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Assembly Language Programming II
Week 5 – Overview of x86 assembly language programming II
Different Addressing modesMOV AL, BL Register addressing mode
MOV AL, 15H immediate
MOV AL, abc
MOV AL, [1234H]
direct
MOV AL, [SI] Register indirect
MOV AL, [BX+SI] Base plus index
MOV AL, [BX+4]
MOV AL, ARRAY[3]
Register relative
MOV AL, [BX+DI+4]
MOV AL, ARRAY[BX+DI]
Base relative plus index
2
How to move the address of a variable to a register ?
• When using indirect addressing, such as– MOV AL, [SI]– SI is an address so how can we initialize SI?
• This is by the instruction called LEA (Load Effective Address)
• LEA is similar to x = &y in C++• Syntax of LEA:– LEA SI, ARRAY
3
Base-plus-index addressing mode• Move a byte or word between a register and the
memory location addressed by a base register (BP or BX) plus an index register (DI or SI)
• Physical address of the operand is obtained by adding a direct or indirect displacement to the contents of either base register BX or base pointer register BP and the current value in DS and SS, respectively.
• Eg MOV [BX+SI], AL• Move value in AL to a location (DS+BX+SI)• If BP is used then use SS instead of DS
4
Application of Base + index
• The base register (BX) often holds the beginning location of a memory array, while the index register (SI) holds the relative position of an element in the array
• Change the value of SI then you can access different elements in the array
5
Register relative addressing mode
• Move a byte or word between a register and memory location addressed by an index or base register plus a displacement
• Eg MOV AL, ARRAY[SI]• EA = value of SI + ARRAY• Physical address = EA + DS• Eg mov AX, [BX+4]• Eg mov AX, array[DI+3]• This is similar to the base-plus-index
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Base relative plus index addressing mode• Transfers a byte or word between a register and
the memory location addressed by a base and an index register plus a displacement
• Combining the based addressing mode and the indexed addressing mode together
• Eg MOV AH, [BX+DI+4]• EA = value of BX + 4 + value of DI• Physical address = DS + EA • This can be used to access data stored as a 2-D
matrix• Eg mov AX, array[BX+DI]
7
Example• Select an instruction for each of the following
tasks:– Copy BL into CL
• Mov CL, BL– Copy DS into AX
• Mov AX, DSSuppose that DS=1100H, BX=0200H, LIST=0250H, and SI=0500H,
determine the address accessed by each of the following instructions:mov LIST[SI], DS (offset address: LIST + SI=0750H, PA = offset +DS = 11750H) mov CL, LIST[BX+SI] ( offset address: LIST+BX+SI = 0950H, PA = 11950H)mov CH, [BX+SI] (offset address: BX+SI = 0700H, PA = 11700H)
8
String addressing mode
• The string instructions of the 8086 instruction set automatically use the source (SI) and destination index registers (DI) to specify the effective addresses of the source and destination operands, respectively.
• Instruction is MOVS• There is no operand after movs• Don’t need to specify the register but SI and
DI are being used during the program execution
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Port addressing mode• Port addressing is used in conjunction with IN and OUT
instruction to access input and output ports. • Any of the memory addressing modes can be used for the
port address for memory-mapped ports.• For ports in the I/O address space, only the direct addressing
mode and an indirect addressing mode using DX are available• Eg IN AL, 15H ; second operand is the port number• Input data from the input port at address 1516 of the I/O
address space to register AL• Eg IN AL, DX• Load AL with port number stored in DX
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Exercises1. Compute the physical address for the specified operand in each of the
following instructions:
MOV [DI], AX (destination operand)
[DI] refers to the address 0200 so PA = 0B200H
MOV DI, [SI] (source operand)
[SI] refers to 0100 so PA = 0B100H)
MOV XYZ[DI], AH (destination operand)
XYZ[DI] refers to 0400+0200, PA = 0B600H
Given CS=0A00, DS=0B00, SI=0100, DI=0200,
BX=0300, XYZ=0400
11
Exercises2. Express the decimal numbers that follows as unpacked and packed BCD bytes
(BCD – binary coded decimal)
a. 29 b. 88
Ans. unpacked BCD takes 1 byte per-digit so 29 becomes
00000010 00001001 as packed 00101001
3. How would the BCD numbers be stored in memory starting at address 0B000
Ans. if it is packed BCD then 29 will occupy one byte at 0B000, if as unpacked BCD then it occupies 2 bytes 00000010 at 0B001 and 00001001 at 0B000)
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Instruction set• A total of 117 basic instructions• 5 groups: data transfer, arithmetic, logic, shift, and
rotate• Data transfer instructions – to move data either
between its internal registers or between an internal register and a storage location in memory
• Cannot move data from one memory location to another memory location
• When the segment register is used as an operand then the move instruction must apply to a 16-bit data
• Mov SS, data ; then data must be 16-bit
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Move instruction Mnemonic Meaning Format Operation Flags affected
Mov Move mov D,S (S) to (D) none
Destination (D) Source (S)
Memory AX
AX Memory
Register Register
Register Memory
Memory Register
Register Immediate
Memory Immediate
Seg-reg Reg16
Seg-reg Mem16
Reg16 Seg-reg
Mem16 Seg-reg14
Special data transfer instructions• XCHG – exchange data between the source and
destination• Eg XCHG AX, DX• Then value in DX is stored in AX and value in AX
stored in DX
15
Special data transfer instruction• LEA – load effective address• LEA is are very important operation
because it allows you to obtain an address of a variable or an array then you can apply the relative addressing modes!!!!
