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APPENDIX
A
80286
SYSTEM INITIALIZATION
flltl.('Swltch
the
80286
from
Re.1
Addr
••
s Mode 10
Prolecled
Mode')
nom.
'witch
80286~mode.
publiC
ldl_de.c,gdt_d
••
c
Switch
the
80286
from
real
addr
•••
mod.
Into
protect.d
mode.
Th.
lnilial
EPROM
GOT,
lOT,
TSS,
.nd
LOT
(If
any)
con.tructed
by
BL0286
will
be
copied
from
EPROM
In[o
RAM.
The
RAM
area
••
~e
~efln.d
by
data
••
gm.nt.
alloc.ted
a.
fixed
entrle.
In
the
GOT.
The
CPU
r.gl.ter.
for
Ihe
GOT,
lOT,
TSS,
and
LOT
will
be
.el
to
point
at
the
RAM-ba.ed
,"gm.nt
••
Th.
ba,e
field.
In
the
RAM-b
••
ed
GOT
will
01.0
be
updat.d
to
pOint
at
Ihe
RAM-ba.ed
segmenls.
Thl.code,
is
u.ed
by
adding
It
to
the.
Il.tof
objeci
module.
glv.n
to
BL0286.
BL0286
must
Ih.n
be
told
10
place
the
••
gment
Inlt_code
al
addre
••
FFFE10H.
Execution
of
the
mod.
switch
code
begins
after
RESET.
This
happ.n.
bec.u.e
the
mode
switch
cod.
will
.•
torl
at
phy.lcal
.ddre.s
FFFFFOH,
which
Is
the
power
up
.dd
•••••
This
code
th.n
.et.
up
RAM
caples
of
Ihe
EPROM-based
.egmenls
before
jumping
to
the
Initial
la.k
placed
at
•
fixed
GOT
.ntry.
Afler
the
Jump,
the
CPU
• x e c
ut
e.
I n
the
s
tate
of
t h •
fir.
t
to.
k d e
fin.
d by B L 0 2 8 6 •
Thl.
code
will
not
us.
any
of
Ih.
EPROM-bas.d
tables
dlr.ctly.
Such
us.
would
r.sull
In
the
80286
writing
Into
EPROM
to
••
1
the
A
bit.
Any
us.
of
a
GOT
or
TSS
will
always
be
In
the
RAM
copy.
The
limit
and
.I,e
of
the
EPROM-based
GOT
and
lOT
mu.t
b.
stored
at
the
public
.ymbols
Idt_d
••
c
and
gdl_d.sc.
The
location
commands
of
BL0286
provld
•.
thl.
function
Intorrupts
are
disabled
during
Ihls
mode
.wltchlng
cod
••
Full
error
ch.cklng
I.
m.de
of
the
EPROM-ba
••
d GOT,
lOT,
TSS,
and
LOT
to
a
••
ur.
th.y
or.
valid
b.for.
copying
th.m
to
RAM.
If
any
of
the
RAM-ba.ed
alia
••
egments
are
.m.ller
Ihan
the
EPRDM
.egm.nts
th.y
or.
to
hold,
halt
or
.huldown
will
occur.
In
g.n.ral,
any
"exc.ptlon
or
Hi'll
will
cau.e
shutdown
to
occur
until
the
first
ta.k
Is
Invok.d.
If
the
RAM
s.gment
I.
larger
Ihan
the
EPROM
segm.nt,
the
RAM
••
gm.nt
will
be
expanded
with
,era
••
If
th.
Initial
TSS
specifies
an
LOT,
th.
LOT
will
also
b.
copied
Into
ldt_alla.
with
zero
fill
If
needed.
The EPRDM-ba
••
d
or
RAM-ba.ed
GOT,
lOT,
TSS,
and
LOT
.egment.
may
b.
locat.d
anywhere
In
phy.lcal
memory.
A-1