Data Addressing Modes

• Base-Plus-Index addressing:
• Effective address computed as:
• seg_base + base + index.

 

• Base registers: Holds starting location of an array.
• ebp (stack)
• ebx (data)
• Any 32-bit register except esp.

 

• Index registers: Holds offset location.
• edi
• esi
• Any 32-bit register except esp .

Data Addressing Modes

• Base-Plus-Index addressing:

Data Addressing Modes

• Register Relative addressing:
• Effective address computed as:
• seg_base + base + constant.

• Same default segment rules apply with respect to ebp , ebx , edi and esi .
• Displacement constant is any 32-bit signed value.

 

• Base Relative-Plus-Index addressing:
• Effective address computed as:
• seg_base + base + index + constant.

• Designed to be used as a mechanism to address a two-dimensional array.

Data Addressing Modes

• Base Relative-Plus-Index addressing:

Data/Code Addressing Modes

• Scaled-Index addressing:
• Effective address computed as:
• seg_base + base + constant*index.

 

Code Memory-Addressing Modes:

• Used in jmp and call instructions.
• Three forms:
• Direct
• PC-Relative
• Indirect

 

• Direct:
• Absolute jump address is stored in the instruction following the opcode.

Code Addressing Modes

• An inter segment jump:

• This far jmp instruction loads cs with 1000H and eip with 00000000H.
• A far call instruction is similar.

 

• PC-Relative:
• A displacement is added to the EIP register.
• This constant is encoded into the instruction itself, as above.

 

• Intra segment jumps:
• Short jumps use a 1-byte signed displacement.
• Near jumps use a 4-byte signed displacement.
• The assembler usually computes the displacement and selects the appropriate form.

Code Addressing Modes

• Indirect:
• Jump location is specified by a register.
• There are three forms:
• Register:
• Any register can be used: eax , ebx , ecx , edx , esp , ebp , edi or esi .

 

• Register Indirect:
• Intra segment jumps can also be stored in the data segment.

 

• Register Relative:

Stack Addressing Modes

• The stack is used to hold temporary variables and stores return addresses for procedures.
• push and pop instructions are used to manipulate it.
• call and ret also refer to the stack implicitly.

 

• Two registers maintain the stack, esp and ss .
• A LIFO (Last-in, First-out) policy is used.
• The stack grows toward lower address.
• Data may be pushed from any of the registers or segment registers.
• Data may be popped into any register except cs.