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384 Fundamentals of Computers NPP

5.2.9 Indexed Adressing Mode 5.2.9 B§S>oŠñS> ES´>oqgJ _moS>
Index Register is specified in the instruc- {ZX}em| _| B§S>oŠg a{OñQ>a {cIm OmVm h¡ {OgHo$ A§Xa
tion which holds the index value. This index B§S>oŠg d¡ë`y hmoVr h¡Ÿ& Bg_| {S>ñßcoñ_|Q> Omo‹S>H$a B\o$ŠQ>rd
value is added to the displacement specified in
the instruction to get the effective address: ES´>og àmßV {H$`m OmVm h¡…
MOV AH, 3 [SI]
In this 8086 instruction SI is a source index CnamoŠV 8086 {ZX}e _| SI EH$ gmog© B§S>oŠg
register and 3 is the displacement. The effective a{OñQ>a h¡ VWm 3 {S>ñßcoñ_|Q> h¡ Omo Amng _| Ow‹S>H$a
address is calculated by adding contents of B’o$p³Q>d ES´>og àXmZ H$aVo h¢&
SI and 3.

Instruction

Index Reg. Displacement

⊕

Effective Address

5.2.10 Base Register Addressing Mode 5.2.10 ~og a{OñQ>a ES´>o{g§J _moS
A Base Register and a displacement may Am°na|S> _| ~og a{OñQ>a VWm {S>ñßcoñ_|Q> ~Vm`m OmVm
be specified in the operand. Addition of both h¡& XmoZm| H$mo Omo‹S>Zo go B\o$p³Q>d ES´>og àmßV hmoVm h¡& {ZåZ
gives the effective address, consider following
8086 instruction: 8086 {ZX}e H$mo XoIm|…
MOV CH, 3 [BX]
BX is a 16- bit Base Register. The displace- BX EH$ 16-{~Q> a{OñQ>a h¡, 3 EH$ {S>ñßcoñ_|Q> h¡Ÿ&
ment specified is 3. The effective address is BX + 3 B\o$p³Q>d ES´>og hmoJm Ÿ& ~og a{OñQ>a _| ~og ES´>og
BX + 3. The base register holds Base address. hmoJmŸ& Hw$N> Am¡a CXmhaU XoImo…
Consider few more examples:
ADD 3 [BX], DX
MOV DH, 42 [BP]
The block diagram representation of this Bg _moS> H$m ãcm°H$ S>m`J«m_ àXe©Z Bg àH$ma go
addressing mode is as follows: {H$`m Om gH$Vm h¡…