MPS Lectures - Chapter 3

Figure 3-3 Operand formats for the addressing modes.

Addressing Mode		Format	Examples

Data-related
	Immediate	Constant Expression	10110B 529 0A9H 'AB' YY-XX+5
	Direct	Variable +- Constant Expression	WGT CNT-5 ARRY+5
	Register	Register	AX DH
	Register indirect	[Register]	[BX]
	Register relative	Variable[Register +- Constant Exp] or [Register +- Constant Exp]	VAR X[BX] MESG[SI+10H] [BX-1]
	Based indexed	[Base register][Index register]	[BP][DI]
	Relative based indexed	Var.[Base reg. +- Const. Exp][Index reg. +- Const. Exp] or [Base reg. +- Const. Exp][Index reg. +- Const. Exp]	E[BX+5][SI-2] DATA[BX][SI] [BP+2][DI-2]

Branch-related
	Intrasegment direct	Label +- Constant Expression	OVFL_ERROR+7
	Intrasegment indirect	Same as data-related formats except cannot be immediate
	Intersegment direct	Label +- Constant Expression	BRANCH_EXIT
	Intersegment indirect	Same as data-related formats except must not be immediate or register

Figure 3-4 Typical assembler language instructions.

assume that

ADD AX,(BX) is a word addition and ADD AL,(BX) a byte addition. Also, if appropriate directives are used to define COST to be a word variable and COUNT to be a byte variable, then INC COST will be a word operation, and INC COUNT will be a byte operation. For some situtations, however, it is impossible for the assembler to deduce the operand type. The instruction INC [BX] increments the quantity whose address is in BX, but should it increment a byte or a word? One of the purposes of the PTR operator is to specify the length of a quantity in this and other ambiguous situations. It is applied by writing the desired type followed by PTR. For the above INC instruction the PTR operator would be used to modify the operand as follows: INC BYTE PTR [BX] if a byte is to be incremented, or INC WORD PTR [BX] if a word is to be incremented.

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