Specifications
1793. In
order
for
the
1793
to
interrupt
the
CPU,
the
CB1
interrupt
on
the
PIA
must be enabled
for
low-to-high
transitions
on
the
CB1
pin.
The
Floppy-disk
Controller's
Data Request output
is
set
each
ti
me
a
byte
is
ready
for
the
CPU
or each
time
the
controller
is
ready
to
receive
a
byte
from
the
CPU.
The
DRQ
output
is
inverted
to
drive
the
'FJRr
input.
When
transferring
data,
the
DRQ
signal
wi
II
go
active
at
a
rate
proportional
to
the
data
transfer
rate
for
the
floppy
disk
drives
in
use.
The
QUARK
is
suffi
cient
ly
fast
so
that
even with
the
worst-case
data
transfer
rate
(SOOkbits/second
for
8-
inch
double-density
disks),
only
70%
of
the
CPU's
time
is
used in moving
data
between
the
controller
and
main
memory.
This
Leaves
30%
of
the
processor's
capacity
avai Lable
to
perform
other
operations.
The
left-over
capacity
is
essentiaL
in
certain
applications,
such as
when
an
interrupt-driven
communications
routine
must
continue
despite
data
transfers
to
or
from
the
disk.
Lesser
transfer
rates
will
of
course
leave
correspondingly
more
of
the
CPU's
time
availabLe.
Version 2.2
operating
systems
pLace
a
jump
instruction
at
location
38hex
in
both bank
A·
and
bank
a In
bank
A,
the
jump
address
points
to
the
interrupt
handLer
for
the
INT
interrupt
in
the
BIOS.
In bank
B,
the
jump address
points
to
the
entry
point
of a
special
routine
in
the
BIOS.
This
routine
saves
the
current
value
in
the
I-register
(which
determines
the
bank-
switching
boundary
when
the
interrupt
occured),
sets
the
bank-switching boundary
to
location
0000 (so
that
alL
CPU
memory
is
in
bank
A),
and
then pushes a speciaL
retum
address onto
the
stack.
The
routine
then
jumps
to
Location
38hex
in
bank
A.
This
causes
the
interrupt
to
proceed as
it
would have
if
the
bank-sw
itching
boundary
had
been
at
location
0000
when
the
interrupt
occured.
When
the
routine
handl ing
the
interrupt
returns,
it
wi
II
pop
the
return
address
off
the
stack.
If
this
retum
address
is
the
special
address pushed earL
ier
by
the
bank B
interrupt
handler,
then
control
will
be
transferred
back
to
this
routine.
It
will
then
restore
the.
former bank-switching boundary,
and
retum
from
the
original
interrupt.
The
version
2.23 and 2.24
operating
systems
distributed
with
the
QUARK
enables only two
interrupt
sources,
the
reaL-time
cLock
interrupt
from
the
CA1
input
on
the
PIA,
and
the
'NMT
interrupt,
whi
ch
is
connected
to
the
1793 fLoppy-disk controL
ler's
DRQ
output.
When
an !NT
interrupt
occurs (which
vectors
to
Location 38hex),
the
interrupt
handler
in
the
distributed
version
of
the
BIOS
does not
verify
that
the
source
of
the
interrupt
is
the
reaL-time clock.
Instead,
it
immediately updates
the
displayed
clock
on
the
screen,
if
the
system
was
instaLLed
with
the
cLock
option.
In
order
to
use
other
interrupt
sources
on
the
QUARK,
the
user
may
either
patch
the
source
file
for
the
BIOS
himseLf, or incLude in
the
appLication program
sufficient
code
to
handle
the
addit
iona L
interrupts.
For
the
latter
approach,
the
general idea would be
for
the
application
program
to
save
the
jump address
at
locations
39hex and
3Ahex,
and
replace
them with a
pointer
to
the
interrupt
entry
point
in
the
program.
When
an
interrupt
occurs,
the
application
program would check
for
the
source of
the
interrupt,
and take
the
appropriate
action
if
the
interrupt
was
intended
for
use
by
the
application.
If
the
interrupt
was
not
for
the
application
program, then
the
program
should
jump
to
the
regular
interrupt
entry
point
in
the
BIOS,
the
address
of
which
was
saved
by
the
application
program
initialLy.
Thus
the
BIOS
routine
wiLL
have
its
opportunity
to
respond
to
the
interrupt.
Notice
that
the
mechanism handLing
interrupts
which occur while
operating
in
bank
B
wiLL
mesh
perfectly
with
the
above
approach.
Of
course,
the
user
is
free
to
handLe bank B
interrupts
in
his
own
way.
The 'flMT
interrupt
is
largely
a
system
rather
than
a
user
feature
on
the
QUARK.
The
operating
system
always
handles
the
NAT
interrupt
from
the
fLoppy
disk
controLler
in
an
appropriate
fashion.
No
mechanism
is
provided
by
the
operating
system
for
trapping
bank
B
NMI
interrupts,
as
none
are
expected.
Megatel
'Computer Technologies Toronto,
Canada
c;...-
Page
H/W-9
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The
QUARK
Video Display
Interfaces