2.5addressing modes in 8086
TRANSCRIPT
register addressing : REGimmediate addressing: within Instructiondirect addressingregister indirect addressingbased addressingindexed addressingbased indexed addressing
◦ Register Operand Addressing Mode can be accessed in byte, word, or double word sizes. MOV AX, BX Byte: AL, AH, BL, BH, CL, CH, DL, DH
Word: AX, BX, CX, DX, SP, BP, SI, DI, CS, DS, SS, ES, FS, GSDouble Word: EAX, EBX, ECX, EDX, ESP, EBP, ESI, EDI
See Fig 3.8 in page 64◦ Immediate Operand Addressing
an operand is part of the instruction MOV AL, 15H 8 bits, 16 bits, and 32 bits in length See Fig 3.10 in page 67
ABCD
0000 IPCSDSSSESFSGS
AXBXCXDXSPBPSIDI
Address MemoryContent
Instruction80386MPU
0100
XXXX ABCD
01000 8B MOV AX, BX01001 C301002 XX
0000 IPCSDSSSESFSGS
AXBXCXDXSPBPSIDI
Address MemoryContent
Instruction80386MPU
0100 01000 B0 MOV AL, 15H01001 1501002 XX
XX15
16-bit Memory Operand Addressing Modes◦ 16-bit addressing modes and 32-bit addressing
modes◦ Physical address = Segment Base: EA(effective
address) Segment Base Address(SBA) : the starting location of
the segment EA : the offset of the operand from the beginning of
the segment of memory EA = Base + Index + Displacement Base = BX or BP, Index = SI or DI, displacement = 8-bit or 16-bit
◦ Direct Addressing Mode PA = Segment Base : Direct Address MOV CX, [1234H]
MOV CX, [1234H]IPCSDSSSESFSGSAXBXCXDXSPBPSIDI
0100001001010020100301004
8B0E3412XX
000001000200
XXXX03234 ED03235 BE
BEED
◦ Register Indirect Addressing Mode PA = Segment Base : Indirect Address {BX,BP,SI,DI} example : MOV AX, [SI]
MOV AX,[SI]IPCSDSSSESFSGSAXBXCXDXSPBPSIDI
0100001001010020100301004
8B04XXXXXX
000001000200
XXXX03234 ED03235 BE
BEED1234
Based Addressing Mode◦ PA = Segment Base : {BX or BP} + {8-bit or 16-bit displacement}◦ Base register : the beginning of a data structure (See Fig 3.16 (b) in
page 74◦ Example: MOV [BX]+1234H, AL
MOV [BX]+1234H, ALIPCSDSSSESFSGSAXBXCXDXSPBPSIDI
0100001001010020100301004
88073412XX
000001000200
04234 04235
1234
ED1000
Displacement
Base Register
+
Element 0Element 1
Data Structure
Element n-1
◦ Indexed Addressing Mode PA = Segment Base : {SI, DI}+{8-bit or 16-bit displacement} Displacement : the starting address of an array; Index: selects the specific
element in the array Example: MOV AL, [SI]+2000H
Index Register
Displacement
+
Element 0Element 1
Array of data
Element n-1
IPCSDSSSESFSGSAXBXCXDXSPBPSIDI
0100001001010020100301004
8A443412XX
000001000200
05234 05235
2000
XX1000
MOV AL, [SI]+1234H
BE
◦ Based-Indexed Addressing Mode PA= Seg Base: {BX, BP}+{SI,DI}+{8-bit or 16-bit
displacement} to access complex data structures See fig 3.20 in page 80 Example: MOV AH, [BX][SI]+1234H
opcode : 8A 44 34 12