Microprocessor System Design
Omid Fatemi
Instructions (1)
(omid@fatemi.net)
University of Tehran 1
Review
• Flag instruction
• ADD and ADC
• A loop program
• Data entering
• MASM
• Directives
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Outline
• Data transfer operations
• Arithmetic operations
• Logic operation
• Control operations
• String operations
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MASM Program Example
(another way to define segments)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;
;
;
This is an example program. It prints the
;
;
character string "Hello World" to the DOS standard output
;
;
using the DOS service interrupt, function 9.
;
;
;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
hellostk SEGMENT BYTE STACK 'STACK'
;Define the stack segment
DB 100h DUP(?)
;Set maximum stack size to 256 bytes (100h)
hellostk ENDS
hellodat
dos_print
strng
hellodat
SEGMENT BYTE 'DATA' ;Define the data segment
EQU 9
;define a constant via EQU
DB 'Hello World',13,10,'$' ;Define the character string
ENDS
hellocod
START:
SEGMENT BYTE 'CODE' ;Define
mov ax, SEG hellodat
mov ds, ax
mov ah, dos_print
mov dx,OFFSET strng
int 21h
mov ax, 4c00h
int 21h
ENDS
END
START
hellocod
the Code segment
;ax <-- data segment start address
;ds <-- initialize data segment register
;ah <-- 9 DOS 21h string function
;dx <-- beginning of string
;DOS service interrupt
;ax <-- 4c DOS 21h program halt function
;DOS service interrupt
; ‘END label’ defines program
entry
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Yet another way to define Segs
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;
Use .stack,.data,.code directives to define segment types
;
;
;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
.stack 100h
; reserve 256 bytes of stack space
.data
dos_print EQU 9
strng
DB 'Hello World',13,10,'$'
;define a constant
;Define the character string
.code
START:
mov
mov
mov
mov
int
mov
int
ax, SEG strng
ds, ax
ah, dos_print
dx,OFFSET strng
21h
ax, 4c00h
21h
END
START
;ax <-- data segment start address
;ds <-- initialize data segment register
;ah <-- 9 DOS 21h string function
;dx <-- beginning of string
;DOS service interrupt
;ax <-- 4c DOS 21h program halt function
;DOS service interrupt
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Masm Assembler Directives
end label
end of program, label is entry point
proc far|near
begin a procedure; far, near keywords
specify if procedure in different code
segment (far), or same code segment (near)
endp
end of procedure
page
set a page format for the listing file
title
title of the listing file
.code
mark start of code segment
.data
mark start of data segment
.stack
set size of stack segment
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Data Allocation Directives
db
define byte
dw
define word (2 bytes)
dd
define double word (4 bytes)
dq
define quadword (8 bytes)
dt
define tenbytes
equ
equate, assign numeric expression to a name
Examples:
db 100 dup (?)
define 100 bytes, with no initial values for bytes
db “Hello”
define 5 bytes, ASCII equivalent of “Hello”.
maxint equ
32767
count
10 * 20
equ
; calculate a value (200)
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Data Transfer Instructions
• Very Common Instruction:
• Allowed Operands
mov desti, source
Destination
Source
Memory
Accumulator
Accumulator Memory
Register
Register
Register
Memory
Memory
Register
Register
Immediate
Memory
Immediate
Seg. Reg.
Register
Seg. Reg.
Memory
Register
Seg. Reg.
Memory
Seg. Reg.
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Arithmetic
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Arithmetic/Logic Instructions
• Basic Mathematical Operations
– Signed/Unsigned Integer Only
– Default is 2’s Complement
– Computes Result AND Modifies Status Flags
• Logic Instructions
– Bit Level
– Word Level
– Computes Results AND Modifies Status Flags
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Arithmetic Instruction Summary
add
adc
inc
aaa
daa
sub
sbb
dec
neg
cmp
das
aas
mul
imul
aam
div
idiv
aad
ax,
ax,
ax
bx
bx
ax,
ax,
ax
ax
ax,
bx
bx
cx
cx
cl
cx
bx
;axax+bx and set flags
;axax+bx+CF(lsb) and set flags
;axax+1 and set flags
;ASCII Adjust after Addition
;Decimal (BCD) Adjust after Addition
;axax-bx and set flags
;ax(ax-CF)-bx and set flags
;axax-1
;ax(-1)*(ax) -- 2’s Complement
;Flags are set according to ax-bx
;Decimal (BCD) Adjust after Subtraction
;ASCII Adjust after Subtraction
;dx:ax ax * cx (unsigned)
;dx:ax ax * cx (2’s complement)
;ASCII Adjust after Multiplication
;alax/cl Quot. AND ahax/cl Rem.
;ax(dx:ax)/cx Quot. AND dx  Rem.
;ASCII Adjust after Division
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Addition Instruction Types
add
adc
inc
aaa
daa
ax,
ax,
ax
Addition
add
al,
add
bx,
add
[bx],
add
cl,
add
al,
add
bx,
add
bx,
bx
bx
;axax+bx and set flags
;axax+bx+CF(lsb) and set flags
;axax+1 and set flags
;ASCII Adjust after Addition
;Decimal (BCD) Adjust after
bl
35afh
al
[bp]
[ebx]
TEMP[di]
[eax+2*ecx]
;alal+bl and set flags
;bxbx+35afh
;ds:(bx)ds:(bx)+al
;clcl+ss:(bp)
;alal+ds:(ebx)
;bxbx+ds:(TEMP+di)
;bxbx+ds:(eax+(2*ecx))
Scaled Index Addressing: 386+
ecx may contain 1, 2 , 4 only
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Increment Examples
inc
bl
;blbl+1 and set flags
inc
BYTE PTR [bx]
;Byte at ds:(bx)ds:(bx)+1
New MASM Directive:
BYTE POINTER
00ffh
inc

[bx]
00ffh
inc
0000h
[DATA1]
;Word at ds:(bx)ds:(bx)+1

0100h
;ds:(DATA1)ds:(DATA1)+1
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Add with Carry
BX
1 1
AX
DX
0 1
CX
CF=1
BX
CF
add
adc
ax,
bx,
cx
dx
AX
;axax+cx and flags set
;bxbx+dx+CF(lsb) and flags set
33-bit Sum Present in CF:bx:ax
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Decimal Adjust after Addition
• For BCD Arithmetic
• “Corrects” Result
0110
+0111
1101
6
7
13should be 0001 0011
(1101 is illegal BCD)
•2 Digits/Word Intel Refers to as “Packed Decimal”
•daa Uses Implicit Operand, al Register
•Follows add, adc to “Adjust”
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Decimal Adjust after Addition Example
mov
mov
mov
add
daa
mov
mov
adc
daa
mov
dx,
bx,
al,
al,
1234h
3099h
bl
dl
cl,
al,
al,
al
bh
dh
ch,
al
;dx1234 BCD
;bx3099 BCD
;al99 BCD
;alcdh illegal BCD, need 34+99=133
;al33h (33 BCD) and CF=1
;cl33 BCD
;al30 BCD
;al30h+12h+1=43h
;al43h (43 BCD) not illegal BCD this time
;cx=4333h BCD for 1234+3099
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ASCII Adjust after Addition
• For Addition Using ASCII Encoded Numbers
30h through 39h Represent ‘0’ through ‘9’
• ax is Default Source and Destination for aaa
31
+39
6a
‘1’
‘9’
‘10’should be 3130h
(6ah is incorrect ASCII result ‘j’)
mov
add
aaa
add
ax,
al,
ax,
31h
39h
;ax0031h=‘1’
;ax31h+39h=006ah=‘<nul>j’
;ax0100h (this is BCD of result)
3030h ;Convert from BCD to ASCII
;ax0100h+3030h=3130h=‘10’
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Subtraction Instruction Types
sub
sbb
dec
neg
cmp
das
aas
ax,
ax,
ax
ax
ax,
bx
bx
bx
;axax-bx and set flags
;ax(ax-CF)-bx and set flags
;axax-1
;ax(-1)*(ax) - 2’s Complement
;Flag is set according to ax-bx
;Decimal (BCD) Adjust after Subtraction
;ASCII Adjust after Subtraction
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Allowable Operands for add, sub
Gen Reg
Gen Reg
+
-
Mem Loc
Immediate
Destination
Source
Gen Reg
Mem Loc
+
Immediate
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Subtract with Borrow, sbb
CF
CF
sub
sbb
ax,
bx,
BX
AX
SI
DI
BX
AX
di
si
;axax-di and CF gets borrow bit
;bx(bx-CF(lsb))-si and flags set
32-bit Difference Present in bx:ax
CF Indicates If Difference is Negative
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Multiplication
• 8086/8088 One of First to Include mul/div Instruction
• Allowable Operands: Bytes, Words, DoubleWords
•Allowable Results: Words, DoubleWords, QuadWords
•OF, CF Give Useful Information
•AF, PF, ZF, SF Change but Contents Unpredictable
•Multiplicand Always in al, ax, eax
•mul - Unsigned Mnemonic
•imul - Signed Mnemonic
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Multiply Instructions
• Product can be Twice the Size
23=6
(same size)
2  8 = 16
(double size, EXT)
•OF=CF=0 means product is same size as result (faster)
•OF=CF=1 means EXT product size (slower)
•AF, PF, ZF, SF Contents Unpredictable
mul
mul
mul
imul
imul
bl
bx
ebx
bl
bx
;axal*bl, Unsigned
;dx:axbx*ax, Unsigned
;edx:eaxebx*eax, Unsigned
;axal*bl, Signed
;dx:axbx*ax, Signed
imul
ebx
;edx:eaxebx*eax, Signed
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Special Immediate Multiply Instruction
• 286+
• Uses imul Mnemonic but with 3 Operands
first:
16-bit dest. register
second: reg/mem location
third:
8/16-bit immediate value
•Always Performs Signed Multiplication
•Product is Limited to 16-bits
imul
cx, dx, 12h
;cxdx*12h
imul
bx, [NUMBER], 12h
;bxds:(NUMBER)*12h
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Division
• 8, 16, 32 bit Operands (32 bit is 386+)
• No Immediate Addressing Mode
• No Flag Bits Change Predictably
• Can Cause Two Types of Error:
1) Divide by 0 (Mathematically Undefined)
2) Divide Overflow (Wordlength Problem)
• Operands: Divisor is Programmer Specified
• Dividend is Implied
• Quotient, Remainder Implied
Size Dividend Quotient Remainder
ax
al
ah
8 bits
ax
dx
16 bits dx:ax
edx
32 bits edx:eax eax
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Division Instruction Examples
• idiv Signed and div Unsigned
dividend / divisor = quotient, rmdr
div
cx
;dx:ax is divided by value in cx
;unsigned quotient is placed in ax
;positive remainder is placed in dx
idiv
ebx
;edx:eax is divided by value in ebx
;signed quotient is placed in eax
;remainder (ALWAYS same sign as
;dividend) is placed in edx
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Logical Instructions
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Logic Instruction Types
not
and
or
xor
test
shl
sal
shr
sar
ax
ax,
ax,
ax,
ax,
ax,
ax,
ax,
ax,
BITWISE LOGICAL
;1’s Complement-Logical Invert
bx
;Bitwise logical and operation
bx
;Bitwise logical inclusive-or operation
bx
;Bitwise logical exclusive-or operation
fffh ;Bitwise and but result discarded
4
3
4
3
SHIFT
;Logical shift left
;Arithmetic shift left
;Logical shift right
;Arithmetic shift right
rol
ror
rcl
bx, 3
cx, 4
ax, 1
ROTATE
;Rotate left
;Rotate right
;Rotate left through carry
rcr
dx, 6
;Rotate right through carry
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Bit Level Logic
and, or, xor, not, test, bt, btc, btc, btr, bts
• Affect Status Flags as Follows:
1) Always Clears CF and OF
2) SF, ZF, AF, PF Change to Reflect Result
• Common Usage:
and
ax, ax
;clear CF and OF
xor
ax, ax
;clear ax=CF=OF=PF=AF=SF=0 and ZF=1
;does more than mov ax, 0h
;faster than push 00h then popf
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Masking Operations
(AND)
XXXX XXXX (unknown word)
0000 1111 (mask word)
0000 XXXX (result)
What if we wanted 1111 XXXX instead?
EXAMPLE: Convert ASCII to BCD to Binary
;First convert to BCD - change 3235h into 0025h
mov
bx,
3235h
;bx ‘25’
and
bx,
0f0fh
;bx0205h
mov
dx,
bx
;dx0205h
shl
bh,
4
;bh20h
or
bl,
bh
; bl = bh or bl = 20 or 05 = 25h
xor
bh,
bh
;zero out bh, so bx = 0025 (BCD value)
;Now convert to binary - change 3235h into 0019h
mov
al,
dh
;al02h
mov
cl,
10
;cl0ah
mul
cl
;ax = 2 * 0Ah = 14h (decimal value is 20)
add
al,
dl
;al14h+05h=19h (decimal value is 25)
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Bit Test Instruction, test
• Same as and But Result is Discarded
• Only Affects Flags (like cmp)
• Use test for Single Bit and cmp for Byte, Word
• ZF=1 if Tested Bit=0 and ZF=0 if Tested Bit=1
test
al,
1
;XXXX XXXX (AND) 0000 0001
test
al,
128
;XXXX XXXX (AND) 1000 0000
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Shifts
shl
shr
sal
sar
-
-
-
-
Logical Shift Left
CF
REG
0
Logical Shift Right
0
REG
CF
Arithmetic Shift Left (same as logical)
CF
REG
0
Arithmetic Shift Right (sign bit is preserved)
REG
CF
MSB
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Simple Arithmetic Using Shifts
;Compute (-3)*VALUE Using Only Shifts and Adds
mov
mov
shl
add
shl
sub
ax,
bx,
ax,
ax,
bx,
ax,
VALUE
ax
2
bx
3
bx
;ax
;bx
;ax
;ax
;bx
;ax






Word from memory with label VALUE
Word from memory with label VALUE
4*VALUE
5*VALUE
8*VALUE
(-3)*VALUE
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Rotates
rol
- Rotate Left
CF
rcl
ror
rcr
REG
- Rotate Through Carry Left
CF
REG
CF
REG
- Rotate Right
- Rotate Through Carry Right
CF
REG
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Example Using Rotates
;Multiply a 48-bit value in dx:bx:ax by 2
shl
rcl
rcl
ax,
bx,
dx,
1
1
1
;ax  2*ax
;bx  2*bx + CF(lsb)
;dx  2*dx + CF(lsb)
;End result is dx:bx:ax  2*(dx:bx:ax)
• Operand for rotates and shifts can be either:
1) Immediate value
2) Quantity in cl
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Program Control Instructions
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Program Control Instructions
•Generally modify CS:IP
•Causes modification in execution sequence (of instructions)
• When such a program flow change occurs:
a) Instructions in the BIU inst. queue become invalid
b) BIU directly fetches CS:IP instruction from memory
c) While EU executes new instruction, BIU flushes/refills inst. queue
• Classification
a) Jumps - Unconditional control transfers (synchronous)
b) Branches - Conditional control transfer
c) Interrupts - Unconditional control transfers (asynchronous)
d) Iteration - More complex type of branch
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Control Instruction Summary
jmp
call
ret
hlt
LABEL
LABEL
UNCONDITIONAL
;next instruction executed has LABEL
;next instruction executed has LABEL
;next instruction executed is after the call
;nothing executed until RESET signal
loop
LABEL
loope/loopz LABEL
loopne/loopnz
ITERATION
;cx  cx - 1, jump to LABEL if cx > 0
;same as loop but ZF=1 also required
;same as loop but ZF=0 also required
int
into
iret
INTERRUPTS
;Invoke the int. handler specified by immed8
;same as int but OF=1 also
;Return from interrupt handler
<immed8>
<immed8>
CONDITIONAL to follow
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Simplest Control Instruction, jmp
jmp
LABEL
;LABEL is offset address of instruction
;in the code segment
3 Forms of jmp
SHORT
- 2 bytes, allows jump to ±127 locations from current address
EB
NEAR
- 3 bytes, allows jump to ±32K locations from current address
E9
FAR
disp
disphi
displo
- 5 bytes anywhere in memory
EA
IP lo
IP hi
CS lo
CS hi
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Example with Short Jump
;Causes bx to count by 1 from 0 to 65535 to 0 to 65535 to …
xor
start: mov
add
jmp
next:
xor
xor
mov
jmp
bx,
ax,
ax,
next
bx
1
bx
bx,
ax,
bx,
start
bx
ax
ax
;Clear
;ax 
;ax 
;add a
;
;Clear
;Clear
;bx 
;add a
;
bx and initialize status flags
1
ax+bx
displacement to IP
(+2 from xor to mov)
bx and initialize flags
ax and initialize flags
ax
displacement to IP
(a negative value - 2’s comp.)
