80C186EB 80C188EB Users Manual 1990 - Al Kossow's Bitsavers

OVERVIEW OF THE 80C186 FAMILY MODULAR
MICROPROCESSOR CORE ARCHITECTURE
With indexed addressing, the effective address is calculated from the sum of a displacement plus the
content of an index register (SI or 01). See Figure 2.17. Indexed addressing is often used to access
elements in an array (see Figure 2.18). The displacement locates the beginning of the array, and the
value of the index register selects one element. If the index register contains OOOOH, the processor
selects the first element. Since all array elements are the same length, simple arithmetic on the register
may select any element.
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Figure 2.17. Indexed Addressing
HIGH ADDRESS
r
I
I
I
I
I
I
I
I
I
INDEX REGISTER
EA
L __________ _
ARRAY (8)
ARRAY (7)
ARRAY (6)
ARRAY (5)
ARRAY (4)
INDEX REGISTER
ARRAY (3) 2
ARRAY (2) ,
I-_A_R_RA_Y_(1_) --I ---1
ARRAY (0)
EA
..,
1 WORD
LOW ADDRESS
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Figure 2.18. Accessing an Array with Indexed Addressing
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