• Eg LEA SI,INPUT• To load a specified register with a 16-bit
offset address• Similar to x = &y in a C++ program
16
Example of LEA
Dat db 11H, 22H, 33H, 44H
LEA BX, datMov AL, [BX+2] ; this is register relative addressing mode
What is being stored in AL ?????
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• LDS – load register and DS• Eg LDS SI, [200]• LDS will load a specified register (SI) and
the DS • 4 bytes of data will be fetched the first two
stored in SI and the following two stored in DS
• LES – load register and ES• LES will load a specified register and the ES
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Arithmetic instructions
• Some examples of available arithmetic operations: add, substract, multiply, etc.
• Can apply to unsigned, signed, binary bytes, words, unpacked, packed decimal bytes, or ASCII numbers
• Packed decimal – two BCD digits are packed into a byte register or memory location
• Unpacked decimal numbers are stored one BCD digit per byte
• After an arithmetic operation, the flags are updated accordingly
19
Arithmetic instructionsADD Add byte or word
ADC Add byte or word with carry
Inc Increment byte or word by 1
AAA ASCII adjust for addition
DAA Decimal adjust for addition
Sub Subtract byte or word
Sbb Subtract byte or word with borrow
DEC Decrement byte or word by 1
NEG Negate byte or word
AAS ASCII adjust for subtraction
DAS Decimal adjust for subtraction
MUL Multiply byte or word unsigned
IMUL Integer multiply byte or word
AAM ASCII adjust for multiply
Div Divide byte or word unsigned
Idiv Integer divide byte or word
AAD ASCII adjust for division
CBW Convert byte to word
CWD Convert word to doubleword20
Addition related operationsMnemonic Meaning Format Operation Flags affected
Add Addition Add D, S D = S+D
CF = carry
OF, SF, ZF, AF, PF, CF
ADC Add with carry ADC D, S
D = S+D+CF
CF = carry
OF, SF, ZF, AF, PF, CF
INC Increment by 1 INC D D = D+1 OF, SF, ZF, AF, PF
AAA ASCII adjust for addition
AAA AF, CF, OF, SF, ZF, PF undefined
DAA Decimal adjust for addition
DAA SF, ZF, AF, PF, CF, OF undefined
Destination Source
Register Register
Register Memory
Memory Register
Register Immediate
Memory Immediate
AX Immediate21
Subtraction
Mnemonic Meaning Format Operation Flags affected
Sub Subtract Sub D,S D = D-S
CF = borrow
OF, SF, ZF, AF, PF, CF
Sbb Subtract with borrow
SBB D,S D = D-S-CF OF, SF, ZF, AF, PF, CF
Dec Decrement by 1 DEC D D=D-1 OF, SF, ZF, AF, PF
Neg Negate Neg D D=0-D
CF =1
OF, SF, ZF, AF, PF, CF
DAS Decimal adjust for subtraction
DAS OF, SF, ZF, AF, PF, CF undefined
AAS ASCII adjust for subtraction
AAS OF, SF, ZF, AF, PF, CF undefined
22
Subtraction operands
Destination Source
Register Register
Register Memory
Memory Register
AX Immediate
Register Immediate
Memory immediate
23
Example
If BX = 3A NEG BXGive FFC6
NEG – make the operand negative (2’s complement)Executing the NEG instruction causes the following 2’s complement subtraction00 - (BX) = 0000 + 2’s complement of 3A
= 0000 + FFC6 = FFC6
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Multiplication• When multiplying 8-bit data, result is 16-bit and
stored in AX• When multiplying 16-bit data, result is 32-bit and
result stored in AX and DX • The AX holds the LSB and DX MSB• Only the source operand is specified and the other
operand is in AL, or AX. This is also the same in division