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Indirect Jump
• Address of target is in register
• Does NOT add disp to IP - Transfer REG contents to IP
;assume that si contains either 0, 1 or 2
add
si, si
;si  2*si
add
si, OFFSET TABLE
;si  si + <address of TABLE>
mov
ax, cs:[si]
;ax gets an address from the jump table
jmp
ax
;ip  ax
;the following jump TABLE is defined in the code segment!!!!
TABLE: DW
ZERO
DW
ONE
DW
TWO
ZERO:
;code for ZERO option
.
.
ONE:
;code for ONE option
.
.
TWO:
;code for TWO option
.
.
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Indirect Addressed Jump
• Address of target is in register
• Does NOT add disp to IP - Transfer MEM contents to IP
;assume that si contains either 0, 1 or 2
add
si, si
;si  2*si
add
si, OFFSET TABLE
;si  si + <address of TABLE>
jmp
cs:[si]
;ip gets an address from the jump table
;the following jump TABLE is defined in the code segment!!!!
TABLE: DW
ZERO
DW
ONE
DW
TWO
ZERO:
;code for ZERO option
.
.
ONE:
;code for ONE option
.
.
TWO:
;code for TWO option
.
.
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Conditional Control Instruction Summary
Simple Flag Branches
Jump based on single flag
CONDITIONAL
jc
jnc
je/jz
jne/jnz
jo
jno
js
jns
jp/jpe
jnp/jpo
LABEL
LABEL
LABEL
LABEL
LABEL
LABEL
LABEL
LABEL
LABEL
LABEL
;jump
;jump
;jump
;jump
;jump
;jump
;jump
;jump
;jump
;jump
on
on
if
if
if
if
on
if
if
if
carry (CF=1)
no carry (CF=0)
ZF=1 - jump if equal/zero
ZF=0 - jump not equal/jump if zero
OF=1 - jump on overflow
OF=0 - jump if no overflow
sign flag set (SF=1)
no sign flag (SF=0)
PF=1 - jump on parity/parity even
PF=0 - jump on no parity/parity odd
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Conditional Control Instruction Summary
Branches for unsigned comparisons
Jump is based on flags used for unsigned number
comparison (based on C, Z flag)
ja/jnbe
jae/jnb
jb/jnae
jbe/jna
LABEL
LABEL
LABEL
LABEL
;jump
;jump
;jump
;jump
Typical use:
cmp al,bl
jb there
if
if
if
if
CONDITIONAL
CF=ZF=0 - jump above-jump not below/equal
CF=0 - jump above/equal-jump not below
CF=1 - jump below-jump not above/equal
CF=1 or ZF=1 - jump equal - jump zero
; jump if al is ‘below’ bl
; unsigned comparison
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Conditional Control Instruction Summary
Branches for signed comparisons
Jump is based on flags used for signed number
comparison (based on Z, S, V flags)
CONDITIONAL
jg/jnle LABEL
jge/jnl LABEL
jl/jnge LABEL
jle/jng LABEL
;jump
;
;jump
;jump
;
;jump
;
Typical use:
cmp al,bl
jl there
if ZF=0 and (SF=OF) - jump greater/not less
