Instruction Representation

1
Instruction Representation

• Assembly Language Programming
• University of Akron
• Dr. Tim Margush

2
Machine Language Codes

• Each instruction is coded as one or more bytes
• The first byte is generally an OpCode
• A numeric code representing a particular
  instruction or class of instructions
• Additional bytes may affect the action of the
  instruction or provide information about the data
  acted upon by the instruction

3
Variable Format Instructions

• The meaning of bits in an instruction varies
  depending on the instruction
• The number of bytes in an instruction varies
  depending on the needs of the instruction
• Basic 8086 instruction format

low disp/data
hi disp/data
low data
hi data
4
Opcode

• 8086 Opcodes are 6, 7, or 8 bits
• d(irection) bit - used for instructions that code
  a register as one of the operands
• 1 register is destination, 0 register is
  source
• w(idth) bit - used to distinguish byte(0)/word(1)
  operands
• 7-bit opcodes do not use d, 8-bit opcodes do not
  use either d or w

5
mod/reg/r/m

• The second byte of some instructions is divided
  into three fields
• The reg field, together with the w bit (in the
  opcode), specifies a register operand
• If there is only one register operand, d
  specifies whether it is the source or destination
• If there are two registers, reg specifies the
  destination register

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reg

• reg w1 w0
• 000 AX AL
• 001 CX CL
• 010 DX DL
• 011 BX BL
• 100 SP AH
• 101 BP CH
• 110 SI DH
• 111 DI BH

• The reg field contains a number between 0 and 7.
• Depending on the w-bit setting, this selects one
  of 8 registers as an operand used by the
  instruction

7
register/memory

• Interpretation depends on mod field contents
• mod 11 r/m(same code as reg)
• second operand is the specified register
• mod 00 r/m110
• second argument is the address specified in the
  the next two bytes of the instruction, direct
  near addressing - DSoffset (Unless segment
  override is in effect)

8
More mod/r/m

• All other combinations specify indirect
  addressing
• Target Address BX/BPSI/DIdisp
• mod specifies the size of the displacement which
  is coded in the next one or two bytes of the
  instruction

• mod r/m mode
• 00 110 direct
• 00 --- no disp
• 01 all byte disp
• 10 all word disp
• 11 all register
• (see next table)

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Indirect Address Modes

• r/m seg addr
• 000 DS BXSI
• 001 DS BXDI
• 010 SS BPSI
• 011 SS BPDI
• 100 DS SI
• 101 DS DI
• 110 SS BP
• 111 DS BX

• if mod 01 or 10
• disp is added
• if mod 00
• no displacement
• Exception mod 00 and r/m 110
• direct addressing using a 16-bit offset and DS
  register

10
Immediate Operands

• Data is stored in the instruction in the byte or
  bytes immediately following the address
  information
• If bytes 3 or 4 contain a displacement the data
  follows the displacement info
• data may be a single byte or a word

11
Disclaimer

• Not all instructions conform to the formats
  described so far! The 8086 instruction set is
  very complex. Some opcodes are only 4-5 bits in
  length, and others are 16-bits. We have hit only
  the highlights here.

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MOV

• mov reg,reg 2 100010dw modregr/m
• mov reg,mem 3-4 100010dw modregr/m disp(1-2)
• mov reg,imm 2-3 1011wreg data(1-2)
• mov acc,mem 3 1010000w disp(2)
• mov mem,acc 3 1010001w disp(2)
• mov mem,imm 4-6 1100011w modregr/m disp(1-2)
  data(1-2)

13
Sample Machine Codes

• 0000 000A a dw 10
• 0002 00 b db ?
• .code
• 0000 8B DF mov bx,di
• 0002 8A F9 mov bh,cl
• 0004 8B 1E 0000 R mov bx,a
• 0008 8A 26 0002 R mov ah,b
• 000C 8B 12 mov dx,sibp
• 000E A0 0002 R mov al,b
• 0011 8A 26 0002 R mov ah,b
• 0015 BB 0003 mov bx,3
• 0018 B1 03 mov cl,3
• 001A C7 06 0000 R 0064 mov a,100
• 0020 C6 06 0002 R FF mov b,255

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PC-Relative Addressing

• The jump instructions use an addressing mode
  called PC-relative or self-relative
• The machine code contains a displacement that is
  added to the current IP to cause a jump to occur
• The assembler must calculate this displacement

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JMP

• Direct intrasegment short
• 11101011 disp
• Direct intrasegment (near)
• 11101001 disp-low disp-high
• Indirect intrasegment
• 11111111 mod 100 r/m
• Indirect intersegment
• 11111111 mod 101 r/m
• Direct intersegment (far)
• 11101010 offset-low offset-high seg-low seg-high

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JMP Example

• 003C LP
• ...
• 0054 JMP LP
• ...
• 010C JMP ELP
• ...
• 0406 ELP
• 0406 JMP XYZ

• disp3C-56E6
• 11101011 11100110
• disp406-10E2F8
• disp406-10F2F7
• 11101001 11110111 00000010
• Assume XYZ is a far label at 01C50094
• EA 94 00 C5 01

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Conditional Jumps

• These are always 2 bytes (8086-80286)
• opcode disp
• JA 01110111 disp
• JE 01110100 disp
• Jwhatever 0111xxxx disp
• 80386 and later processors allow near jumps
• opcode is 1000xxxx disp-lo disp-hi