• In division, AH is the remainder and AL is the quotient
• For 16-bit, AX contains the quotient and DX contains the remainder
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Example
If AL is –1 and CL is –2, what will be the result produced In AX forMUL CLIMUL CL
MUL CL is unsigned multiply = FD02IMUL CL is signed multiply = 2
26
Special functions• CBW – convert byte (8-bit) to word (16-bit)• CBW – fill AH with 0 if AL is positive; fill AH with 1s
if AL is negative then AH becomes FF• CWD – convert word (16-bit) to double word (32-
bit) and the DX register is used to store the high order word
• CWD – fill DX with 0 if AL is positive; fill DX with 1s if AX is negative then DX becomes FF
• Use CBW when doing 8-bit operation• Use CWD when doing 16-bit operation
27
Example
What is the result of executing the following sequence Of instructions?MOV AL, A1CBWCWD
Ans.AX = FFA1 after CBWDX = FFFF after CWD
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Multiplication and division functions
Mnemonic Meaning Format Operation Flags affected
MUL Multiply
(Unsigned)
MUL S AX = AL*S8
DX,AX = AX*S16
OF, CF, SF, ZF, AF, PF undefined
DIV Division
(unsigned)
DIV S AL = Q (AX/S8)
AH = R(AX/S8)
OF, CF, SF, ZF, AF, PF undefined
IMUL Integer multiply
(signed)
IMUL S AX = AL*S8
DX,AX=AX*S16
OF, CF, SF, ZF, AF, PF undefined
IDIV Integer divide
(signed)
IDIV S AX = Q(AX/S8)
AH=R(AX/S8)
AX = Q(DX,AX)/S16
DX=R(DX,AX)/S16
If Q is positive and exceeds 7FFF or if Q is negative and becomes less than 8001 then type 0 interrupt occurs
OF, CF, SF, ZF, AF, PF undefined
29
Multiplication related functions
Mnemonic Meaning Format Operation Flags affected
AAM Adjust AL for multiplication
Ascii adjust after Multiplication for BCD values
AAM AH=Q(AL/10)
AL=R(AL/10)
SF, ZF, PF, OF, AF, CF undefined
AAD Adjust AX for division
Prepare 2 BCD values for division
AAD AL=(AH*10+AL)
AH =00
SF, ZF, PF, OF, AF, CF undefined
CBW Convert byte to word
CBW All bits of AH = (MSB of AL)
None
CWD Convert word to double word
CWD All bits of DX = MSB of AX
none
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A simple application
• Try to write a simple assembly program to convert from Centigrade (Celsius) to Fahrenheit
• The equation for the conversion is • f = c * 9 / 5 + 32• What operations are required?• Do you need to define variables?
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Logic Instructions
• Logic operations include: AND, OR, exclusive-OR (XOR) and NOT
• Eg AND AX, BX• Result is stored in AX• Operation is applied to bits of the operands
32
Exercise• Describe the result of executing the
following:• MOV AL, 01010101B • AND AL, 00011111B (AL = 00010101B)• OR AL, 11000000B (AL = 11010101B)• XOR AL, 00001111B (AL = 11011010B)• NOT AL (AL = 00100101B)
Ans. AL = 00100101
NOTE: NOT != NEG
33
Application of logical operations• If you only want to examine a particular bit
in a 8-bit pattern you can do– AND AL, #00010000
• If you want to clear a register to 0 – XOR BL, BL
34
Shift Instructions• There are 4 shift operations • Can perform: logical shift and arithmetic shift• Can shift left as well as right• Logical shift – the vacated bit will be filled with ‘0’• Arithmetic shift – the vacated bit will be filled with the
original most significant bit (this only applies in right shift, in left shift vacated bit is filled with ‘0’ as well.)