nor equal
if SF=OF - jump greater-equal/not less than
if SF  OF - jump less than/not greater nor
equal
if ZF=1 or SF  OF - jump less or equal/not
greater than
; jump if al is less than bl
; signed comparison
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SET condition Instruction
• Sets a byte operand to 1 if a given condition is true, or
it set the byte to 0 if the condition is false
• Useful for saving flag contents
• Syntax is SETcondition reg8 or mem8
• condition includes the suffixes of all conditional jump
instructions
EXAMPLE
setb
T1
;T1  1 if CF=1 else T1  0
seto
T1
;T1  1 if OF=1 else T1  0
setz
al
;AL  1 if ZF=1 else AL  0
setnc
myFlag ;myFlag  1 if CF=0 else myFlag  0
setge
byte ptr [si]
;set [si] to 1 if SF = OF
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Iteration Instruction, loop
• Combination of decrement cx and conditional Jump
• Decrements cx and if cx0 jumps to LABEL
• 386+ loopw (cx operation) and loopd (ecx operation)
Example:
ADDS
PROC
mov
mov
mov
cld
AGAIN: mov
lodsw
add
mov
ADDS
stosw
loop
ret
ENDP
NEAR
cx,
si,
di,
bx,
ax,
di,
AGAIN
100
;cx  64h - number of words to add
OFFSET BLOCK1 ;si  offset of BLOCK1 (in ds)
OFFSET BLOCK2 ;di  offset of BLOCK2 (in es)
;Auto-increment si and di, DF=0
di
;bx  di, save offset of BLOCK2
;ax  ds:[si], sisi+2, didi+2
[bx]
;ax  ax + ds:[bx]
bx
;di  bx, restore di with
;
offset in BLOCK2
;es:[di]  ax, sisi+2, didi+2
;cx  cx - 1, if cx0 jump to AGAIN
;ip  ss:[sp]
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Procedures
• Group of instructions that perform single task
– (can be used as) a SUBROUTINE
call
ret
- invokes subroutine - pushes ip
- returns from subroutine - pops ip
• Uses MASM directives:
PROC and ENDP
• Must specify
NEAR
FAR
- intrasegment
- intersegment
• Difference is op-code of ret
NEAR
FAR
- c3h - pops IP
- cbh - pops CS, pops IP
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call Instruction
• Differs from jmp since return address on stack
NEAR call:
FAR call:
3 bytes - 1 opcode and 2 for IP
5 bytes - 1 opcode, 2 for IP and 2 for CS
• call with operand - can use 16-bit offset in any register
except segment registers
call
bx
;pushes ip then jumps to cs:[bx]
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call Instruction - Example
mov
call
COMP
COMP
PROC
push
mov
in
inc
out
pop
ret
ENDP
si,
OFFSET COMP
si
.
.
.
NEAR
dx
dx, 03f8h
al,
dx
dx
dx,
al
dx
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call Instruction - Example Explained
mov
call
COMP
COMP
PROC
push
mov
in
si,
OFFSET
si
.
.
.
