Flags are special indicators used by the processor to signal the outcome of operations, these are stored in a special register called the flags register. Some common flags include:
Zero Flag (ZF) – 1 if the result of an operation is zero; otherwise, it’s 0.
Sign Flag (SF) – 1 if the result is negative; otherwise, it’s 0.
Overflow Flag (OF) – 1 if an arithmetic overflow occurs, meaning the result is too large to fit the destination operand.
Carry Flag (CF) – 1 if there’s a carry-out (for addition) or borrow (for subtraction) from the most significant bit.
Parity Flag (PF) – 1 if the number of set bits in the result is even; otherwise, it’s 0.
Auxiliary Carry Flag (AF) – Used for binary-coded decimal (BCD) arithmetic; it’s set when there’s a carry-out from the lower nibble (4 bits).
Memory addressing in assembly language is a method by which instructions refer to data stored in memory.
Immediate Addressing – The operand is a constant value directly specified in the instruction.
E.g. MOV EAX, 1234h ; Move the immediate value 1234h into EAX
Direct Addressing – The operand specifies the memory address directly
E.g. MOV EAX, [1234h] ; Move the value at memory address 1234h into EAX
Register Addressing – The operand is a register, and the data is in the register.
E.g. MOC EAX, EBX ; Move the value in register EBX into EAX
Register Indirect Addressing – The operand is a register containing the address of the data.
E.g. MOV EBX, 1234h
MOV EAX, [EBX] ; Move the value at the address in EBX into EAX
Based Addressing – Combines a base register with an offset to form the effective address.
E.g. MOV EBX, 100h
MOV EAX, [EBX + 10h] ; Move the value at the address (BX + 10h) into EAX
Indexed Addressing – Use an index register to modify the address.
E.g. MOV ESI, 10h
MOV EAX, [ESI] ; Move the value at the address in ESI into EAX
Base-Indexed Addressing – Combines both a base register and an index register to form the address.
E.g. MOV EBX, 100h
MOV ESI, 20h
MOV EAX, [EBX + ESI] ; Move the value at the address (EBX + ESI) into EAX
Based-Indexed with Displacement – Combines a base register, an index register, and a displacement to form the address.
MOV EBX, 1000h
MOV ESI, 20h
MOV EAX, [EBX + ESI + 10h] ; Move the value at the address (EBX + ESI + 10h) into EAX
