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Assembly Language Programming

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15
Protected Mode
Programming
15.1. INTRODUCTION
Till now we have been discussing the 8088 architecture which was a 16bit
processor. Newer processors of the Intel series provide 32bit architecture. Till
now we were in real mode of a newer processor which is basically a
compatibility mode making the newer processor just a faster version of the
original 8088. Switching processor in the newer 32bit mode is a very easy
task. Just turn on the least significant bit of a new register called CR0
(Control Register 0) and the processor switches into 32bit mode called
protected mode. However manipulations in the protected mode are very
different from those in the read mode.
All registers in 386 have been extended to 32bits. The new names are EAX,
EBX, ECX, EDX, ESI, EDI, ESP, EBP, EIP, and EFLAGS. The original names
refer to the lower 16bits of these registers. A 32bit address register can
access upto 4GB of memory so memory access has increased a lot.
As regards segment registers the scheme is not so simple. First of all we
call them segment selectors instead of segment registers and they are still
16bits wide. We are also given two other segment selectors FS and GS for no
specific purpose just like ES.
The working of segment registers as being multiplied by 10 and added into
the offset for obtaining the physical address is totally changed. Now the
selector is just an index into an array of segment descriptors where each
descriptor describes the base, limit, and attributes of a segment. Role of
selector is to select on descriptor from the table of descriptors and the role of
descriptor is to define the actual base address. This decouples the selection
and actual definition which is needed in certain protection mechanisms
introduced into this processor. For example an operating system can define
the possible descriptors for a program and the program is bound to select
one of them and nothing else. This sentence also hints that the processor
has some sense of programs that can or cannot do certain things like change
this table of descriptors. This is called the privilege level of the program and
varies for 0 (highest privilege) to 3 (lowest privilege). The format of a selector
is shown below.
The table index (TI) is set to 0 to access the global table of descriptors
called the GDT (Global Descriptor Table). It is set to 1 to access another
table, the local descriptor table (LDT) that we will not be using. RPL is the
requested privilege level that ranges from 0-3 and informs what privilege level
img
Computer Architecture & Assembly Language Programming
Course Code: CS401
CS401@vu.edu.pk
the program wants when using this descriptor. The 13bit index is the actual
index into the GDT to select the appropriate descriptor. 13 bits mean that a
maximum of 8192 descriptors are possible in the GDT.
The GDT itself is an array of descriptors where each descriptor is an 8byte
entry. The base and limit of GDT is stored in a 48bit register called the
GDTR. This register is loaded with a special instruction LGDT and is given a
memory address from where the 48bits are fetched. The first entry of the
GDT must always be zero. It is called the null descriptor. After that any
number of entries upto a maximum of 8191 can follow. The format of a code
and data descriptor is shown below.
The 32bit base in both descriptors is scattered into different places
because of compatibility reasons. The limit is stored in 20 bits but the G bit
defines that the limit is in terms of bytes of 4K pages therefore a maximum of
4GB size is possible. The P bit must be set to signal that this segment is
present in memory. DPL is the descriptor privilege level again related to the
protection levels in 386. D bit defines that this segment is to execute code is
16bit mode or 32bit mode. C is conforming bit that we will not be using. R
signals that the segment is readable. A bit is automatically set whenever the
segment is accessed. The combination of S (system) and X (executable) tell
that the descriptors is a code or a data descriptor. B (big) bit tells that if this
data segment is used as stack SP is used or ESP is used.
Our first example is a very rudimentary one that just goes into protected
mode and prints an A on the screen by directly accessing 000B8000.