NEAR
dx
dx, 03f8h
al,
dx
inc
out
dx
dx,
pop
ret
ENDP
dx
al
COMP
;get offset of COMP subroutine
;push ip, ipsi
;Save current contents of dx
;dx  03f8h (an immediate data Xfer)
;al receives 1 byte of data from I/O
; device with output port address 03f8h
;dx03f9h
;send 1 byte of data to I/O device
; input port with address 03f9h
;restore dx to value at call time
;ipss:[sp], spsp+2
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call Instruction with Indirect Address
• Useful for choosing different subroutines at runtime
• Can use a table (like the jump table example)
;Assume bx contains 1, 2 or 3 for subroutine desired
TABLE
DW
ONE
DW
TWO
DW
THREE
dec
bx
add
bx,
bx
mov
di, OFFSET TABLE
call
cs:[bx+di]
jmp
CONT
ONE
PROC
NEAR
…
ONE
ENDP
TWO
PROC
NEAR
…
TWO
ENDP
THREE
PROC
NEAR
…
THREE
ENDP
CONT:
nop
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call Instruction with Indirect Address
;Table of addresses of subroutines
TABLE
DW
ONE
DW
TWO
DW
THREE
;bx contains 1, 2 or 3 - desired subroutine
dec
bx
;bx  0, 1 or 2
add
bx,
bx
;bx  0, 2 or 4
mov
di, OFFSET TABLE
;di  TABLE offset
call
cs:[bx+di]
;push ip, ipoffset of subroutine
jmp
CONT
;ip  offset of nop instruction
ONE
PROC
NEAR
…
ONE
ENDP
TWO
PROC
NEAR
…
TWO
ENDP
THREE
PROC
NEAR
…
THREE
ENDP
CONT:
nop
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ret Instruction
NEAR FAR •
•
pops 16-bit value places in IP
pops 32-bit value places in CS:IP
Type is determined by PROC directive
Other form of ret has immediate operand (8 bit)
The immediate operand is added to the SP after popping the
return address
Example
ret
6
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University of Tehran 54
String Transfer Instructions
• String Forms:
movsb
movsw
;move string byte by byte
;move string word by word
EXAMPLE:
movsb
;Copies 8 bits at DS:SI to ES:DI
• New String Form (386+):
movsd
;move string in double words
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String Transfer Instructions
New mov forms (386+):
movsx
;move string with sign extended
- Reads source as byte or word and sign extends
to word or double word before storing in destination
EXAMPLE:
movsx cx, al
movzx
;cl get al
;if MSB of al=0 then
;ch gets 00h
;else ch gets ffh
;move string with zero extended
- Reads source as byte or word and zero extends
to word or doub. word before storing in destination
EXAMPLE:
movzx cx, al
;ch gets 00h and cl gets
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University
Repeated String Move Example
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;
;
;
This is an example program which shows how
;
;
the string move instruction works.
;
;
;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
strngstk
strngstk
SEGMENT BYTE STACK 'STACK'
DB 100h DUP(?)
ENDS
strngdat
strng1
strng2
crlf
strngdat
SEGMENT BYTE 'DATA' ;Define the data segment
DB 'This is string 1',13,10,'$' ;Define the first string
DB 'THIS IS STRING 2',13,10,'$' ;Define the second string
DB 13,10,'$'
;Space to new line string
ENDS
strngcod
START:
SEGMENT
mov ax,
mov ds,
mov es,
;
;
;
BYTE 'CODE' ;Define
SEG strngdat
ax
ax
;Define the stack segment
;Set stack size to 100 bytes
the
;ax
;ds
;es
Code segment
<-- data segment start address
<-- initialize data segment register
<-- initialize extra segment register
Print the strings to the display before moving them
mov
lea
int
lea
int
lea
int
ah,
dx,
21h
dx,
21h
dx,
21h
9
strng1
strng2
crlf
;ah <-- 9 DOS 21h string function
;dx <-- offset of first string
;DOS service interrupt
;dx <-- offset of second string
;DOS service interrupt
;dx <-- offset of crlf string
;DOS service interrupt
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Repeated String Move Example (Cont.)
;
;
;
Now do a repeated string move byte by byte
cld
lea si, strng2
lea di, strng1
mov cx, 19
rep movsb
;
;
;
;Autoincrement set DF=0
;Source is second string
;Destination is first string
;Strings have 19 (decimal) chars.
