Specifications
lower
haLf
of the logical address space
is
now
mapped
into the upper
haLf
of
bank
B.
With
the
MAP
TYPE
bit
cleared
and
the
PAGE
MODE
bit
set,
it
becomes
possible
to
access the
otherw ise-hidden upper
Bk
of
bank
B.
Setting the
bank-sw
itch
boundary
to
8000H,
as in the
description above, forces logical addresses
from
0000
to
7FFFhex
to
be
mapped
onto physical
address
in
bank
B
from
8000hex
to
FFFFhex,
a
range
which
includes the hidden
Bk.
Thus
the
logical address range
6000hex
to
7FFFhex
wiLL
be
mapped
onto the upper
8k
of
bank
B.
By
appropriately
setting
the
F-bits
in
the
SAM
controL
register
and
bit
0
of
the
1-
register,
it
is possibLe to position the
Video
Display
Memory
so
that
this
hidden
8k
region
is
used
for
all
or part of the
Video
DispLay
Memory.
Table V in the
Appendix
gives suggested
vaLues
for the F-bits
and
the resultant
Video
Memory
address boundaries.
-----------------------------------------------------------,--------------.-------
QUARK
Boot
Mode
Memory
Mapping
------------------------------
The
QUARK
has
a speciaL
memory-mapping
mode
which
is
used
after
the system
has
been
reset
by
a low-level signaL
at
the
RESET
input, or
when
the internaL
BOOT
MODE
Line
has
been
set.
This
memory
mapping
mode
is
caLLed
the
Boot
Mode.
This
mode
is
used
when
the system
is
Loaded
from
the
fLoppy
disk drive (in a so-caLled 'bootstrap"
manner),
and
whenever
the
SAM
control
registers
are to
be
written.
In
this
mode,
any
memory
read operation
by
the
CPU
(incLuding instruction fetching)
wiLL
read
from
the contents of the 512-byte Bootstrap
PROM,
rather than
from
Main
Memory.
Memory
write operations
wi
LL
write to
Main
Memory,
using the
memory
mapping
process described. I/O
read or write operations
wiLL
reference I/O ports in the usual
way,
except that the Character
Generator (I/O addresses 00-3F) cannot
be
written
when
in the
Boot
Mode.
When
the
QUARK
is
reset,
the
registers
of the
ZSOB,
incLuding the
Program
Counter, are
cLeared.
ALso,
various I/O
Lines
on
the
VIA
and
PIA
are cLeared
and
set
to
act as inputs. In
particular,
the
BOOT
MODE
controL
line,
which
is the
CB2
controL line
on
the
PIA,
will
be
reset
to
act
as
an
input,
aLLowing
a pullup
resistor
on
this
line
to
assert
a
"BOOT
MODE"
condition.
The
Z8(B
wi
Ll
now
fetch the instruction at location
0000.
Since the
BOOT
MODE
control
line is high,
this
wi
LL
cause the instruction at Location
0000
of the Bootstrap
PROM
to
be
fetched. Successive instructions
wi
II
be
fetched
from
the
PROM
unti l the
BOOT
MODE
controL
line
is
cleared to a
Low-LeveL
output condition. This puts the
QUARK
into the
NormaL
Mode
of
operation,
where
all
memory
read or write operations
wi
LL
reference the
Main
Memory.
In
the
NormaL
Mode,
the
PROM
cannot
be
accessed.
The
Logical
address
space occupied
by
the
Bootstrap
PROM
consists
of
the
first
64
addresses
(i.e.
00-3Fhex) of each 256-byte page
in
memory.
Thus
the
entire
PROM
can be
accessed
in
eight 64-byte
"chunks"
in
the bottom
2k
of
memory.
This logicaL address space
is
aliased
between
0000
and
073F(hex)
are
used
in
accessing the Bootstrap
PROM.
Addresses between
the
64-byte
chunks cause I/O read or
write
operations.
It
is
not
recomended
that
these addresses
be
used. I/O operations
may
be
performed
in
the usual
manner
using
I/O
instructions while in the
Boot
Mode.
It
shouLd
be
pointed out that
it
is unlikely
that
the user
wilL
ever
need
to
directly
use
the
Boot
Mode
of
operation,
nor
is
it
ever
necessary
to
directly
access
any
part
of
the
Bootstrap
PROM.
After the operating system
has
been
booted
from
the floppy disk, the only
operation requiring a return
to
the
Boot
Mode
is
when
the
SAM
controL
registers
are
to
be
re-
written. In
this
case, a special routine called the
SAM
Loader
Access
Routine, resident on the
bottom
page
of
Main
Memory,
wiLL
automaticaLLy
handLe
the entry into
and
return
from
the
Boot
Mode.
Megatel
Computer
TechnoLogies
Toronto,
Canada
Page
H/W-6