Example 15.1
001
[org 0x0100]
002
jmp
start
003
004
gdt:
dd
0x00000000, 0x00000000
; null descriptor
005
dd
0x0000FFFF, 0x00CF9A00
; 32bit code
006
;
\--/\--/
\/||||\/
007
;
|
|
| ||||+--- Base (16..23)=0 fill later
008
;
|
|
| |||+--- X=1 C=0 R=1 A=0
009
;
|
|
| ||+--- P=1 DPL=00 S=1
010
;
|
|
| |+--- Limit (16..19) = F
011
;
|
|
| +--- G=1 D=1 r=0 AVL=0
168
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Computer Architecture & Assembly Language Programming
Course Code: CS401
CS401@vu.edu.pk
012
;
|
|
+--- Base (24..31) = 0
013
;
|
+--- Limit (0..15) = FFFF
014
;
+--- Base (0..15)=0 fill later
015
dd
0x0000FFFF, 0x00CF9200
; data
016
;
\--/\--/
\/||||\/
017
;
|
|
| ||||+--- Base (16..23) = 0
018
;
|
|
| |||+--- X=0 E=0 W=1 A=0
019
;
|
|
| ||+--- P=1 DPL=00 S=1
020
;
|
|
| |+--- Limit (16..19) = F
021
;
|
|
| +--- G=1 B=1 r=0 AVL=0
022
;
|
|
+--- Base (24..31) = 0
023
;
|
+--- Limit (0..15) = FFFF
024
;
+--- Base (0..15) = 0
025
026
gdtreg:
dw
0x17
; 16bit limit
027
dd
0
; 32bit base (filled later)
028
029
stack:
times 256 dd 0 ; for use in p-mode
030
stacktop:
031
032
start:
mov
ax, 0x2401
033
int
0x15
; enable A20
034
035
xor
eax, eax
036
mov
ax, cs
037
shl
eax, 4
038
mov
[gdt+0x08+2], ax
039
shr
eax, 16
040
mov
[gdt+0x08+4], al
; fill base of code desc
041
042
xor
edx, edx
043
mov
dx, cs
044
shl
edx, 4
045
add
edx, stacktop
; edx = stack top for p-
046
mode
047
048
xor
eax, eax
049
mov
ax, cs
050
shl
eax, 4
051
add
eax, gdt
052
mov
[gdtreg+2], eax
; fill phy base of gdt
053
lgdt
[gdtreg]
; load gdtr
054
055
mov
eax, cr0
056
or
eax, 1
057
058
cli
; MUST disable interrupts
059
mov
cr0, eax
; P-MODE ON
060
jmp
0x08:pstart
; load cs
061
062
;;;;; 32bit protected mode ;;;;;
063
064
[bits 32] ; ask assembler to generate 32bit code
065
pstart:
mov  eax, 0x10
066
mov  ds, ax
067
mov  es, ax
; load other seg regs
068
mov  fs, ax
; flat memory model
069
mov  gs, ax
070
mov  ss, ax
071
mov  esp, edx
072
073
mov
byte [0x000b8000], 'A'
; direct poke at video
074
jmp
$
; hang around
075
Gate A20 is a workaround for a bug that is not detailed here. The BIOS call
will simply enable it to open the whole memory for us. Another important
thing is that the far jump we used loaded 8 into CS but CS is now a selector
so it means Index=1, TI=0, and RPL=0 and therefore the actual descriptor
loaded is the one at index 1 in the GDT.
169
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Computer Architecture & Assembly Language Programming
Course Code: CS401
CS401@vu.edu.pk
15.2. 32BIT PROGRAMMING
Our next example is to give a falvour of 32bit programming. We have
written the printstr function for read and for protected mode. The availability
of larger registers and flexible addressing rules allows writing a much
comprehensive version of the code. Also offsets to parameters and default
widths change.
Example 15.2
001
[org 0x0100]
002
jmp
start
003
004
gdt:
dd
0x00000000, 0x00000000
; null descriptor
005
dd
0x0000FFFF, 0x00CF9A00
; 32bit code
006
dd
0x0000FFFF, 0x00CF9200
; data
007
008
gdtreg:
dw
0x17
; 16bit limit
009
dd
0
; 32bit base
010
011
rstring:
db
'In Real Mode...', 0
012
pstring:
db
'In Protected Mode...', 0
013
014
stack:
times 256 dd 0
; 1K stack
015
stacktop:
016
017
printstr:
push
bp
; real mode print string
018
mov
bp, sp
019
push
ax
020
push
cx
021
push
si
022
push
di
023
push
es
024
025
mov  di,[bp+4]
;load string address
026
mov  cx,0xffff
;load maximum possible size in cx
027
xor  al,al
;clear al reg
028
repne scasb
;repeat scan
029
mov ax,0xffff
;
030
sub ax,cx
;calculate length
031
dec ax
;off by one, as it includes zero
032
mov cx,ax
;move length to counter
033
034
mov
ax, 0xb800
035
mov
es, ax
; point es to video base
036
mov
ax,80
;its a word move, clears ah
037
mul
byte [bp+8]
