Intel Assembly

• Format of an assembly instruction:

 

• LABEL:
• Stores a symbolic name for the memory location that it represents.

 

• OPCODE:
• The instruction itself.

 

• OPERANDS:
• A register, an immediate or a memory address holding the values on which the operation is performed.

 

• There can be from 0 to 3 operands.

Data Addressing Modes

• Data movement instructions move data (bytes, words and doublewords) between registers and between registers and memory.

 

• Only the movs instruction can have both operands in memory.

 

• Most data transfer instructions do not change the EFLAGS register.

 

• Format:
• opcode destination , source

 

 

• Register

Data Addressing Modes

• Immediate

 

• Direct (eax), Displacement (other regs)

 

• Register Indirect

• Any of eax , ebx , ecx , edx , ebp , edi or esi may be used.

Data Addressing Modes

• Base-plus-index

• Any combination of eax , ebx , ecx , edx , ebp , edi or esi .

 

• Register relative

• A second variation includes: mov eax , [ ARR+ ebx ]

Data Addressing Modes

• Base relative-plus-index

• A second variation includes: mov eax , [ ebx + edi +4]

 

• Scaled-index

• A second variation includes: mov eax , ebx *2+ ecx +offset
• Scaling factors can be 2X, 4X or 8X.

Data Addressing Modes

• Register addressing:

• Note: mov really COPIES data from the source to destination register.
• Never mix an 16-bit register with a 32-bit, etc.
• For example

• None of the mov instruction effect the EFLAGS register.

Data Addressing Modes

• Immediate addressing:
• The value of the operand is given as a constant in the instruction stream.

• Use b for binary, q for octal and nothing for decimal.

 

• ASCII data requires a set of apostrophes:

 

• Register and immediate addressing example:

Data Addressing Modes

• Direct addressing:
• Transfers between memory and al , ax and eax .
• Usually encoded in 3 bytes, sometime 4:

 

• Displacement:

• Displacement instructions are encoded with up to 7 bytes (32 bit register and a 32 bit displacement).

Data Addressing Modes

• Direct and displacement addressing example:

• Note: Direct addressing (using al) requires 3 bytes to encode while Displacement (using bx) requires 4.

 

• Register Indirect addressing:
• Offset stored in a register is added to the segment register.

• The memory to memory mov is allowed with string instructions.

Data Addressing Modes

• Register Indirect addressing (cont)
• Any register EXCEPT esp for the 80386 and up.
• For eax , ebx , ecx , edx , edi and esi : The data segment is the default.
• For ebp : The stack segment is the default.

 

• Some versions of register indirect require special assembler directives byte, word , or dword

• Does [ edi ] address a byte, a word or a double-word?

 

• The assembler can't determine the size of 0x10 !
• Use: