IBM 770 manual Memory, ROM Subsystem, RAM Subsystem, System Board

Memory

The ThinkPad computers use the following types of memory:

Ÿ Read-only memory (ROM)

Ÿ Random access memory (RAM)

ŸReal-time clock/complementary metal-oxide semiconductor RAM (RT/CMOS RAM)

ROM Subsystem

The	ROM	subsystem consists				of	four banks of 128 KB memory.
ROM	is	active		when	power	is	turned on and is assigned to the top of
the	first		and	last	1 MB	of	address		space (hex 000F0000–000FFFFF
and hex FFFF0000–FFFFFFFF). After POST							checks that system
memory		is	operating		correctly,		the	ROM	code is copied to RAM at
the	same		address		space,	and	ROM	is	disabled.

RAM Subsystem

The

RAM

subsystem

on the system board starts at address

hex 00000000

of

the

address space. The RAM subsystem for the

ThinkPad

770

is

64

bits

wide.

The

32

MB

base

memory is on the system board. Two 144-pin

8-byte

dual inline

memory module

(DIMM)

connectors

are

provided

on

the

system

board. Both

connectors

accept

an

8 MB, a 16 MB, a

32 MB, a 64 MB,

or a

128

MB

DIMM. The memory

capacity can be

increased

up

to

256 MB. When two 128 MB DIMMs

are

installed,

the

base

32

MB

memory

on

the

system

board

becomes inoperative.

The total amount of usable memory is less than the amount of memory installed because of ROM-to-RAM remapping and power management.

2-16 System Board