Apple PowerBook 180 User Manual download pdf (Page 18)

Macintosh PowerBook 180c

Hardware Overview

Figure 3

32-bit and 24-bit memory map comparison

RAM 1

The PowerBook 180c includes 4 MB of 85 ns pseudostatic RAM, consisting of eight

512K x 8-bit chips. The RAM chips are located on the RAM/ROM board to provide

room for the color display circuitry on the secondary logic board.

The RAM expansion slot is located on the RAM/ROM board, and supports up to 10 MB

of PSRAM, for a total of 14 MB. See the section “RAM Expansion Interface” on page 10

for information about designing RAM expansion cards.

RAM is always contiguous because only one size of RAM chip (4 Mbits) is used. As a

result, software does not have to “stitch” memory. The RAM array is located in the

system memory map between addresses $0000 0000 and $00DF FFFF, except following a

system reset or sleep cycle, when it is overlaid by system ROM. However, the overlay is

removed following access to normal ROM space, and the RAM space is then accessible.

Both RAM and ROM memory spaces provide DSACK signals to the processor even if

memory is not actually installed. The RAM data path is 32 bits wide.

RAM Wait States 1

RAM accesses require 2 wait states when using 85 ns PSRAM exclusively. Installing

100 ns expansion RAM causes the memory controller to insert an additional wait state

in all memory accesses (both main and expansion).

$FF FFFF

I/O space

Slot E (external video)

ROM

RAM

8 MB maximum,

one continuous bank

$F0 0000

$E0 0000

$90 0000

$80 0000

$00 0000

32-bit memory map 24-bit memory map

$6000 0000

$5000 0000

$4400 0000

$4000 0000

$0400 0000

$00E0 0000

$0000 0000

$FFFF FFFF

$FEF0 FFFF

I/O

Reserved

ROM

Expansion RAM

(bus error)

Illegal

RAM

14 MB maximum,

one continuous bank

Reserved

Slot E (external video)

Reserved

$FE00 0000

$FD00 0000

$FC00 0000

Reserved

Slot C (internal video)

Reserved

$D0 0000

$C0 0000

Reserved

Slot C (internal video)

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