register-indirect-with-displacement

Structured data (structs, records) have fields at fixed offsets from a base pointer. Accessing object.field requires base address + field offset. Can addressing mode handle this?

Register Indirect with Displacement computes the effective address by adding a register value (base) and a constant displacement (offset). Ideal for accessing structure fields and stack variables.

1. Instruction specifies a register (base) and a constant offset (displacement)
2. CPU reads the register to get base address
3. Effective address = register value + displacement
4. CPU accesses memory at computed address
5. Example: MOV AX, [BX + 4] — accesses memory at (BX + 4)

• Perfect for struct/record field access: base = struct pointer, displacement = field offset
• Displacement is fixed at compile time (field offsets don’t change)
• Used for stack variables (SP/BP + offset), struct fields, array elements
• More flexible than pure indirect or indexed addressing

• Built from: Addressing Mode, Indirect Addressing
• Related: Struct, Stack, Effective Address
• Contrasts with: Indexed Addressing — uses register for index, not constant
• Builds into: Stack Frame — accessing local variables

• Displacement is limited (typically 8 or 16 bits in instruction encoding)
• Register must contain valid base address
• Common in RISC: lw $t0, 4($sp) (MIPS load with displacement)