, because these addresses are not the same for all models. ▲

Memory Map

Like all Macintosh PowerBook computers that use the 68030 microprocessor, the PowerBook 165c always operates in 32-bit addressing mode. To maintain compatibility with software that uses 24-bit addressing conventions, the memory management unit (MMU) in the 68030 is used to map 24-bit addresses to their 32-bit equivalent.

In 32-bit mode, the 68030 supports a 4 GB address space. In 24-bit mode, however, the upper 8 address bits are ignored, and the maximum address space is limited to 16 MB. The MMU remaps addresses so that RAM, ROM, VRAM, and I/O all appear within this 16 MB range. Although the address translation is transparent to software, it has the effect of limiting the amount of addressable RAM to 8 MB.

Figure 1-2 shows the 32-bitmemory map of the PowerBook 165c. Figure 1-3 compares the 24-bit and 32-bit memory maps.

6Hardware Overview