• Arithmetic shift will maintain the ‘sign’ of the original data
• If shift more than 1 bit then the number of bits to be shifted is stored in CL
35
Shift operationsMnemonic Meaning Format Operation Flags affected
SAL/SHL Shift arithmetic left/shift logical left
SAL/SHL D, Count
Shift the (D) left by the number of bit positions equal to count and fill the vacated bits positions on the right with zeros
CF, PF, SF, ZF, OF, AF undefined
OF undefined if count not equal to 1
SHR Shift logical right SHR D, Count Shift the (D) right by the number of bit positions equal to count and fill the vacated bits positions on the left with zeros
CF, PF, SF, ZF, OF, AF undefined
OF undefined if count not equal to 1
SAR Shift arithmetic right SAR D, Count Shift the (D) right by the number of bit positions equal to count and fill the vacated bits positions on the left with the original most significant bit
CF, PF, SF, ZF, OF, AF undefined
OF undefined
36
Rotate Instructions
• Rotate is similar to shift • The vacate bit is filled with the bit rotated out
from the other side• Rotate left through carry (RCL) – bit rotated out is
stored in the carry flag, the vacated bit is filled by the carry flag
• Eg RCR BX, CL• If CL = 0416 and BX = 123416 what is the result of
the above operation? – 1000 0001 0010 0011 if C is 0 at the beginning
37
Rotate Operations
Mnemonic Meaning Format Operation Flags affected
ROL Rotate left ROL D, Count Rotate the D left by the number of bit positions equal to Count. Each bit shifted out from the leftmost bit goes back into the rightmost bit position
CF
OF undefined if count 1
ROR Rotate
right
ROR D, Count Rotate the D right by the number of bit positions equal to Count. Each bit shifted out from the rightmost bit goes back into the leftmost bit position
CF
OF undefined if count 1
38
Rotate operationMnemonic Meaning Format Operation Flags affected
RCL Rotate left thro’ carry
RCL D, Count Same as ROL except carry is attached to D for rotation
CF
OF undefined if count 1
RCR Rotate right thro’ carry
RCR D, Count Same as ROL except carry is attached to D for rotation
CF
OF undefined if count 1
39
Applications of shift/rotate• To examine bit 6 of the AL register
– RCL AL, 2• To implement a multiply
– Shift a pattern to left = x2– 00000100 = 4 after shifting to left 00001000 = 8 – Shift to right = /2– 00000100 after shifting to right 00000010 = 2
• 3x9– Binary pattern of 9 = 1001 – 3x9 = 3 x 23 + 3x20
– 3x23 = shifting pattern of 3 to left 3 times + the original value (shift 0 time)– Binary pattern of 3 00000011– 00011000 (after 3 shift) + 00000011– Result = 00011011 = ???
40
Exercise
If AL = 128, BL = -4After the following:
ADD AL, 3ADC BL, ALRCL BL, 1 ; this is rotate left thro’ carry, this does not change the
; sign or overflow
Q. What will be the carry, sign and overflow flag?
41
Exercise
AX = 0010H BX = 0020H CX = 0030H DX = 0040HSI = 0100H DI = 0200H CF = 1HDS:100 = 10H DS:101 = 00H DS:120 = FFHDS:121 = FFH DS:130 = 08H DS:131 = 00HDS:150 = 02H DS:151 = 00H DS:200 = 30HDS:201 = 00H DS:210 = 40H DS:211 = 00HDS:220 = 30H DS:221 = 00H
42
ADD AX, 00FF ( AX = 010F H)
ADC SI, AX (SI = 0111H)
INC BYTE PTR [0100]; Byte ptr indicate the size of memory data addressed by the memory pointer
(the value refers to is 10H so it becomes 11H)
SUB DL, BL (DL = 40 BL =20 so result = 20H)
SBB DL, [0200] (DL = 40H, [0200] refers to 30H so result is 09H )
DEC BYTE PTR [DI+BX] (data refers to by byte ptr [DI+BX] is 30H after DEC = 29H )
NEG BYTE PTR [DI] + 0010 ( NEG 40H = 1100 0000 )
MUL DX (10x40 so AX = 0400H DX = 0000H )
IMUL BYTE PTR [BX + SI] (AX = 0FF0H , DX = 0000H)
DIV BYTE PTR[SI] + 0030 (value refers to by Byte PTR[SI]+0030 is 08H (one byte) so after the division AH = 00 AL = 02)
IDIV BYTE PTR [BX][SI]+0030 ( the value refers to is 02H so after div AH =00 AL =08)
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Exercise1. How would the decimal number 1234 be coded in ASCII and stored in
memory starting at address 0C000H? Assume that least significant digit is stored at the lower addressed memorylocation1 = (31H in 0C003H) 2 = (32H in 0C002H) 3 = (33H in 0C001H) 4 = (34H in 0C000H)
2. If register BX contains the value 010016, register DI contains 0010 and register DS contains 1075, what physical memory location is swapped when the following instruction is executedXCHG [BX+DI], AXPA = 10750+0100+0010 = 10860H
3. Compute the physical address for the specified operand in each of the following instructions:MOV [DI], AX (destination operand) (0B000+0200 = 0B200)MOV DI, [SI] (source operand) (0B000+0100 = 0B100)MOV DI+XYZ, AH (destination operand) (0B000+0200+0400 = 0B600)
Given CS=0A00, DS=0B00, SI=0100, DI=0200, BX=0300, XYZ=0400
44