ChapterS
More
out
the
Bank
anagerll
11
1111
Extensions
to
BASIC
allowing access
to
the second 64K bank
of
RAM.
Subjects covered:
* Storing screen images
* Pseudo-file operation
The memory map for BASIC 1.1 (shown
in
Chapter 7
part
14) indicates
that
64K of
the
128K RAM is unused. The BASIC
and
firmware themselves reside
in
a
ROM
which, together with
the
disc ROM, increase
the
normal memory provided from 64K
to 112K (64K RAM, 48K ROM).
Each section of16K is called a 'block', and any particular selection offour blocks (to
make 64K
in
total) is called a 'bank'. The technique of selecting blocks is therefore
called
'bank
switching'.
The
Z80 microprocessor can accomodate only 64K of memory
at
anyone
time,
so
the
operating system contains instructions to switch
the
firmware
ROM
into play instead
of Block
0 of RAM,
and
to switch
either
the
BASIC
ROM
or
the
disc
ROM
into play
instead of Block
3.
This switching
takes
place automatically when
the
BASIC or
firmware is required.
Bank
switching of RAM merely extends this concept to include
the
overlaying of RAM,
rather
than
ROM. The switching is
taken
care of by
an
assembler program.
The program
BAN
K M AN.
BA
S is provided on Side 1 of
the
system discs package.
If
the
program is
run
after BASIC
has
started,
it
will install the
standard
bank
management
RSX code. Hence,
the
program is known as
the
'BANK MANAGER'.
One use for
the
memory
in
the
second 64K is as temporary storage space for picture
screens.
Such applications
may
include, for example, a 'Screen Designer' program
which stores a
number
of different screens, or a video game which may have a number
of different screens already prepared.
Another use for
the
second 64K is
as
an
extension to
the
variable workspace, which
can be regarded as
either
an
extension to
the
string
array
space, or as a simple
'RAMdisc'.
More About the
Bank
Manager
....
Chapter 8 Page 1
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