;Repeated stirng move from 2 to 1
Print the strings to the display after moving them
mov
lea
int
lea
int
lea
int
;
;
;
ah,
dx,
21h
dx,
21h
dx,
21h
9
strng1
strng2
crlf
;ah <-- 9 DOS 21h string function
;dx <-- offset of first string
;DOS service interrupt
;dx <-- offset of second string
;DOS service interrupt
;dx <-- offset of crlf string
;DOS service interrupt
Invoke DOS interrupt that returns processor to OS
mov ax, 4c00h
;ax <-- 4c DOS 21h program halt
int 21h
strngcod
START
;DOS service interrupt
function
END
ENDS
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Assembling/Linking
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Running the String Move Program
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Other String Instructions
lodsb
;loads al with contents of ds:si
;Inc/Dec si by 1 depending on DF
lodsw
;loads ax with ds:si
;Inc/Dec si by 2 depending on DF
lodsd
;loads eax with ds:si
;Inc/Dec si by 4 depending on DF
;386+
stosb
;loads es:di with contents of al
;Inc/Dec di by 1 depending on DF
stosw
;loads es:di with contents of ax
;Inc/Dec di by 2 depending on DF
stosd
;loads es:di with contents of eax
;Inc/Dec di by 4 depending on DF
;386+
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Logic Instruction Types (386+)
shld
shrd
SHIFT
ax, 12 ;Double precision logical shift left
ax, 14 ;Double precision logical shift right
bt
bts
btr
btc
BIT TEST
ax, 12 ;CF12th bit from right in ax
bx, 8 ;CF8th bit of bx and bx[8]1
cx, 1 ;CF1st bit in cx and cx[1]0
dx, 2 ;CF2nd bit of dx and dx[2]dx[2]’
bsf
bsr
BIT SCAN
ax, bx ;ZF=1 if all bits in bx=0
;else ZF=0 and ax gets index of first
;set bit (1) starting from right (LSB) of bx
ax, bx ;ZF=1 if all bits in bx=0
;else ZF=0 and ax gets index of first
;set bit (1) starting from left (MSB) of bx
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Double Precision Shifts
• 386+
•shld - Logical Shift Left
•shrd - Logical Shift Right
• Uses 3 Operands Instead of 2
• Example
shrd
ax,
bx,
12
;logical right shift of ax by 12
;rightmost 12 bits of bx into
;leftmost 12 bits of ax
• Contents of bx remain unchanged !!!!!!!
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String Scan Instruction, scas
•scasb, scasw, scasd (386+)
• Compares al, ax, eax with memory data
• Does an integer subtraction - result not saved
• Generally used with a REPEAT prefix
•DF controls auto-increment/decrement
•Example:
mov
di,
cld
mov
cx,
xor
al,
repne scasb
OFFSET BLOCK
100
al
;di  address of memory location BLOCK
;DF  0, auto-increment mode
;cx  64h, initialize counter to 100
;clear al
;test for 00h in location es:di
;if es:di not equal to 00h then
; cx  cx - 1, di di + 1, repeat
;else if cx = 00h
; do not repeat test
;else if es:di equals 00h
; ZF = 1, do not repeat test
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Skip ASCII Space Character
lea
di,
cld
mov
cx,
mov
al,
repe scasb
STRING ;di  offset of memory location labeled STRING
;DF=0 auto-increment mode
256
;cx  ffh, initialize counter to 256
20h
;al  ‘ ’, an ASCII <space> Character
;while es:di=20h, continue scanning
;when cx=0 or es:di not equal 20h stop
;after stopping cx contains offset from
;STRING where first non-20h resides (if not 0)
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Compare String Instruction, cmps
•cmpsb, cmpsw, cmpsd (386+)
• Compares 2 sections of memory
• Does an integer subtraction - result not saved
• Generally used with a REPEAT prefix
•si, di auto-increment/decrement depending on DF
•Example: Test two strings for equivalence
;Assume that ds and es are already set-up (NOTE:ds can equal es)
lea
si,
LINE
;si gets offset of location labeled LINE
lea
di,
TABLE
;di gets offset of location labeled TABLE
cld
;DF=0, auto-increment mode
moc
cx,
10
;initialize counter register to 10
repe cmpsb
;while ds:si=es:di decrement cx and incr. si, di
;if cx=0 stop testing
;after complete, if cx not equal 0, then
;strings do not match
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Skip ASCII Space Character
lea
di,
cld
mov
cx,
mov
al,
repe scasb
STRING ;di  offset of memory location labeled STRING
;DF=0 auto-increment mode
256
;cx  ffh, initialize counter to 256
20h
;al  ‘ ’, an ASCII <space> Character
;while es:di=20h, continue scanning
;when cx=0 or es:di not equal 20h stop
;after stopping cx contains offset from
;STRING where first non-20h resides (if not 0)
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Summary
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