;its a byte mul to calc y offset
038
add
ax,[bp+10]
;add x offset
039
shl
ax,1
;mul by 2 to get word offset
040
mov
di,ax
;load pointer
041
042
mov si, [bp+4]
; string to be printed
043
mov ah, [bp+6]
; load attribute
044
045
cld
; set auto increment mode
046
nextchar:
lodsb
;load next char and inc si by 1
047
stosw
;store ax and inc di by 2
048
loop nextchar
049
050
pop
es
051
pop
di
052
pop
si
053
pop
cx
054
pop
ax
055
pop
bp
056
ret
8
057
058
start:
push byte 0
; 386 can directly push
059
immediates
060
push
byte 10
061
push
byte 7
062
push
word rstring
063
call
printstr
064
170
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Computer Architecture & Assembly Language Programming
Course Code: CS401
CS401@vu.edu.pk
065
mov
ax, 0x2401
066
int
0x15
; enable a20
067
068
xor
eax, eax
069
mov
ax, cs
070
shl
eax, 4
071
mov
[gdt+0x08+2], ax
072
shr
eax, 16
073
mov
[gdt+0x08+4], al
; set base of code desc
074
075
xor
edx, edx
076
mov
dx, cs
077
shl
edx, 4
078
add
edx, stacktop
; stacktop to be used in p-mode
079
080
xor
ebx, ebx
081
mov
bx, cs
082
shl
ebx, 4
083
add
ebx, pstring
; pstring to be used in p-mode
084
085
xor
eax, eax
086
mov
ax, cs
087
shl
eax, 4
088
add
eax, gdt
089
mov
[gdtreg+2], eax
; set base of gdt
090
lgdt
[gdtreg]
; load gdtr
091
092
mov
eax, cr0
093
or
eax, 1
094
095
cli
; disable interrupts
096
mov
cr0, eax
; enable protected mode
097
jmp
0x08:pstart
; load cs
098
099
;;;;; 32bit protected mode ;;;;;
100
101
[bits 32]
102
pprintstr:
push
ebp
; p-mode print string routine
103
mov
ebp, esp
104
push
eax
105
push
ecx
106
push
esi
107
push
edi
108
109
mov  edi, [ebp+8]  ;load string address
110
mov  ecx, 0xffffffff  ;load maximum possible size in cx
111
xor  al, al
;clear al reg
112
repne scasb
;repeat scan
113
mov  eax, 0xffffffff
;
114
sub  eax, ecx
;calculate length
115
dec  eax
;off by one, as it includes zero
116
mov  ecx, eax
;move length to counter
117
118
mov
eax, 80
;its a word move, clears ah
119
mul
byte [ebp+16]
;its a byte mul to calc y
120
offset
121
add
eax,
[ebp+20]
;add x offset
123
shl
eax,
1
;mul by 2 to get word offset
124
add
eax,
0xb8000
125
mov
edi,
eax
;load pointer
126
127
mov esi, [ebp+8]
; string to be printed
128
mov ah, [ebp+12]
; load attribute
129
130
cld
; set auto increment mode
131
pnextchar:
lodsb
;load next char and inc si by 1
132
stosw
;store ax and inc di by 2
133
loop pnextchar
134
135
pop
edi
136
pop
esi
137
pop
ecx
138
pop
eax
139
pop
ebp
140
ret
16
; 4 args now mean 16 bytes
141
171
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Computer Architecture & Assembly Language Programming
Course Code: CS401
CS401@vu.edu.pk
142
pstart:
mov
ax, 0x10
; load all seg regs to 0x10
143
mov
ds, ax
; flat memory model
144
mov
es, ax
145
mov
fs, ax
146
mov
gs, ax
147
mov
ss, ax
148
mov
esp, edx
; load saved esp on stack
149
150
push
byte 0
133
push
byte 11
134
push
byte 7
135
push
ebx
136
call
pprintstr
; call p-mode print string
137
routine
138
139
mov
eax, 0x000b8000
140
mov
ebx, '/-\|'
141
142
nextsymbol:
mov
[eax], bl
143
mov
ecx, 0x00FFFFFF
144
loop
$
145
ror
ebx, 8
146
jmp
nextsymbol
15.3. VESA LINEAR FRAME BUFFER
As an example of accessing a really large area of memory for which
protected mode is a necessity, we will be accessing the video memory in high
resolution and high color graphics mode where the necessary video memory
is alone above a megabyte. We will be using the VESA VBE 2.0 for a standard
for these high resolution modes.
VESA is the Video Electronics Standards Association and VBE is the set of
Video BIOS Extensions proposed by them. The VESA VBE 2.0 standard
includes a linear frame buffer mode that we will be using. This mode allows
direct access to the whole video memory. Some important VESA services are
listed below.
