IBM 770 manual System Board, Bits, Expansion, Memory Bytes, Date-Century, Base, Usable

Bits	3–2	These bits			are	reserved.
Bit	1	If	set	to 1,		this	bit	indicates that a coprocessor is
		installed.
Bit	0	If	set	to	1,	this	bit	indicates that physical diskette drive 0
		is	installed.

Low

and

High

Base

Memory Bytes (Hex 015 and

Hex

016):

The

low

and high

base

memory

bytes define the amount of memory

below

the

640

KB

address

space.

The

value

in

these

bytes

represents

the

number

of 1

KB blocks of

base

memory.

For

example,

hex 0280

indicates 640

KB. The

low

byte

is

hex

015;

the high byte

is

hex

016.

Low

and

High

Expansion

Memory Bytes

(Hex

017

and

Hex

018):

The

low

and

high

expansion

memory

bytes

define

the

amount

of

memory

above

the

1

MB

address space.

The value in these bytes represents

the number of 1 KB blocks of

expansion

memory. For

example,

hex

0800

indicates

2048

KB. The

low byte is hex 017;

the

high

byte

is

hex

018.

Reserved

Bytes

(Hex

01D–02D):

These bytes are reserved.

Configuration Checksum Bytes (Hex 02E and Hex 02F):

The

configuration checksum bytes contain the

checksum

character

for

bytes hex 010 through hex

02D

of

the

64-byte

CMOS

RAM. The

high

byte

is

hex

02E;

the

low

byte

is

hex

02F.

Low

and

High

Usable

Memory

Bytes

(Hex 030

and

Hex

031):

The low and high usable memory bytes

define

the

total amount of

contiguous

memory

from

1

MB

to

20

MB.

The

hexadecimal

values

in

these

bytes

represent the number of 1

KB

blocks

of

usable

memory. For

example,

hex

0800

is

equal

to

2048

KB. The

low

byte

is

hex

30;

the

high

byte

is hex

31.

Date-Century

Byte

(Hex

032):

Bits

7

through

0

of

the

date-century

byte contain the binary-coded decimal value for the century. For information about reading and setting this IBMbyte, refer to the Personal System/2 and Personal Computer BIOS .Interface

Reserved Bytes (Hex 033–07F): These bytes are reserved.

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