<s>
The	O
Zero	B-General_Concept
Page	I-General_Concept
(	O
or	O
Base	B-General_Concept
Page	I-General_Concept
)	O
is	O
a	O
data	B-General_Concept
structure	I-General_Concept
used	O
in	O
CP/M	B-Application
systems	O
for	O
programs	O
to	O
communicate	O
with	O
the	O
operating	O
system	O
.	O
</s>
<s>
In	O
8-bit	O
CP/M	B-Application
versions	O
it	O
is	O
located	O
in	O
the	O
first	B-General_Concept
256	I-General_Concept
bytes	I-General_Concept
of	I-General_Concept
memory	I-General_Concept
,	O
hence	O
its	O
name	O
.	O
</s>
<s>
The	O
equivalent	O
structure	O
in	O
DOS	B-Device
is	O
the	O
Program	O
Segment	O
Prefix	O
(	O
PSP	O
)	O
,	O
a	O
256-byte	O
structure	O
,	O
which	O
,	O
however	O
,	O
is	O
by	O
default	O
located	O
at	O
offset	O
0	O
in	O
the	O
program	O
's	O
load	O
segment	O
(	O
rather	O
than	O
in	O
segment	O
0	O
)	O
immediately	O
preceding	O
a	O
loaded	O
program	O
.	O
</s>
<s>
In	O
8-bit	O
CP/M	B-Application
,	O
it	O
has	O
the	O
following	O
structure	O
:	O
</s>
<s>
OffsetSizeContents00	O
–	O
02CodeExit	O
program	O
(	O
jumps	O
to	O
the	O
BIOS	B-Operating_System
,	O
and	O
is	O
also	O
used	O
to	O
find	O
BIOS	B-Operating_System
entry	O
points	O
)	O
.03ByteI/O	O
byte	B-Application
,	O
an	O
optional	O
feature	O
allowing	O
device	O
reassignment	O
in	O
CP/M	B-Application
2.04ByteCurrent	O
command	O
processor	O
drive	O
(	O
low	O
4	O
bits	O
)	O
and	O
user	O
number	O
(	O
high	O
4	O
bits	O
)	O
.05	O
–	O
07CodeJump	O
to	O
CP/M	B-Application
BDOS	O
entry	O
-	O
main	O
system	O
call	O
entry	O
point	O
.	O
</s>
<s>
This	O
is	O
also	O
the	O
address	O
of	O
the	O
first	O
byte	B-Application
of	O
memory	O
not	O
usable	O
by	O
the	O
program.08	O
–	O
3ACode8080	O
restart/interrupt	O
vectors.3B	O
–	O
4FBytesReserved50ByteThe	O
drive	O
from	O
which	O
the	O
program	O
was	O
loaded	O
(	O
CP/M	B-Application
3	O
)	O
51	O
–	O
52WordAddress	O
of	O
the	O
password	O
for	O
the	O
first	O
FCB	B-Operating_System
(	O
CP/M	B-Application
3	O
)	O
53ByteLength	O
of	O
the	O
password	O
for	O
the	O
first	O
FCB	B-Operating_System
(	O
CP/M	B-Application
3	O
)	O
54	O
–	O
55WordAddress	O
of	O
the	O
password	O
for	O
the	O
second	O
FCB	B-Operating_System
(	O
CP/M	B-Application
3	O
)	O
56ByteLength	O
of	O
the	O
password	O
for	O
the	O
second	O
FCB	B-Operating_System
(	O
CP/M	B-Application
3	O
)	O
57	O
–	O
5BBytesReserved5C	O
–	O
6BDefault	O
FCB	B-Operating_System
16C	O
–	O
7FDefault	O
FCB	B-Operating_System
2	O
(	O
overwritten	O
if	O
FCB	B-Operating_System
1	O
is	O
opened	O
)	O
80ByteNumber	O
of	O
characters	O
in	O
command	O
tail.81	O
–	O
FFBytesCommand	O
tail	O
(	O
everything	O
after	O
the	O
program	O
name	O
)	O
.	O
</s>
<s>
In	O
CP/M	B-Operating_System
-86	I-Operating_System
,	O
the	O
structure	O
is	O
:	O
</s>
<s>
OffsetSizeContents00	O
–	O
02BytesLength	O
of	O
code	O
group	O
in	O
bytes03	O
–	O
04WordSegment	O
address	O
of	O
code	O
group05Byte8080	O
model	O
flag	O
-	O
set	O
if	O
program	O
only	O
has	O
one	O
segment06	O
–	O
08BytesLength	O
of	O
data	O
group	O
in	O
bytes09	O
–	O
0AWordSegment	O
address	O
of	O
data	O
group0BByteReserved0C	O
–	O
11Descriptor	O
for	O
extra	O
group	O
-	O
same	O
format	O
as	O
for	O
data12	O
–	O
17Descriptor	O
for	O
stack	O
group18	O
–	O
1DDescriptor	O
for	O
X1	O
group1E	O
–	O
23Descriptor	O
for	O
X2	O
group24	O
–	O
29Descriptor	O
for	O
X3	O
group2A	O
–	O
2FDescriptor	O
for	O
X4	O
group30	O
–	O
4FBytesReserved50ByteThe	O
drive	O
from	O
which	O
the	O
program	O
was	O
loaded	O
(	O
CP/M	B-Application
3	O
)	O
51	O
–	O
52WordAddress	O
of	O
the	O
password	O
for	O
the	O
first	O
FCB	B-Operating_System
(	O
CP/M	B-Application
3	O
)	O
53ByteLength	O
of	O
the	O
password	O
for	O
the	O
first	O
FCB	B-Operating_System
(	O
CP/M	B-Application
3	O
)	O
54	O
–	O
55WordAddress	O
of	O
the	O
password	O
for	O
the	O
second	O
FCB	B-Operating_System
(	O
CP/M	B-Application
3	O
)	O
56ByteLength	O
of	O
the	O
password	O
for	O
the	O
second	O
FCB	B-Operating_System
(	O
CP/M	B-Application
3	O
)	O
57	O
–	O
5BBytesReserved5C	O
–	O
6BDefault	O
FCB	B-Operating_System
16C	O
–	O
7FDefault	O
FCB	B-Operating_System
2	O
(	O
overwritten	O
if	O
FCB	B-Operating_System
1	O
is	O
opened	O
)	O
80ByteNumber	O
of	O
characters	O
in	O
command	O
tail.81	O
–	O
FFBytesCommand	O
tail	O
(	O
everything	O
after	O
the	O
program	O
name	O
)	O
.	O
</s>