INT 10 ­ VESA ­ Get
SuperVGA Infromation
AX = 4F00h
ES:DI -> buffer for SuperVGA information
Return:
AL = 4Fh if function supported
AH = status
INT 10 ­ VESA ­ Get SuperVGA Mode Information
AX = 4F01h
CX = SuperVGA video mode
ES:DI -> 256-byte buffer for mode information
Return:
AL = 4Fh if function supported
AH = status
ES:DI filled if no error
INT 10 ­ VESA ­ Set VESA Video Mode
AX = 4F02h
BX = new video mode
Return:
AL = 4Fh if function supported
AH = status
One of the VESA defined modes is 8117 which is a 1024x768 mode with
16bit color and a linear frame buffer. The 16 color bits for every pixel are
organized in 5:6:5 format with 5 bits for red, 6 for green, and 5 for blue. This
makes 32 shades of red and blue and 64 shades of green and 64K total
172
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Computer Architecture & Assembly Language Programming
Course Code: CS401
CS401@vu.edu.pk
possible colors. The 32bit linear frame buffer base address is available at
offset 28 in the mode information buffer. Our example will produces shades
of green on the screen and clear them and again print them in an infinite
loop with delays in between.
Example 15.3
001
[org 0x0100]
002
jmp
start
003
004
modeblock:
times 256 db 0
005
006
gdt:
dd
0x00000000, 0x00000000
; null descriptor
007
dd
0x0000FFFF, 0x00CF9A00
; 32bit code
008
dd
0x0000FFFF, 0x00CF9200
; data
009
010
gdtreg:
dw
0x17
; 16bit limit
011
dd
0
; 32bit base
012
013
stack:
times 256 dd 0
; 1K stack
014
stacktop:
015
016
start:
mov
ax, 0x4f01
; get vesa mode information
017
mov
cx, 0x8117
; 1024*768*64K linear frame
018
buffer
019
mov
di, modeblock
020
int
0x10
021
mov
esi, [modeblock+0x28]
; save frame buffer base
022
023
mov
ax, 0x4f02
; set vesa mode
024
mov
bx, 0x8117
025
int
0x10
026
027
mov
ax, 0x2401
028
int
0x15
; enable a20
029
030
xor
eax, eax
031
mov
ax, cs
032
shl
eax, 4
033
mov
[gdt+0x08+2], ax
034
shr
eax, 16
035
mov
[gdt+0x08+4], al
; set base of code desc
036
037
xor
edx, edx
038
mov
dx, cs
039
shl
edx, 4
040
add
edx, stacktop
; stacktop to be used in p-mode
041
042
xor
eax, eax
043
mov
ax, cs
044
shl
eax, 4
045
add
eax, gdt
046
mov
[gdtreg+2], eax
; set base of gdt
047
lgdt
[gdtreg]
; load gdtr
048
049
mov
eax, cr0
050
or
eax, 1
051
052
cli
; disable interrupts
053
mov
cr0, eax
; enable protected mode
054
jmp
0x08:pstart
; load cs
055
056
;;;;; 32bit protected mode ;;;;;
057
058
[bits 32]
059
pstart:
mov
ax, 0x10
; load all seg regs to 0x10
060
mov
ds, ax
; flat memory model
061
mov
es, ax
062
mov
fs, ax
063
mov
gs, ax
064
mov
ss, ax
065
mov
esp, edx
; load saved esp on stack
066
067
l1:
xor
eax, eax
173
img
Computer Architecture & Assembly Language Programming
Course Code: CS401
CS401@vu.edu.pk
068
mov
edi, esi
069
mov
ecx, 1024*768*2/4
; divide by 4 as dwords
070
cld
071
rep
stosd
072
073
mov
eax, 0x07FF07FF
074
mov
ecx, 32
; no of bands
075
mov
edi, esi
076
077
l2:
push ecx
078
mov  ecx, 768*16
; band width = 32
079
lines
080
cld
081
rep stosd
082
083
mov  ecx, 0x000FFFFF
; small wait
084
loop $
085
pop  ecx
086
087
sub  eax, 0x00410041
088
loop l2
089
090
mov  ecx, 0x0FFFFFFF
; long wait
091
loop $
092
jmp  l1
093
15.4. INTERRUPT HANDLING
Handling interrupts in protected mode is also different. Instead of the IVT
at physical address 0 there is the IDT (interrupt descriptor table) located at
physical address stored in IDTR, a special purpose register. The IDTR is also
a 48bit register similar in structure to the GDTR and loaded with another
special instruction LGDT. The format of the interrupt descriptor is as shown
below.
The P and DPL have the same meaning as in data and code descriptors.
