Assembly Data Transfer & Addressing

Data Transfer Instructions

• MOV instruction copies data from a source to a destination.
  • Operands must be the same size.
  • Memory to memory moves are not allowed directly.
  • Standard formats include MOV reg, reg; MOV mem, reg; MOV reg, mem; MOV mem, imm; MOV reg, imm.
• MOVZX instruction zero-extends a smaller source operand to a larger destination, which is register only, and is used for unsigned integers.
• MOVSX instruction sign-extends a smaller source operand to a larger destination, which is register only, and is used for signed integers.
• XCHG instruction exchanges the contents of two operands, which can be register or memory, but does not allow immediate operands.

Addressing Modes

• Direct Memory Operands access memory locations using variable names, or labels.
• Direct-Offset Operands access memory locations by adding a displacement to a variable name like arrayB+1, which is useful for accessing array elements.

Arithmetic Instructions

• INC instruction increments a register or memory operand by 1.
• DEC instruction decrements a register or memory operand by 1.
• ADD instruction adds a source operand to a destination operand, and the result is stored in the destination.
• SUB instruction subtracts a source operand from a destination operand, and the result is stored in the destination.
• Carry, Zero, Sign, Overflow, Auxiliary Carry, and Parity flags are all affected by arithmetic operations.
• NEG Instruction reverses the sign of a number by converting it using two's complement, which affects Carry, Zero, Sign, Overflow, Auxiliary Carry, and Parity flags.
• Arithmetic Expressions can be implemented in assembly using instructions like MOV, NEG, SUB, and ADD.

Status Flags

• Arithmetic instructions affect CPU status flags, which indicate information about the result like negative, positive, zero, or overflow, and are used for error detection and conditional branching.
• Zero Flag (ZF) is set when the result is zero.
• Sign Flag (SF) is set when the result is negative.
• Carry Flag (CF) indicates unsigned integer overflow, which means out of range.
• Overflow Flag (OF) indicates signed integer overflow, which means out of range.

Carry Flag (CF) Details

• During addition, the Carry Flag (CF) is set if the sum exceeds the destination operand's storage size, indicating unsigned overflow.
• During subtraction, the Carry Flag (CF) is set when a larger unsigned integer is subtracted from a smaller one.

Overflow Flag (OF) Details

• The Overflow Flag (OF) is set when a signed arithmetic operation overflows or underflows the destination operand.
• Overflow occurs when adding two positive numbers and getting a negative result, or adding two negative numbers and getting a positive result.
• The CPU sets all status flags based on Boolean rules, regardless of whether the operation is signed or unsigned; the programmer decides which flags to interpret.

Data-Related Operators and Directives

• OFFSET returns the distance of a variable from the beginning of its segment.
• PTR overrides an operand's default size.
• TYPE returns the size in bytes of an operand or array element.
• LENGTHOF returns the number of elements in an array.
• SIZEOF returns the number of bytes used by an array initializer (LENGTHOF * TYPE).
• LABEL redefines a variable with different size attributes.

Indirect Addressing

• Indirect addressing uses registers as pointers to access memory locations.
• Indirect operands are enclosed in brackets such as [ESI], and the register contains the address of the data.
• Indirect addressing is useful for traversing arrays by incrementing the register, and the increment value must match the array element size.
• Indexed operands add a constant to a register to generate an effective address like arrayW[ESI].
• Suitable for array processing, the index register should be initialized to zero.
• Indexed scaling is used to account for the size of array elements when calculating offsets like arrayB[ESI * TYPE arrayB].

Pointers

• Pointers store the memory address of another variable.
• Pointers are useful for manipulating arrays and data structures.
• Pointer size depends on the processor mode, either 32-bit or 64-bit.
• The OFFSET operator can be used to clarify pointer declarations.

JMP and LOOP Instructions

• The JMP instruction provides an unconditional jump to a specified label, creating loops.
• The LOOP instruction repeats a block of code a specific number of times, using ECX as a counter.
  • ECX is decremented with each iteration.
  • The loop continues as long as ECX is not zero.
  • Nested loops require saving the outer loop counter's ECX value to avoid conflicts.