Two Dimensional Arrays

Two operand types, base-index and base-index-displacement, work well when dealing with two-dimensional arrays.

Base-Index Operands

• Adds the values of two registers (base and index), producing an offset address.

• Any two general purpose registers may be used in 32-bit mode(in 16-bit mode, only bx with di or si, and bp with di or si)

.dataarray WORD 1000h, 2000h, 3000h.code

mov ebx, OFFSET arraymov esi, 2mov ax, [bx + si] ;ax = 2000h

Table Example

• For two dimensional tables, a base register usually contains a row offset, and an index register contains a column offset.

• Defining a table that has three rows and five columns:

tableB BYTE 10h, 20h, 30h, 40h, 50h

BYTE 60h, 70h, 80h, 90h, 0A0h

BYTE 0B0h, 0C0h,0D0h, 0E0h, 0F0h

NumCols = 5

Memory

• In memory this table is a continuous stram of bytes as if it were a one-dimensional array.

• The physical storage of the array is in row-major order, where the last byte in the first row is followed by the first byte in the second row, and so on.

How to move through the tableAssuming that the rows begin with 0, Columns begin with 0.

RowNumber = 1

Column Number =2

Mov ebx, OFFSET tableB

Add ebx, NumCols * RowNumber

Mov esi, ColumnNumber

Mov al, [ebx + esi] ;AL = 80h

150 155 15710 20 30 40 50 60 70 80 90 A0 B0 C0 D0 E0 F0

[ebx] [ebx + si]

Looking at Memory Locations• Move into BX, OFFSET of tableB• Each row is 5 bytes long (NumCols)• Data is on 1st row (row nums begins with 0)• Multiply Rownumber * NumCols• Add result to bx.• Move into SI, ColumnNumber (data is in 2nd

column)• Move into al, value

150 155 15710 20 30 40 50 60 70 80 90 A0 B0 C0 D0 E0 F0

[ebx] [ebx + si]