The S bit tells that this is a system descriptor while the 1110 following it tells
that it is a 386 interrupt gate. Our example hooks the keyboard and timer
interrupts and displays certain things on the screen to show that they are
working.
Example 15.4
001
[org 0x0100]
002
jmp
start
003
004
gdt:
dd
0x00000000, 0x00000000
; null descriptor
005
dd
0x0000FFFF, 0x00CF9A00
; 32bit code
006
dd
0x0000FFFF, 0x00CF9200
; data
007
008
gdtreg:
dw
0x17
; 16bit limit
009
dd
0
; 32bit base
174
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Computer Architecture & Assembly Language Programming
Course Code: CS401
CS401@vu.edu.pk
010
011
idt:
times 8 dw unhandled, 0x0008, 0x8e00, 0x0000
012
dw
timer, 0x0008, 0x8e00, 0x0000
013
\---/  \----/
||\/  \----/
014
|
|
|| |
+----- offset bits 16..32
015
|
|
|| +----- reserved
016
|
|
|+------ Type=E 386 Interrupt Gate
017
|
|
+--- P=1 DPL=00 S=0
018
|
+------- selector
019
+-------- offset bits 0..15
020
dw
keyboard, 0x0008, 0x8e00, 0x0000
021
times 246 dw unhandled, 0x0008, 0x8e00, 0x0000
022
023
idtreg:
dw
0x07FF
024
dd
0
025
026
stack:
times 256 dd 0
; 1K stack
027
stacktop:
028
029
start:
mov
ax, 0x2401
030
int
0x15
; enable a20
031
032
xor
eax, eax
033
mov
ax, cs
034
shl
eax, 4
035
mov
[gdt+0x08+2], ax
036
shr
eax, 16
037
mov
[gdt+0x08+4], al
; set base of code desc
038
039
xor
edx, edx
040
mov
dx, cs
041
shl
edx, 4
042
add
edx, stacktop
; stacktop to be used in p-mode
043
044
xor
eax, eax
045
mov
ax, cs
046
shl
eax, 4
047
add
eax, gdt
048
mov
[gdtreg+2], eax
; set base of gdt
049
lgdt
[gdtreg]
; load gdtr
050
051
xor
eax, eax
052
mov
ax, cs
053
shl
eax, 4
054
add
eax, idt
055
mov
[idtreg+2], eax
; set base of idt
056
057
cli
; disable interrupts
058
lidt [idtreg]
; load idtr
059
060
mov
eax, cr0
061
or
eax, 1
062
mov
cr0, eax
; enable protected mode
063
064
jmp
0x08:pstart
; load cs
065
066
;;;;; 32bit protected mode ;;;;;
067
068
[bits 32]
069
unhandled:
iret
070
071
timer:
push eax
072
073
inc
byte [0x000b8000]
074
075
mov  al, 0x20
076
out  0x20, al
077
pop  eax
078
iret
079
080
keyboard:
push eax
081
082
in
al,
0x60
083
mov
ah,
al
084
and
al,
0x0F
085
shr
ah,
4
175
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Computer Architecture & Assembly Language Programming
Course Code: CS401
CS401@vu.edu.pk
086
add
ax, 0x3030
087
cmp
al, 0x39
088
jbe
skip1
089
add
al, 7
090
skip1:
cmp
ah, 0x39
091
jbe
skip2
092
add
ah, 7
093
skip2:
mov
[0x000b809C], ah
094
mov
[0x000b809E], al
095
096
skipkb:
mov  al, 0x20
097
out  0x20, al
098
pop  eax
099
iret
100
101
pstart:
mov
ax, 0x10
; load all seg regs to 0x10
102
mov
ds, ax
; flat memory model
103
mov
es, ax
104
mov
fs, ax
105
mov
gs, ax
106
mov
ss, ax
107
mov
esp, edx
; load saved esp on stack
108
109
mov
al, 0xFC
110
out
0x21, al
; no unexpected int comes
111
112
sti
; interrupts are okay now
113
114
jmp
$
EXERCISES
1. Write very brief and to-the-point answers.
a. Why loading idtr with a value appropriate for real mode is
necessary while gdtr is not?
b. What should we do in protected mode so that when we turn
protection off, we are in unreal mode?
c. If the line jmp code:next is replaced with call code:somefun,
the prefetch queue is still emptied. What problem will occur
when somefun will return?
d. How much is ESP decremented when an interrupt arrives.
This depends on weather we are in 16-bit mode or 32-bit.
