ADDRESSING MODES SUMMARY

The iAPX88 processor supports seven modes of memory access. Remember that immediate is not an addressing mode but an operand type. Operands can be immediate, register, or memory. If the operand is memory one of the seven addressing modes will be used to access it. The memory access mechanisms can also be written in the general form “base + index + offset” and we can define the possible addressing modes by saying that any one, two, or none can be skipped from the general form to form a legal memory access.

There are a few common mistakes done in forming a valid memory access. Part of a register cannot be used to access memory. Like BX is allowed to hold an address but BL or BH are not. Address is 16bit and must be contained in a 16bit register. BX-SI is not possible. The only thing that we can do is addition of a base register with an index register. Any other operation is disallowed. BS+BP and SI+DI are both disallowed as we cannot have two base or two index registers in one memory access. One has to be a base register and the other has to be an index register and that is the reason of naming them differently.

Direct

A fixed offset is given in brackets and the memory at that offset is accessed. For example “mov [1234], ax” stores the contents of the AX registers in two bytes starting at address 1234 in the current data segment. The instruction “mov [1234], al” stores the contents of the AL register in the byte at offset 1234.

Based Register Indirect

A base register is used in brackets and the actual address accessed depends on the value contained in that register. For example “mov [bx], ax” moves the two byte contents of the AX register to the address contained in the BX register in the current data segment. The instruction “mov [bp], al” moves the one byte content of the AL register to the address contained in the BP register in the current stack segment.

Indexed Register Indirect

An index register is used in brackets and the actual address accessed depends on the value contained in that register. For example “mov [si], ax” moves the contents of the AX register to the word starting at address contained in SI in the current data segment. The instruction “mov [di], ax” moves the word contained in AX to the offset stored in DI in the current data segment.

Based Register Indirect + Offset

A base register is used with a constant offset in this addressing mode. The value contained in the base register is added with the constant offset to get the effective address. For example “mov [bx+300], ax” stores the word contained in AX at the offset attained by adding 300 to BX in the current data segment. The instruction “mov [bp+300], ax” stores the word in AX to the offset attained by adding 300 to BP in the current stack segment.

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Indexed Register Indirect + Offset

An index register is used with a constant offset in this addressing mode. The value contained in the index register is added with the constant offset to get the effective address. For example “mov [si+300], ax” moves the word contained in AX to the offset attained by adding 300 to SI in the current data segment and the instruction “mov [di+300], al” moves the byte contained in AL to the offset attained by adding 300 to DI in the current data segment.

Base + Index

One base and one index register is used in this addressing mode. The value of the base register and the index register are added together to get the effective address. For example “mov [bx+si], ax” moves the word contained in the AX register to offset attained by adding BX and SI in the current data segment. The instruction “mov [bp+di], al” moves the byte contained in AL to the offset attained by adding BP and DI in the current stack segment. Observe that the default segment is based on the base register and not on the index register. This is why base registers and index registers are named separately. Other examples are “mov [bx+di], ax” and “mov [bp+si], ax.” This method can be used to access a two di mensional array such that one dimension is in a base register and the other is in an index register.

Base + Index + Offset

This is the most complex addressing method and is relatively infrequently used. A base register, an index register, and a constant offset are all used in this addressing mode. The values of the base register, the index register, and the constant offset are all added together to get the effective address. For example “mov [bx+si+300], ax” moves the word contents of the AX register to the word in memory starting at offset attained by adding BX, SI, and 300 in the current data segment. Default segment association is again based on the base register. It might be used with the array base of a two dimensional array as the constant offset, one dimension in the base register and the other in the index register. This way all calculation of location of the desired element has been delegated to the processor.