Arrays and Addressing Modes

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Arrays and Addressing Modes

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

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What Is an Array?

• Collection of components
• All components are the same size
• Components are located in consecutive storage
  locations
• The base address of the array is the address of
  the first component
• Components are numbered 0, 1, ...

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Random Access

• The address of any component (CA) can be
  determined from the base address (BA), component
  size (CS), and the component index (CI)
• CA BA CI CS
• Time to access any component is the same (does
  not depend on component index)
• Contrast access in a linked list

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Array Declarations

• int_arry dw 10, 20, 30
• byt_arry db 12,'a','xy',0FFh,-1
• un_arry dd 25 dup (?)
• zro_arry dw 10 dup (0)
• spc_arry db 80 dup (" ")
• The text says array components are numbered 1, 2,
  etc. Ignore this and use C conventions (0, 1, 2,
  )

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8086 Addressing Modes

• There are 24 different addressing modes!
• Fortunately, they break down into a smaller
  number of distinctive categories
• Addressing Mode means how the address information
  in the instruction is used to select a physical
  address
• Common modes specify one or more of Base
  register, Index register, and Displacement

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Direct Addressing

• Instruction contains a displacement into a
  segment
• Segment may be implied or specified using the
  segment override operator
• mov ax,abc
• mov cl,ss12
• mov dl,es200

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Register Indirect

• Offset is contained in a register - it is a
  pointer to a memory location
• BP, BX, SI, DI only
• JMP and CALL allow any GP register to be used
• The segment register is implied or may be
  specified

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Register Indirect Examples

• mov ax,bp
• mov cl,ssbx
• mov dl,essi
• mov di,ax
• DS is assumed for BX, SI, and DI
• SS is assumed for BP

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Base and Indexed Addressing

• mov ax,numbersSI
• mov cl,BP4
• mov ES6DI,AX
• This is the same as Register Indirect, except a
  displacement is added into the mixture
• Based BP or BX are used
• Indexed SI or DI are used

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Base-Index Addressing

• mov cl,BXSI
• mov ax,numbersSIBP
• mov ES6SIBX,dh
• One base register and one index register
• If a displacement is specified, the addressing
  mode is called Base-Index with Displacement

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PTR and LABEL

• Ambiguous size operands
• Use PTR to resolve
• mov bx,1
• mov word ptr bx,1
• mov si, 'A'
• mov si,byte ptr 'A'

• X label byte
• M label word
• DW 100
• X and M refer to the same address
• mov AH,X
• mov AH,byte ptr M
• mov AX,M

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Summing Words In an Array

• mov cx,10 number of words
• mov ax,0 initialize total
• lea bx,arry pointer to array
• top uses register indirect
• add ax,bx add next
• add bx,2 point to next
• loop top

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Summing Them Again

• mov cx,10 number of words
• mov ax,0 initialize total
• mov si,0 index into array
• top uses indexed addressing
• add ax,arrysi add next
• add si,2 point to next
• loop top

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And Again

• mov cx,10 number of words
• mov ax,0 initialize total
• lea bx,arry base of array
• mov si,0 index into array
• top base-index addressing
• add ax,bxsi add next
• add si,2 point to next
• loop top

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Access to the Stack

• mov bp,sp BP points to stack top
• mov ax,bp0 top of stack
• mov bx,bp2 next to top of stack
• mov bp3,ah byte at offset 3 in stack
• mov bp-1,ch byte above top of stack

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Homework

• Pg 201
• 1, 2, 3, 4 (b,c)