Does it depend on any other thing as well? If yes, what?
e. Give two instructions that change the TR register.
2. Name the following descriptors like code descriptor, data descriptor,
interrupt gate etc.
gdt:
dd
0x00000000,
0x00000000
dd
0x00000000,
0x00000000
dd
0x80000fA0,
0x0000820b
dd
0x0000ffff,
0x00409a00
dd
0x80000000,
0x0001d20b
3. Using the above GDT, which of the following values, when moved into
DS will cause an exception and why.
0x00
0x08
0x10
0x18
0x28
0x23
4. Using the above GDT, if DS contains 0x20, which of the following
offsets will cause an exception on read access?
0x0ffff
0x10000
0x10001
176
img
Computer Architecture & Assembly Language Programming
Course Code: CS401
CS401@vu.edu.pk
5. The following function is written in 32-bit code for a 16-bit stack.
Against every instruction, write the prefixes generated before that
instruction. Prefixes can be address size, operand size, repeat, or
segment override. Then rewrite the code such that no prefixes are
generated considering that this is assembled and executed in 32-bit
mode. Don't care for retaining register values. The function copies
specified number of DWORDs between two segments.
[bits 32]
memcpy:
mov
bp, sp
lds
esi, [bp+4]
;
source address
les
edi, [bp+10]
;
destination address
mov
cx, [bp+16]
;
count of DWORDs to move
shl
cx, 1
;
make into count of WORDs
L1:
mov
dx, [si]
mov
[es:di], dx
dec
cx
jnz
L1
ret
6. Rewrite the following scheduler so that it schedules processes stored
in readyQ, where enque and deque functions are redefined and
readyQ contains TSS selectors of processes to be multitasked.
Remember you can't use a register as a segment in a jump (eg jmp
ax:0) but you can jump to an indirect address (eg jmp far [eax]) where
eax points to a six-byte address. Declare any variables you need.
mov al, 0x20
scheduler:
jmp USERONESEL:0
out  0x20, al
mov  byte [USERONEDESC+5], 0x89
jmp  USERTWOSEL:0
out  0x20, al
mov  byte [USERTWODESC+5], 0x89
jmp  scheduler
7. Protected mode has specialized mechanism for multitasking using task state
segments but the method used in real mode i.e. saving all registers in a PCB,
selecting the next PCB and loading all registers from there is still applicable.
Multitask two tasks in protected mode multitasking without TSS. Assume that
all processes are at level zero so no protection issues arise. Be careful to
save the complete state of the process.
8. Write the following descriptors.
a. 32 bit, conforming, execute-only code segment at level 2, with base
at 6MB and a size of 4MB.
b. 16 bit, non-conforming, readable code segment at level 0, with base
at 1MB and a size of 10 bytes.
c.  Read only data segment at level 3, with base at 0 and size of 1MB.
d. Interrupt Gate with selector 180h and offset 11223344h.
9. Write physical addresses for the following accesses where CS points to the
first descriptor above, DS to the second, ES to the third, EBX contains
00010000h, and ESI contains 00020000h
a. [bx+si]
b. [ebx+esi-2ffffh]
c.  [es:ebx-10h]
10. Which of the following will cause exceptions and why. The registers have the
same values as the last question.
a. mov eax, [cs:10000h]
b. mov [es:esi:100h], ebx
c. mov ax, [es:ebx]
11. Give short answers.
a. How can a GPF (General protection fault) occur while running
the following code
push es
pop  es
177
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Computer Architecture & Assembly Language Programming
Course Code: CS401
CS401@vu.edu.pk
b. How can a GPF occur during the following instruction? Give
any two reasons.
jmp
10h:100h
c. What will happen if we call interrupt 80h after loading out IDT
and before switching to protected mode?
d. What will happen if we call interrupt 80h after switching into
protected mode but before making a far jump?
12. Write the following descriptors. Assume values for attributes not
specifically mentioned.
a. Write able 32-bit data segment with 1 GB base and 1 GB limit
and a privilege level of 2.
b. Readable 16-bit code descriptor with 1 MB base and 1 MB
limit and a privilege level of 1.
c. Interrupt gate given that the handler is at 48h:12345678h and
a privilege level of 0.
13. Describe the following descriptors. Give their type and the value of all
their fields.
dd 01234567h, 789abcdeh
dd 30405060h, 70809010h
dd 00aabb00h, 00ffee00h
14. Make an EXE file, switch into protected mode, rotate an asterisk on
the border of the screen, and return to real mode when the border is
traversed.
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