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CHAPTER 1
Hardware
6
Hardware Overview
Hardware Overview 1
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.
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Memory Map 1
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-bit memory map of the PowerBook 165c. Figure 1-3 compares the
24-bit and 32-bit memory maps.