Figure 2-10 General instruction format.
- Immediate
- The datum is either 8 bits or 16 bits long and is part of the instruction.
- Direct
- The 16-bit effective address of the datum is part of the instruction.
- Register
- The datum is in the register that is specified by the instruction. For a 16-bit operand, a register may be AX, BX, CX, DX, SI, DI, SP, or BP, and for an 8-bit operand a register may be AL, AH, BL, BH, CL, CH, DL, or DH.
- Register Indirect
- The effective address of the datum is in the base register BX or an index register that is specified by the instruction, i.e.,
| EA = |  | (BX)
(DI)
(SI) |  |
- Register Relative
- The effective address is the sum of an 8- or 16-bit displacement and the contents of a base register or an index register, i.e.,
| EA = | | (BX)
(BP)
(SI)
(DI) | | + | | | 8-bit displacement
(sign extended)
16-bit displacement | |
- Based Indexed
- The effective address is the sum of a base register and an index register, both of which are specified by the instruction, i.e.,
- Relative Based Indexed
- The effective address is the sum of an 8- or 16-bit displacement and a based indexed address, i.e.,
| EA = | | (BX)
(BP) | | + | | | (SI)
(DI) | | + | | | 8-bit displacement
(sign extended)
16-bit displacement | |
Figure 2-11 Data-related addressing modes.
 |
| (a) Immediate |
|
 |
| (b) Direct |
|
 |
| (c) Register |
|
 |
| (d) Register indirect |
|
 |
| (e) Register relative |
|
 |
| (f) Based indexed |
|
 |
| (g) Relative based indexed |
| *EA is added to 1610 times the contents of the appropriate segment register. |
For example, if
(BX) = 0158 (DI) = 10A5 Displacement = 1B57 (DS) = 2100
and DS is used as the segment register, then the effective and physical addresses produced by these quantities and the various addressing modes would be
- Direct: EA = 1B57
- Physical address = 1B57 + 21000 = 22B57
- Register: No effective address - datum is in specified register.
- Register indirect assuming register BX:
- EA = 0158
Physical address = 0158 + 21000 = 21158
- Register relative assuming register BX:
- EA = 0158 + 1B57 = 1CAF
Physical address = 1CAF + 21000 = 22CAF
- Based indexed assuming registers BX and DI:
- EA = 0158 + 10A5 = 11FD
Physical address = 11FD + 21000 = 221FD
- Relative Based indexed assuming BX and DI:
- EA = 0158 + 10A5 + 1B57 = 2D54
Physical address = 2D54 + 21000 = 23D54
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