Command Setup and Execution Under CPIM-86
2-6
The Compact Memory Model
The
Compact
Memory Model
is
assumed when code
and
data
groups are present,
along
with one
or
more
of
the stack, extra,
or
auxiliary groups.
In
this case, the CS,
DS,
and
ES registers are set
to
the base addresses
of
their respective areas. Figure
2-3 shows the initial configuration
of
segment registers in the
Compact
Model. The
segment register values can be changed during program execution by loading from
the initial values placed in base page by the
CCP, thus allowing access
to
the entire
memory space.
SS:
CCP
SS
+ SP:
CCP
Stack
CS:
IP
=
OOOOH
code
DS:
base
page
DS+OI00H:
data
ES:
data
Figure 2-3 CP/M-86 Compact Memory Model
Local stacks are required in programs
that
make BDOS calls since the BDOS may
change information in the system stack.
If
the transient
program
intends
to
use the
stack
group
as a stack area, registers
SS
and
SP must be set
upon
entry. The
SS
and
SP registers remain set
to
the
CCP
area, even
if
a stack group
is
defined. Although it
may
appear
that
the
SS
and
SP registers should be set to address the stack
group,
there are two reasons this cannot be done. First, the transient program may be using
the stack
group
as a
data
area.
In
that
case, the
Far
Call instruction used by the
CCP
to
transfer
control
to
the transient
program
could write over
data
in the stack area.
Second, the
SS
register would logically be set
to
the base
of
the group, while the SP
register would be set
to
the offset
of
the end
of
the group. However,
if
the stack
group
exceeds 64K, the address range from the base
to
the end
of
the
group
could
exceed a 16-bit offset value.
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